Re: [time-nuts] 20Hz p-p / 24 hours with a Mini-T

[Bruce Griffiths]

Miguel Checa wrote:
> Hi group!
>
> I need some opinions, counsel, help, whatever you can send my way:
>
> I need to maintain the Mini-T within 20 Hz p-p in a 24 h period. I use 
> T=450 and all is well until the thing starts to lose satellites one by 
> one and goes into holdover and after a while, the VCO voltage jumps to 
> the start value and with it the frequency (up to -150Hz).
>
> The antenna is not in a good place and it can't be moved, but if I 
> restart the Mini-T, all satellites that were grayed out come alive and 
> strong. Any clues?
>
> Thanks,
>
> Miguel
> W4/LU9AXC
>
>
>   
Miguel

20Hz is a fairly wide tolerance that one could easily maintain with a
good OCXO with periodic manual frequency adjustments of perhaps once per
year.

If the Mini-T never sees a sufficient number of SVs uninterrupted for
somewhat longer than 450 sec then you need to reduce T to a value such
that sufficient SVs are seen for longer than T.

Do you have anyu idea of sky coverage and maximum observed SV track lenghts?

Bruce

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Re: [time-nuts] 10 MHz Band-Pass Filter Needed

[Steve Rooke]

2009/1/9 Bruce Griffiths :
> A crystal filter may not be a good choice for phase stability unless an
> oven is used.
> Some of the older frequency standards used crystal filters within an
> oven to cleanup the output signal.

There is the other issue of xtal jumps occuring randomly (?) which
could cause rapid phase changes.

73, Steve

> The limited crystal dissipation as may raise the phase noise floor of a
> low noise OCXO significantly.
> The implementation shown in AN9815 is particularly poor in this respect.
>
> A low Q resonant LC filter using low tempco components will have greater
> phase stability.
> Whilst obtaining capacitors with relatively low tempcos is relatively
> easy, the tempco of most off the shelf inductors is usually unspecified.
>
> The significance of the increased phase instability and /or raised phase
> noise floor depends on the application.
> If one is multiplying the output to 10GHz then even minor temperature
> fluctuations will be a concern.
>
> Bruce
>
> Darrell Robinson wrote:
>> If you're simply looking for purity of signal maybe a crystal filter.
>>
>> Intersil application note AN9815 is interesting.
>>
>> Darrell
>>
>>
>> - Original Message -
>> From: "Richard W. Solomon" 
>> To: 
>> Sent: Thursday, January 08, 2009 10:11 AM
>> Subject: [time-nuts] 10 MHz Band-Pass Filter Needed
>>
>>
>>
>>> The GPSDO I want to use has an output rich in harmonics. In some
>>> cases that is good, but Murphy rules and in the application I have
>>> today, it is not good.
>>>
>>> I need a 10 MHz Band-Pass Filter, Bandwidth is not critical, something
>>> small with SMA connectors would be ideal, but I can live with BNC.
>>>
>>> Anyone have such a beast or know where I can get one ? I checked
>>>
>> Mini-Circuits
>>
>>> and choked on the price !!
>>>
>>> Thanks, Dick, W1KSZ
>>>
>>> ___
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>>>
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>>>
>>
>>
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-- 
Steve Rooke - ZL3TUV & G8KVD & JAKDTTNW
Omnium finis imminet

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[time-nuts] 20Hz p-p / 24 hours with a Mini-T

[Miguel Checa]

Hi group!

I need some opinions, counsel, help, whatever you can send my way:

I need to maintain the Mini-T within 20 Hz p-p in a 24 h period. I use 
T=450 and all is well until the thing starts to lose satellites one by 
one and goes into holdover and after a while, the VCO voltage jumps to 
the start value and with it the frequency (up to -150Hz).

The antenna is not in a good place and it can't be moved, but if I 
restart the Mini-T, all satellites that were grayed out come alive and 
strong. Any clues?

Thanks,

Miguel
W4/LU9AXC



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Re: [time-nuts] GPSDO TC & Damping

[Bruce Griffiths]

Richard Moore wrote:
> On Jan 8, 2009, at 2:46 PM, time-nuts-requ...@febo.com wrote:
>
>   
>> Message: 1
>> Date: Fri, 09 Jan 2009 10:28:35 +1300
>> From: Bruce Griffiths 
>> Subject: Re: [time-nuts] GPSDO TC
>> To: Discussion of precise time and frequency measurement
>>  
>>
>> Richard Moore wrote:
>> 
>>> On Jan 8, 2009, at 2:58 AM, time-nuts-requ...@febo.com wrote:
>>>
>>>
>>>   
 Message: 6
 Date: Thu, 08 Jan 2009 11:51:50 +0100
 From: Magnus Danielson 
 Subject: Re: [time-nuts] GPSDO time constant
 To: Tom Van Baak ,Discussion of precise  
 time and
frequency measurement 

 For ThunderBolt owners it is pretty straightforward to adjust the
 TC and
 damping, which is very nice. Use this oppertunity!

 
>>> So, Magnus (and Tom), what damping factor do you suggest for a TBolt?
>>> I'm running a verrry long TC now. If 1.2 is not actually critically
>>> damped, what value would be? Any guesses? BTW, I really like that
>>> plot of Tom's that tracks the oven and then gets better from the  
>>> GPS...
>>>
>>> Dick Moore
>>>
>>>
>>>   
>> Richard
>>
>> As always, the problem is how do you know that the time constant  
>> you are
>> using is anywhere near optimum?
>>
>>
>> Bruce
>> 
>
> Well, like many here, I don't actually have the equipment, especially  
> the reference std., to do these MDEV, ADEV and other analyses, so,  
> since I use the GPSDO for a frequency standard and not for UTC, I  
> thought I'd get the expert opinions. Magnus has several times  
> indicated here that a TC laying somewhere in and around 100 to 1000  
> secs is probably optimum. When I enquired some time back about  
> damping in the TBolt, the consensus seemed to be "leave it at 1.2". I  
> have, but it just seems to me that won't be optimum for a fixed- 
> position, lab-located frequency standard -- at the moment, I'm  
> leaning toward the 0.7to 1.0 area.
>
>   
Why, since it has been demonstrated that a damping factor of 1.2 is
better than one of 0.7 for a particular Thunderbolt this would tend to
indicate that adjusting the damping without good justification is
somewhat foolhardy.
If in fact the phase noise characteristics of your OCXO are similar toi
the one in the Thunderbolt that Tom measured this would degrade the
performance.

With no way of measuring the effect of such adjustments you are just
hoping that your particular Thunderbolt is similar to the one Tom measured.
Thats not engineering its more like witchcraft.

> Tom's recent chart was quite helpful, especially the 1000 sec curve.  
> Now, I hope that Tom or someone else follows up on the suggestion to  
> track performance vs. damping factor. I do understand that the  
> results for any one GPSDO don't *necessarily* translate to other  
> devices, but they don't necessarily don't, either. At least for the  
> TBolts a lot of us are playing with, one good example (like Tom's)  
> may well put mine in a better ballpark than the ballpark the factory  
> wants it to play in, given the factors that you all have described.  
> Thx everyone for the comments. Look forward to the next round!
>
> Dick Moore
>   
The probability that you will improve the performance significantly
without a means of measuring the resultant performance is fairly low.
You will never know if either an improvement or a degradation in
performance has occurred.
The one saving grace being that the factory defaults can always be restored.

Bruce

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Re: [time-nuts] 10 MHz Band-Pass Filter Needed

[Bruce Griffiths]

A crystal filter may not be a good choice for phase stability unless an
oven is used.
Some of the older frequency standards used crystal filters within an
oven to cleanup the output signal.

The limited crystal dissipation as may raise the phase noise floor of a
low noise OCXO significantly.
The implementation shown in AN9815 is particularly poor in this respect.

A low Q resonant LC filter using low tempco components will have greater
phase stability.
Whilst obtaining capacitors with relatively low tempcos is relatively
easy, the tempco of most off the shelf inductors is usually unspecified.

The significance of the increased phase instability and /or raised phase
noise floor depends on the application.
If one is multiplying the output to 10GHz then even minor temperature
fluctuations will be a concern.

Bruce

Darrell Robinson wrote:
> If you're simply looking for purity of signal maybe a crystal filter.
>
> Intersil application note AN9815 is interesting.
>
> Darrell
>
>
> - Original Message - 
> From: "Richard W. Solomon" 
> To: 
> Sent: Thursday, January 08, 2009 10:11 AM
> Subject: [time-nuts] 10 MHz Band-Pass Filter Needed
>
>
>   
>> The GPSDO I want to use has an output rich in harmonics. In some
>> cases that is good, but Murphy rules and in the application I have
>> today, it is not good.
>>
>> I need a 10 MHz Band-Pass Filter, Bandwidth is not critical, something
>> small with SMA connectors would be ideal, but I can live with BNC.
>>
>> Anyone have such a beast or know where I can get one ? I checked
>> 
> Mini-Circuits
>   
>> and choked on the price !!
>>
>> Thanks, Dick, W1KSZ
>>
>> ___
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>> 
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
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>> 
>
>
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>   


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Re: [time-nuts] GPSDO TC & Damping

[Richard Moore]

On Jan 8, 2009, at 2:46 PM, time-nuts-requ...@febo.com wrote:

> Message: 1
> Date: Fri, 09 Jan 2009 10:28:35 +1300
> From: Bruce Griffiths 
> Subject: Re: [time-nuts] GPSDO TC
> To: Discussion of precise time and frequency measurement
>   
>
> Richard Moore wrote:
>> On Jan 8, 2009, at 2:58 AM, time-nuts-requ...@febo.com wrote:
>>
>>
>>> Message: 6
>>> Date: Thu, 08 Jan 2009 11:51:50 +0100
>>> From: Magnus Danielson 
>>> Subject: Re: [time-nuts] GPSDO time constant
>>> To: Tom Van Baak , Discussion of precise  
>>> time and
>>> frequency measurement 
>>>
>>> For ThunderBolt owners it is pretty straightforward to adjust the
>>> TC and
>>> damping, which is very nice. Use this oppertunity!
>>>
>>
>> So, Magnus (and Tom), what damping factor do you suggest for a TBolt?
>> I'm running a verrry long TC now. If 1.2 is not actually critically
>> damped, what value would be? Any guesses? BTW, I really like that
>> plot of Tom's that tracks the oven and then gets better from the  
>> GPS...
>>
>> Dick Moore
>>
>>
> Richard
>
> As always, the problem is how do you know that the time constant  
> you are
> using is anywhere near optimum?
>
>
> Bruce

Well, like many here, I don't actually have the equipment, especially  
the reference std., to do these MDEV, ADEV and other analyses, so,  
since I use the GPSDO for a frequency standard and not for UTC, I  
thought I'd get the expert opinions. Magnus has several times  
indicated here that a TC laying somewhere in and around 100 to 1000  
secs is probably optimum. When I enquired some time back about  
damping in the TBolt, the consensus seemed to be "leave it at 1.2". I  
have, but it just seems to me that won't be optimum for a fixed- 
position, lab-located frequency standard -- at the moment, I'm  
leaning toward the 0.7to 1.0 area.

Tom's recent chart was quite helpful, especially the 1000 sec curve.  
Now, I hope that Tom or someone else follows up on the suggestion to  
track performance vs. damping factor. I do understand that the  
results for any one GPSDO don't *necessarily* translate to other  
devices, but they don't necessarily don't, either. At least for the  
TBolts a lot of us are playing with, one good example (like Tom's)  
may well put mine in a better ballpark than the ballpark the factory  
wants it to play in, given the factors that you all have described.  
Thx everyone for the comments. Look forward to the next round!

Dick Moore


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Re: [time-nuts] 10 MHz Band-Pass Filter Needed

[Brian Kirby]

A quick filter would be a series resonance circuit.  My notes show that 
on a circuit that was 10 Mhz and terminated into 50 ohms load, I used a 
10uH choke and a 22 pF cap.  Both of these items were off the shelf , 
the cap was a silver mica 5%.

Brian KD4FM

Darrell Robinson wrote:
> If you're simply looking for purity of signal maybe a crystal filter.
> 
> Intersil application note AN9815 is interesting.
> 
> Darrell
> 
> 
> - Original Message - 
> From: "Richard W. Solomon" 
> To: 
> Sent: Thursday, January 08, 2009 10:11 AM
> Subject: [time-nuts] 10 MHz Band-Pass Filter Needed
> 
> 
>> The GPSDO I want to use has an output rich in harmonics. In some
>> cases that is good, but Murphy rules and in the application I have
>> today, it is not good.
>>
>> I need a 10 MHz Band-Pass Filter, Bandwidth is not critical, something
>> small with SMA connectors would be ideal, but I can live with BNC.
>>
>> Anyone have such a beast or know where I can get one ? I checked
> Mini-Circuits
>> and choked on the price !!
>>
>> Thanks, Dick, W1KSZ
>>
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Re: [time-nuts] 10 MHz Band-Pass Filter Needed

[Darrell Robinson]

If you're simply looking for purity of signal maybe a crystal filter.

Intersil application note AN9815 is interesting.

Darrell


- Original Message - 
From: "Richard W. Solomon" 
To: 
Sent: Thursday, January 08, 2009 10:11 AM
Subject: [time-nuts] 10 MHz Band-Pass Filter Needed


> The GPSDO I want to use has an output rich in harmonics. In some
> cases that is good, but Murphy rules and in the application I have
> today, it is not good.
>
> I need a 10 MHz Band-Pass Filter, Bandwidth is not critical, something
> small with SMA connectors would be ideal, but I can live with BNC.
>
> Anyone have such a beast or know where I can get one ? I checked
Mini-Circuits
> and choked on the price !!
>
> Thanks, Dick, W1KSZ
>
> ___
> time-nuts mailing list -- time-nuts@febo.com
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Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops

[David I. Emery]

On Thu, Jan 08, 2009 at 08:51:45AM +, Poul-Henning Kamp wrote:
> In message <49657762.5060...@rubidium.dyndns.org>, Magnus Danielson writes:
> 
> 
> >> Was there a big bang?  What was the source of the 600 amps?
> >
> >I think there (with some delay) was some awfull scream of dispare.
> >The cost of Ethernet interfaces where much more significant back then.
> 
> The most expensive one we lost was in a UNISYS 2200, where three
> microprocessors worked together to limit bandwidth to 100 kB/s.
> I belive the sticker prices as $15k.

I'm somewhat confused about how this took out Ethernet
transceivers or interfaces... from the beginning even vampire tap RG-8
yellow cable Ethernet transceivers were ground isolated from chassis
ground of the computer system just exactly to avoid ground loops and
back path ground currents.   Both power and transmit/receive and control
signals are isolated... and usually transformer coupled... and as I
remember it rather a substantial voltage difference between shield on
the cable and computer system ground had to be tolerated (hundreds of
volts at least)...

I guess, however, if someone grounded the yellow cable at more
than one point enough current could flow on its outer conductor  to
induce substantial voltage between the shield and the center conductor
which could trash the driver/receiver/carrier sense chips or protective
clamp diodes ...

One was never, of course, supposed to ground the yellow cable at
more than one point...

-- 
  Dave Emery N1PRE/AE, d...@dieconsulting.com  DIE Consulting, Weston, Mass 
02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."


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Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops

[Joseph M Gwinn]

time-nuts-boun...@febo.com wrote on 01/08/2009 03:47:29 AM:

> In message  00817...@mck.us.ray.com>, Joseph M Gwinn writes:
> 
>
> >Was there a big bang?  What was the source of the 600 amps?
> 
> They replaced the separation transformer with a UPS, and they
> connected the two sides ground together at the UPS.
> 
> Unfortunately the grounding on our secondary side was much better
> than the power companys grounding on the primary side, which was the
> entire point of having the the transformer in the first place.

One assumes that there were too many cooks.

 
> Yes, there were a significant bang and his two-hand wire-cutter was
> recategorized from "tool" to "industrial art".

He probably needed a stiff drink after that.


Joe

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Re: [time-nuts] GPSDO TC

[Lux, James P]

> -Original Message-
> From: time-nuts-boun...@febo.com
> [mailto:time-nuts-boun...@febo.com] On Behalf Of Magnus Danielson
> Sent: Thursday, January 08, 2009 4:15 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] GPSDO TC
>
> If someone gives me a good RF frontend we could fool around some.
>
>

Maxim MAX2741 L1 GPS receiver IC
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/4323

MAX2741EVKIT is maybe available..

2745 is another
MAX2745EVKIT (not available for purchase)

Maxim recommends:
MAX2769 Universal GPS receiver
 but it's one of those "request the data sheet" parts..

 Anyway, there's probably something out there.

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Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops

[Joseph M Gwinn]

Magnus,


time-nuts-boun...@febo.com wrote on 01/07/2009 10:47:46 PM:

> Joseph,
> 
> >>> Could be a differential TX and RX.  I recall that they send a RS422 
> > signal.
> >> Depending on the speed, RS422 works fine with transformers.
> > 
> > Yes.  It would be 10 MHz or 20 MHz, depending on coding.  Or 5 MHz, so 
the 
> > transitions are at 10 MHz.  I don't recall, or never knew.
> 
> RS422 does not imply any encoding as such so it would be 10 MHz but 
> naturally there is twice that many transitions, but it is the frequency 
> of the signal you are interested in for this case.

I know that RS422 is not the encoding.  I cheated, and talked to the 
relevant engineer.

For digital signals (1PPS, various triggers), it's RS422 over 100 ohm 
twinax (fancy shielded twisted pair).

The 10 MHz sinewave is sent over a pair of 50 ohm coax links, with the 
signals 180 degrees out of phase.  This is acheived with a pair of hybrid 
transformers which convert from one-cable to two-cable and then back to 
one-cable, where all cables are 50 ohm coax.



> >>> I imagine that the shield is grounded at both ends, if only for
> >>> safety reasons.
> >> That is actually a very unsafe practice, unless there is another
> >> much thicker and reliable ground connection between the two domains.
> > 
> > There is a very heavy grounding grid, and such systems almost always 
> > ground the (outer) shields at every connector.
> 
> Which would imply that if the signal passes through a connector jack or 
> through a wall, much of the current would be sent back to its EMF source 

> locally in the room. This does have its merits.

Yes, but that isn't the reason.  It's really a safety and EMC rationale.

 
> >> But you should never let the screen float in the far end, you should
> >> terminate it with a 10M resistor and a sparkgap in parallel to the
> >> local ground.
> >>
> >> The resistor takes care of static electricity and the sparkgap will
> >> do lightnings.
> > 
> > I've done such things, but with a 100 ohm resistor (and a safety 
ground to 
> > ensure that the voltage doesn't get too large.  But this was 
> a lab lashup.
> 
> The trouble with 100 ohm is that still can be a little low in relation 
> to ground loop impedances, it still allow some fair current to roll down 

> the cable. A capacitor in parallel would cut most of the transient 
> energy straight through and allow for a higher resistive path for the 
> low frequency energy.

The ground grid impedance between any two points is well less than one 
ohm, so 100 ohms will pretty much abolish all ground loops.  I've used 10 
ohms in like labs, with success.  I'll grant that this would not work with 
long wires outside.


By the way, I also finally talked to one of our most experienced EMI/EMC 
engineers.  He suggested using MIL-STD-461 test CS109, even though CS109 
was developed for enclosures.  It turns out he was involved in developing 
CS109 when he worked for the US Navy.

 

Joe

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Re: [time-nuts] GPSDO TC

[Magnus Danielson]

Lux, James P skrev:
>> -Original Message-
>> From: time-nuts-boun...@febo.com
>> [mailto:time-nuts-boun...@febo.com] On Behalf Of Poul-Henning Kamp
>> Sent: Thursday, January 08, 2009 3:27 PM
>> To: Discussion of precise time and frequency measurement
>> Subject: Re: [time-nuts] GPSDO TC
>>
>>
>>
>> And timekeeping lends itself well to experiment with this,
>> not the least because getting it wrong doesn't smash anything
>> valuable :-)
> 
> But you might show up early or late for dinner, which could have dire 
> consequences.

True. I was more expecting that when you get it wrong, GPS satellites 
starts falling from the sky, and they are pretty expensive things to 
smash flat into the atmosphere.

>> For instance, one thing to think about in the context of
>> GPSDO's is that in addition to the PPS signal, we also have
>> all the other information.
>>
>> For intance it would make sense to loosen the PLL a bit when
>> satellites enter and leave the solution, because that often
>> moves the GPS signal a few nanoseconds abrubtly, which is
>> enough to throw most PLLs into thinking you had a phase jump.
> 
> 
> This brings up an interesting question.  A lot of the discussion here is
> about taking an off the shelf GPS receiver of one sort or another, and
> then putting something around it to "improve" the system.  A goodly part
> of what's in the "around it" is essentially deconvolving (conceptually)
> the peculiarities of the receiver.
> 
> These days, it's not that hard to build the RF section of a GPS receiver,
> and one can do the processing in an FPGA and attached CPU.  Is there an
> "open source" signal processing chain (i.e. to acquire and track the PN
> codes, and generate the raw observables, and then to do the timing/nav
> solution)?  If such a thing exists, or can be created, then you can do a
> fancier nav solution that explicitly accounts for all the satellites and
> weights them differently as they appear and disappear.

I happens to have some VHDL code lying around. However, the digital 
front-end is not that much magic involved with. The real work is in the 
tracking-processing, for which I have some partial C code lying around 
and there is open source code available.

If someone gives me a good RF frontend we could fool around some.

> Navsys sells a product that generates GPS signals by simulation and then
> you load them into a USRP and play them with Gnuradio. They also sell the
> receiver software.
> Here's a review of Matlab toolboxes:
> http://www.constell.org/Downloads/gpsmatlab.article.pdf
> 
> Xilinx has an article:
> http://www.xilinx.com/publications/magazines/dsp_01/xc_pdf/p50-53_dsp-gps.pdf
> That describes a GPS receiver implemented using SystemGenerator, etc., but I
> suspect that they're not distributing the source code.
> 
> There's this paper, too:
> http://old.gps.aau.dk/downloads/IONGNSS2005_BorreAkos_paper.pdf
> 
> Here's someone who did it as a project in school:
> http://cegt201.bradley.edu/projects/proj2008/gps/
> He tried to convert the Matlab from Akos, et al., to C++

We are a few that has a GNSS Sampler, which is basically a GPS frontend 
hooked to a USB chip and you do correlation etc in the computer. It is a 
bit of a challenge to get it to track in real time thought there are 
those that do that.

>> There is also the complex 12h signal in most GPS receivers
>> PPS, should that be notched out of the PLL so that it will
>> not react to offsets that have a 12h period ?  (obviously
>> only in stationary
>> applications.)
>>
>> So many things to try, so little time...
> 
> Indeed...
> 
> 
> But hey.. Why not start hooking up a USRP or Xilinx Eval board..

Let's do that... some day.

Cheers,
Magnus

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Re: [time-nuts] GPSDO TC

[Lux, James P]

> -Original Message-
> From: time-nuts-boun...@febo.com
> [mailto:time-nuts-boun...@febo.com] On Behalf Of Poul-Henning Kamp
> Sent: Thursday, January 08, 2009 3:27 PM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] GPSDO TC
>
>
>
> And timekeeping lends itself well to experiment with this,
> not the least because getting it wrong doesn't smash anything
> valuable :-)

But you might show up early or late for dinner, which could have dire 
consequences.

>
> For instance, one thing to think about in the context of
> GPSDO's is that in addition to the PPS signal, we also have
> all the other information.
>
> For intance it would make sense to loosen the PLL a bit when
> satellites enter and leave the solution, because that often
> moves the GPS signal a few nanoseconds abrubtly, which is
> enough to throw most PLLs into thinking you had a phase jump.


This brings up an interesting question.  A lot of the discussion here is about 
taking an off the shelf GPS receiver of one sort or another, and then putting 
something around it to "improve" the system.  A goodly part of what's in the 
"around it" is essentially deconvolving (conceptually) the peculiarities of the 
receiver.

These days, it's not that hard to build the RF section of a GPS receiver, and 
one can do the processing in an FPGA and attached CPU.  Is there an "open 
source" signal processing chain (i.e. to acquire and track the PN codes, and 
generate the raw observables, and then to do the timing/nav solution)?  If such 
a thing exists, or can be created, then you can do a fancier nav solution that 
explicitly accounts for all the satellites and weights them differently as they 
appear and disappear.

Navsys sells a product that generates GPS signals by simulation and then you 
load them into a USRP and play them with Gnuradio. They also sell the receiver 
software.
Here's a review of Matlab toolboxes:
http://www.constell.org/Downloads/gpsmatlab.article.pdf

Xilinx has an article:
http://www.xilinx.com/publications/magazines/dsp_01/xc_pdf/p50-53_dsp-gps.pdf
That describes a GPS receiver implemented using SystemGenerator, etc., but I 
suspect that they're not distributing the source code.

There's this paper, too:
http://old.gps.aau.dk/downloads/IONGNSS2005_BorreAkos_paper.pdf

Here's someone who did it as a project in school:
http://cegt201.bradley.edu/projects/proj2008/gps/
He tried to convert the Matlab from Akos, et al., to C++


>
> There is also the complex 12h signal in most GPS receivers
> PPS, should that be notched out of the PLL so that it will
> not react to offsets that have a 12h period ?  (obviously
> only in stationary
> applications.)
>
> So many things to try, so little time...

Indeed...


But hey.. Why not start hooking up a USRP or Xilinx Eval board..

Jim

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Re: [time-nuts] GPSDO TC

[Poul-Henning Kamp]

>> So what is optimum... from control theory we learn, that with an even
>> better model of your system, you can push performance to the edge! But
>> you always loose robustness in doing that. 

One thing to remember: control theory is akin to a taxinomy which
has only "elephants" and "non elephants", in that sense that
non-linear control theory is not widely taught and generally
considered an evil to stay away from at all cost.

Traditional PLL methods are inherently linear and consequently
limited in what they can do, but you can achive suprisingly good
results by combining a classical PLL with non-linear higher modes
or mode switches.

You can of course, in the absense of solid theory for what you
are attempting, also get it horribly wrong, as in the NTP PLL :-)

But the machine-control and robotics community has finally realized
that to get machines to "have the moves" they have to abandon
classical control theory, drop the poles and zeros of the plane
and instead build physical predictive models.  The first area
to pick up on this big time were disk drive arm actuators, which
are nowhere near a simple differential equation any more.

And timekeeping lends itself well to experiment with this,
not the least because getting it wrong doesn't smash anything
valuable :-)

For instance, one thing to think about in the context of GPSDO's
is that in addition to the PPS signal, we also have all the
other information.

For intance it would make sense to loosen the PLL a bit when
satellites enter and leave the solution, because that often moves
the GPS signal a few nanoseconds abrubtly, which is enough to
throw most PLLs into thinking you had a phase jump.

There is also the complex 12h signal in most GPS receivers PPS,
should that be notched out of the PLL so that it will not react
to offsets that have a 12h period ?  (obviously only in stationary
applications.)

So many things to try, so little time...

Poul-Henning

-- 
Poul-Henning Kamp   | UNIX since Zilog Zeus 3.20
p...@freebsd.org | TCP/IP since RFC 956
FreeBSD committer   | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

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Re: [time-nuts] 10 MHz Band-Pass Filter Needed (Roberto Barrios)

[Steve Rooke]

Hi Roberto,

Congratulations! On your very first visit here, you have hit the Bruce
Effect. Do not be alarmed! Just place your filter in a temperature
controlled oven regulated to a change of 1E-99C for external temp
changes of -100C to +200C and you may be alright, I did say "may".

But then, for ham purposes, the phase/frequency changes should make no
difference to you. If you were NIST, well that would be another
matter. But this is a time-nuts forum and people here think in the
1E-16 range. Some of us here just strive for the best and find that
some of our needs are easily covered with much more modest equipment.
That does not mean we are happy to accept this limited capability for
projects we may wish to do in the future.

You have made a wise choice coming here, it has low noise (accept for
my posts) and some of the best minds contributing. Keep watching this
space and join in to ask question and give ideas. Do not be put off by
any comments on what you post, there are high ideals here and some may
not be the most tactful people when they make their comments. You will
learn a great deal here and you will certainly get the time bug.

73, Steve - ZL3TUV & G8KVD

2009/1/9 Bruce Griffiths :
> Roberto Barrios wrote:
>>> The GPSDO I want to use has an output rich in harmonics. In some> cases 
>>> that is good, but Murphy rules and in the application I have> today, it is 
>>> not good.> > I need a 10 MHz Band-Pass Filter, Bandwidth is not critical, 
>>> something> small with SMA connectors would be ideal, but I can live with 
>>> BNC.> > Anyone have such a beast or know where I can get one ? I checked 
>>> Mini-Circuits> and choked on the price !!> > Thanks, Dick, W1KSZ>
>>>
>> Hi Dick,
>>
>> There are plans for a simple nine section low pass filter here:
>> http://jwmeng.com/AppNote/AppNote003.html
>>
>> You can also use (I did) a filter from an old 10mbit ethernet network card, 
>> as suggested here:
>> http://www.uhf-satcom.com/misc/10MHz_dist/
>>
>> I joined this list just yesterday, so hello to everyone. I live in Madrid 
>> and I'm interested in frequency measurement for HAM purposes. Not 
>> understanding most of the things you guys discuss here, I thought I would 
>> never have the opportunity to give my 2 cents... I´m currently working on a 
>> simple GPSDO that (luckily) some  of you may find interesting. I'll send you 
>> details in a few days so I can get your opinions and (hopefully) it 
>> inspires/helps someone.
>>
>> Bests Regards & 73's
>> Roberto Barrios, EB4EQA
>> Madrid, Spain
>>
>>
>>
> Roberto
>
> But what is the filter phase shift tempco?
> In the presence of temperature changes the filter phase shift variation
> is equivalent to phase and frequency modulation of the output of the filter.
> One cure is to use low tempco filter parts and place the filter in an oven.
>
> Bruce
>
>
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>



-- 
Steve Rooke - ZL3TUV & G8KVD & JAKDTTNW
Omnium finis imminet

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Re: [time-nuts] GPSDO TC

[Magnus Danielson]

>> As always, the problem is how do you know that the time constant you are
>> using is anywhere near optimum?
>>
>>
>> Bruce
> 
> So what is optimum... from control theory we learn, that with an even
> better model of your system, you can push performance to the edge! But
> you always loose robustness in doing that. 

Trouble is, we have many more variables and our set of goodness 
measurements are the ADEV and friends, which is much more troublesome to 
analyse than traditional variance and noise bandwidth values.

The variance for a PLL is an integral over f multiplying the noise power 
function of f with the square of the amplitude response over f. It is 
traditional to simplify this by assume a white noise power N0 (V²/Hz) in 
which case the noise level creeps out of the integral (since it now is a 
variable independent of f) and the remaining integral is that of the 
squared amplitude response of the filter. This can then further be 
generalized to the noise bandwidth formula.

Notice how we started from the variance measure, our traditional sigma 
value. We already know that it is insufficient to qualify the noise 
since the the f^-n noise powers does not converge on integration. Using 
derivate formulations like noise bandwidth those inherit the analysis 
problems. It even becomes hard achieving something similar as it now is 
a balance between different noise powers and filtering combines them in 
new interesting ways.

I think we need to either do hard analysis or we notice the tendencies 
in measures and try to explain them in other general tendencies and 
knowledge and draw some simplified conclusions and get away with it.

> So what is the implication of a to large TC here? Nothing going instable
> in the control loop? We are just following the "freerunning" OCXO curve
> past the point where GPS goes downhill?

For a second degree loop it would mean loosing lock or not be able to 
pull in properly. The actual problem is dynamics. Loop bandwidth will 
scale drift rate numbers with the square.

Cheers,
Magnus

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Re: [time-nuts] 10 MHz Band-Pass Filter Needed (Roberto Barrios)

[Bruce Griffiths]

Roberto Barrios wrote:
>> The GPSDO I want to use has an output rich in harmonics. In some> cases that 
>> is good, but Murphy rules and in the application I have> today, it is not 
>> good.> > I need a 10 MHz Band-Pass Filter, Bandwidth is not critical, 
>> something> small with SMA connectors would be ideal, but I can live with 
>> BNC.> > Anyone have such a beast or know where I can get one ? I checked 
>> Mini-Circuits> and choked on the price !!> > Thanks, Dick, W1KSZ> 
>> 
> Hi Dick,
>  
> There are plans for a simple nine section low pass filter here:
> http://jwmeng.com/AppNote/AppNote003.html
>  
> You can also use (I did) a filter from an old 10mbit ethernet network card, 
> as suggested here:
> http://www.uhf-satcom.com/misc/10MHz_dist/
>  
> I joined this list just yesterday, so hello to everyone. I live in Madrid and 
> I'm interested in frequency measurement for HAM purposes. Not understanding 
> most of the things you guys discuss here, I thought I would never have the 
> opportunity to give my 2 cents... I´m currently working on a simple GPSDO 
> that (luckily) some  of you may find interesting. I'll send you details in a 
> few days so I can get your opinions and (hopefully) it inspires/helps someone.
>  
> Bests Regards & 73's
> Roberto Barrios, EB4EQA
> Madrid, Spain
>  
>
>   
Roberto

But what is the filter phase shift tempco?
In the presence of temperature changes the filter phase shift variation
is equivalent to phase and frequency modulation of the output of the filter.
One cure is to use low tempco filter parts and place the filter in an oven.

Bruce


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Re: [time-nuts] GPSDO TC

[John Ackermann N8UR]

You can cure it by accepting the certificate...

Since I'm not doing ecommerce from febo.com, I'm not willing to pay the
hundred dollar plus per year fee for an "official" SSL certificate, and
use a home-grown one, which is fine for stopping eavesdropping on the
wire.  It will give the "self-signed" error because, well, it is. :-)

John


Dave Ackrill said the following on 01/08/2009 05:46 PM:
> Can someone cure this fail message please?
> 
> Thanks...
> 
> Secure Connection Failed
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> www.febo.com uses an invalid security certificate.
> 
> The certificate is not trusted because it is self signed.
> The certificate is only valid for febo.com.
> The certificate expired on 11/11/2008 14:57.
> 
> (Error code: sec_error_expired_issuer_certificate)
> 
> 
> 
> 
> 
> 
> 
> 
> 
> * This could be a problem with the server's configuration, or it could 
> be someone trying to impersonate the server.
> 
> 
> * If you have connected to this server successfully in the past, the 
> error may be temporary, and you can try again later.

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Re: [time-nuts] GPSDO TC

[Bruce Griffiths]

Hej Magnus

Magnus Danielson wrote:
> Bruce Griffiths skrev:
>   
>> Magnus Danielson wrote:
>> 
>>> Dick,
>>>
>>> Richard Moore skrev:
>>>   
>>>   
 On Jan 8, 2009, at 2:58 AM, time-nuts-requ...@febo.com wrote:

 
 
> Message: 6
> Date: Thu, 08 Jan 2009 11:51:50 +0100
> From: Magnus Danielson 
> Subject: Re: [time-nuts] GPSDO time constant
> To: Tom Van Baak ,   Discussion of precise time and
>   frequency measurement 
> For ThunderBolt owners it is pretty straightforward to adjust the  
> TC and
> damping, which is very nice. Use this oppertunity!
>   
>   
 So, Magnus (and Tom), what damping factor do you suggest for a TBolt?  
 I'm running a verrry long TC now. If 1.2 is not actually critically  
 damped, what value would be? Any guesses? BTW, I really like that  
 plot of Tom's that tracks the oven and then gets better from the GPS...
 
 
>>> Assuming that damping factors match classical analysis of damping, then 
>>> the square root of 2 is the answer... 1.414 or there abouts.
>>>
>>> I would be more conservative than that. I would consider damping factors 
>>> such as 3-4 or so. I have no support from measurements on GPSDOs but it 
>>> is reasonable that the rise of gain at and near the PLL frequency we see 
>>> for other systems will occur and result in similar effects even here.
>>> This gain raises the noise floor and amount of gain is directly coupled 
>>> to the damping factor. It's just standard PLL stuff all over again. The 
>>> only difference is that we view the result in ADEV or MDEV views.
>>>
>>> Cheers,
>>> Magnus
>>>
>>>   
>>>   
>> Hej Magnus
>>
>> For a second order loop, the noise bandwidth is minimised for a fixed
>> time constant by choosing a damping factor of 0.5.
>> Using a damping factor of 1.414 increases the noise bandwidth by about 60%.
>> Using a damping factor of 0.7071 only increases the loop noise bandwidth
>> by about 6%.
>> A damping factor of 0.3 increases the noise bandwidth by about 13%.
>> 
>
> Yes, but the bump comes from the increase gain around the resonance and 
> spoils the OCXO/GPS cross-over. The simplified noise-bandwidth measure 
> does not really comply here since they usually build on a simplified 
> model of noise type (white noise - gaussian). A simple check in Gardiner 
> provides both the generic integrating formula, simplified results and a 
> graph showing the smae numbers that you give.
>   
Whilst the phase noise of a sawtooth corrected M12+T GPS timing receiver
approximates white phase noise (at least for tau < 1 day), this may not
be so for the receiver used in the Thunderbolt.
The phase noise of the OCXO certainly cannot be accurately modeled as
white phase noise for large tau.
> I rather beleive what ADEV, MDEV and TDEV experience in this context.
>
>   
Yes measurements are the key but if one doesnt have a suitable
statistically independent low noise frequency reference it isnt possible
to optimise the loop parameters for an individual GPSDO.
> We could go back to the real integration formula, adapt it to various 
> powers of f^-n noises and analyse it for the same set of PLL loop 
> filters as analysed by Gardiner. Similarly we could cook up a simulation 
> and do the ADEV, MDEV and TDEV measures. Traditional noise bandwidth 
> measures can be calculated alongside.
>
> I am somewhat surprised that you missed the opportunity to correct me as 
> I was giving the incorrect value for damping factor of a critically 
> damped system. It is the square root of 1/2 and not 2, thus 0.7071 is 
> the appropriate damping factor for critically damped systems.
>
>   
I had noted that your quoted damping factor was incorrect but I
suspected that you would realise that.

Actually according to Gardener critical damping factor is 1 ( minimum
settling with no overshoot for a phase step).
However a damping factor of 0.7071 is widely used.
> I am somewhat surprised that when we have been discussing the bandwidth 
> of the PLLs and considering OCXOs being running with fairly high drift 
> rate we have been assuming second degree loops. This form of 
> acceleration requires third degree responses for proper handling, as 
> being well documented in literature such as Gardiner. Going for third 
> degree response the bandwidth of the loop can be (at least more freely) 
> disconnected from tracking requirements due to drift rate.
>   
I only mentioned second order type II loops as the analysis is somewhat
simpler and there is no indication from the number of tuning parameters
for the Thunderbolt that a higher order loop is involved.
> Another aspect worth mentioning is that a pure PLL with a small 
> bandwidth has a very long trackin period. Heuristics to use wider 
> bandwidths or use frequency measure aided bootstrapping or a diffrential 
> element (PID rather than PI regulator) which is equivalent to also feed 
> a frequency err

Re: [time-nuts] 10 MHz Band-Pass Filter Needed (Roberto Barrios)

[Roberto Barrios]

> The GPSDO I want to use has an output rich in harmonics. In some> cases that 
> is good, but Murphy rules and in the application I have> today, it is not 
> good.> > I need a 10 MHz Band-Pass Filter, Bandwidth is not critical, 
> something> small with SMA connectors would be ideal, but I can live with 
> BNC.> > Anyone have such a beast or know where I can get one ? I checked 
> Mini-Circuits> and choked on the price !!> > Thanks, Dick, W1KSZ> 
Hi Dick,
 
There are plans for a simple nine section low pass filter here:
http://jwmeng.com/AppNote/AppNote003.html
 
You can also use (I did) a filter from an old 10mbit ethernet network card, as 
suggested here:
http://www.uhf-satcom.com/misc/10MHz_dist/
 
I joined this list just yesterday, so hello to everyone. I live in Madrid and 
I'm interested in frequency measurement for HAM purposes. Not understanding 
most of the things you guys discuss here, I thought I would never have the 
opportunity to give my 2 cents... I´m currently working on a simple GPSDO that 
(luckily) some  of you may find interesting. I'll send you details in a few 
days so I can get your opinions and (hopefully) it inspires/helps someone.
 
Bests Regards & 73's
Roberto Barrios, EB4EQA
Madrid, Spain
 
_
See how Windows® connects the people, information, and fun that are part of 
your life
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Re: [time-nuts] GPSDO TC

[Dave Ackrill]

Can someone cure this fail message please?

Thanks...

Secure Connection Failed













www.febo.com uses an invalid security certificate.

The certificate is not trusted because it is self signed.
The certificate is only valid for febo.com.
The certificate expired on 11/11/2008 14:57.

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Re: [time-nuts] GPSDO TC

[Bruce Griffiths]

Björn Gabrielsson wrote:
> On Fri, 2009-01-09 at 11:09 +1300, Bruce Griffiths wrote:
> [...snip...
>
>   
>> My point was that measurements are required to establish the optimum for
>> each individual OCXO not just for a given OCXO model.
>>
>> Bruce
>> 
>
> Are those measurements possible to do in real time with only the GPSDO?
>
> --
>
>Björn
>
>
>   

Björn

Not really, however one can measure the relative ADEV as a function of
tau for the free running OCXO and the timing receiver.
There will be a broad minimum at some tau. The optimum time constant may
lie somewhere in the vicinity of that minimum.
Its probably impossible to do much better than that without a nice
stable statistically independent (for all tau) frequency standard.

Bruce

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Re: [time-nuts] GPSDO TC

[Björn Gabrielsson]

On Fri, 2009-01-09 at 11:09 +1300, Bruce Griffiths wrote:
[...snip...

> My point was that measurements are required to establish the optimum for
> each individual OCXO not just for a given OCXO model.
> 
> Bruce

Are those measurements possible to do in real time with only the GPSDO?

--

   Björn


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Re: [time-nuts] GPSDO TC

[Magnus Danielson]

Bruce Griffiths skrev:
> Magnus Danielson wrote:
>> Dick,
>>
>> Richard Moore skrev:
>>   
>>> On Jan 8, 2009, at 2:58 AM, time-nuts-requ...@febo.com wrote:
>>>
>>> 
 Message: 6
 Date: Thu, 08 Jan 2009 11:51:50 +0100
 From: Magnus Danielson 
 Subject: Re: [time-nuts] GPSDO time constant
 To: Tom Van Baak ,Discussion of precise time and
frequency measurement 
 For ThunderBolt owners it is pretty straightforward to adjust the  
 TC and
 damping, which is very nice. Use this oppertunity!
   
>>> So, Magnus (and Tom), what damping factor do you suggest for a TBolt?  
>>> I'm running a verrry long TC now. If 1.2 is not actually critically  
>>> damped, what value would be? Any guesses? BTW, I really like that  
>>> plot of Tom's that tracks the oven and then gets better from the GPS...
>>> 
>> Assuming that damping factors match classical analysis of damping, then 
>> the square root of 2 is the answer... 1.414 or there abouts.
>>
>> I would be more conservative than that. I would consider damping factors 
>> such as 3-4 or so. I have no support from measurements on GPSDOs but it 
>> is reasonable that the rise of gain at and near the PLL frequency we see 
>> for other systems will occur and result in similar effects even here.
>> This gain raises the noise floor and amount of gain is directly coupled 
>> to the damping factor. It's just standard PLL stuff all over again. The 
>> only difference is that we view the result in ADEV or MDEV views.
>>
>> Cheers,
>> Magnus
>>
>>   
> Hej Magnus
> 
> For a second order loop, the noise bandwidth is minimised for a fixed
> time constant by choosing a damping factor of 0.5.
> Using a damping factor of 1.414 increases the noise bandwidth by about 60%.
> Using a damping factor of 0.7071 only increases the loop noise bandwidth
> by about 6%.
> A damping factor of 0.3 increases the noise bandwidth by about 13%.

Yes, but the bump comes from the increase gain around the resonance and 
spoils the OCXO/GPS cross-over. The simplified noise-bandwidth measure 
does not really comply here since they usually build on a simplified 
model of noise type (white noise - gaussian). A simple check in Gardiner 
provides both the generic integrating formula, simplified results and a 
graph showing the smae numbers that you give.

I rather beleive what ADEV, MDEV and TDEV experience in this context.

We could go back to the real integration formula, adapt it to various 
powers of f^-n noises and analyse it for the same set of PLL loop 
filters as analysed by Gardiner. Similarly we could cook up a simulation 
and do the ADEV, MDEV and TDEV measures. Traditional noise bandwidth 
measures can be calculated alongside.

I am somewhat surprised that you missed the opportunity to correct me as 
I was giving the incorrect value for damping factor of a critically 
damped system. It is the square root of 1/2 and not 2, thus 0.7071 is 
the appropriate damping factor for critically damped systems.

I am somewhat surprised that when we have been discussing the bandwidth 
of the PLLs and considering OCXOs being running with fairly high drift 
rate we have been assuming second degree loops. This form of 
acceleration requires third degree responses for proper handling, as 
being well documented in literature such as Gardiner. Going for third 
degree response the bandwidth of the loop can be (at least more freely) 
disconnected from tracking requirements due to drift rate.

Another aspect worth mentioning is that a pure PLL with a small 
bandwidth has a very long trackin period. Heuristics to use wider 
bandwidths or use frequency measure aided bootstrapping or a diffrential 
element (PID rather than PI regulator) which is equivalent to also feed 
a frequency error measurement into the loop will significantly aid the 
loop in quick lock-in. A Kalman filter approach is just along the same 
lines and when considered next to some more elaborate schemes of 
heuristic aided PLL seems to much simpler. While the Kalman filter 
approach isn't as optimum as claimed to be it is still a useful tool.

Cheers,
Magnus

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Re: [time-nuts] GPSDO TC

[Bruce Griffiths]

Björn Gabrielsson wrote:
> On Fri, 2009-01-09 at 10:28 +1300, Bruce Griffiths wrote:
>   
>> Richard Moore wrote:
>> 
>>> On Jan 8, 2009, at 2:58 AM, time-nuts-requ...@febo.com wrote:
>>>
>>>   
>>>   
 Message: 6
 Date: Thu, 08 Jan 2009 11:51:50 +0100
 From: Magnus Danielson 
 Subject: Re: [time-nuts] GPSDO time constant
 To: Tom Van Baak ,Discussion of precise time and
frequency measurement 
 
 For ThunderBolt owners it is pretty straightforward to adjust the  
 TC and
 damping, which is very nice. Use this oppertunity!
 
 
>>> So, Magnus (and Tom), what damping factor do you suggest for a TBolt?  
>>> I'm running a verrry long TC now. If 1.2 is not actually critically  
>>> damped, what value would be? Any guesses? BTW, I really like that  
>>> plot of Tom's that tracks the oven and then gets better from the GPS...
>>>
>>> Dick Moore
>>>
>>>   
>>>   
>> Richard
>>
>> As always, the problem is how do you know that the time constant you are
>> using is anywhere near optimum?
>>
>>
>> Bruce
>> 
>
> So what is optimum... from control theory we learn, that with an even
> better model of your system, you can push performance to the edge! But
> you always loose robustness in doing that. 
>
> So what is the implication of a to large TC here? Nothing going instable
> in the control loop? We are just following the "freerunning" OCXO curve
> past the point where GPS goes downhill?
>
> --
>
>Björn
>
>
>   

Björn


My point was that measurements are required to establish the optimum for
each individual OCXO not just for a given OCXO model.

Bruce

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Re: [time-nuts] GPSDO TC

[Björn Gabrielsson]

On Fri, 2009-01-09 at 10:28 +1300, Bruce Griffiths wrote:
> Richard Moore wrote:
> > On Jan 8, 2009, at 2:58 AM, time-nuts-requ...@febo.com wrote:
> >
> >   
> >> Message: 6
> >> Date: Thu, 08 Jan 2009 11:51:50 +0100
> >> From: Magnus Danielson 
> >> Subject: Re: [time-nuts] GPSDO time constant
> >> To: Tom Van Baak ,Discussion of precise time and
> >>frequency measurement 
> >> 
> >> For ThunderBolt owners it is pretty straightforward to adjust the  
> >> TC and
> >> damping, which is very nice. Use this oppertunity!
> >> 
> >
> > So, Magnus (and Tom), what damping factor do you suggest for a TBolt?  
> > I'm running a verrry long TC now. If 1.2 is not actually critically  
> > damped, what value would be? Any guesses? BTW, I really like that  
> > plot of Tom's that tracks the oven and then gets better from the GPS...
> >
> > Dick Moore
> >
> >   
> Richard
> 
> As always, the problem is how do you know that the time constant you are
> using is anywhere near optimum?
> 
> 
> Bruce

So what is optimum... from control theory we learn, that with an even
better model of your system, you can push performance to the edge! But
you always loose robustness in doing that. 

So what is the implication of a to large TC here? Nothing going instable
in the control loop? We are just following the "freerunning" OCXO curve
past the point where GPS goes downhill?

--

   Björn


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Re: [time-nuts] GPSDO TC

[Bruce Griffiths]

Magnus Danielson wrote:
> Dick,
>
> Richard Moore skrev:
>   
>> On Jan 8, 2009, at 2:58 AM, time-nuts-requ...@febo.com wrote:
>>
>> 
>>> Message: 6
>>> Date: Thu, 08 Jan 2009 11:51:50 +0100
>>> From: Magnus Danielson 
>>> Subject: Re: [time-nuts] GPSDO time constant
>>> To: Tom Van Baak , Discussion of precise time and
>>> frequency measurement 
>>> For ThunderBolt owners it is pretty straightforward to adjust the  
>>> TC and
>>> damping, which is very nice. Use this oppertunity!
>>>   
>> So, Magnus (and Tom), what damping factor do you suggest for a TBolt?  
>> I'm running a verrry long TC now. If 1.2 is not actually critically  
>> damped, what value would be? Any guesses? BTW, I really like that  
>> plot of Tom's that tracks the oven and then gets better from the GPS...
>> 
>
> Assuming that damping factors match classical analysis of damping, then 
> the square root of 2 is the answer... 1.414 or there abouts.
>
> I would be more conservative than that. I would consider damping factors 
> such as 3-4 or so. I have no support from measurements on GPSDOs but it 
> is reasonable that the rise of gain at and near the PLL frequency we see 
> for other systems will occur and result in similar effects even here.
> This gain raises the noise floor and amount of gain is directly coupled 
> to the damping factor. It's just standard PLL stuff all over again. The 
> only difference is that we view the result in ADEV or MDEV views.
>
> Cheers,
> Magnus
>
>   
Hej Magnus

For a second order loop, the noise bandwidth is minimised for a fixed
time constant by choosing a damping factor of 0.5.
Using a damping factor of 1.414 increases the noise bandwidth by about 60%.
Using a damping factor of 0.7071 only increases the loop noise bandwidth
by about 6%.
A damping factor of 0.3 increases the noise bandwidth by about 13%.

Bruce

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Re: [time-nuts] GPSDO TC

[Bruce Griffiths]

Richard Moore wrote:
> On Jan 8, 2009, at 2:58 AM, time-nuts-requ...@febo.com wrote:
>
>   
>> Message: 6
>> Date: Thu, 08 Jan 2009 11:51:50 +0100
>> From: Magnus Danielson 
>> Subject: Re: [time-nuts] GPSDO time constant
>> To: Tom Van Baak ,  Discussion of precise time and
>>  frequency measurement 
>> 
>> For ThunderBolt owners it is pretty straightforward to adjust the  
>> TC and
>> damping, which is very nice. Use this oppertunity!
>> 
>
> So, Magnus (and Tom), what damping factor do you suggest for a TBolt?  
> I'm running a verrry long TC now. If 1.2 is not actually critically  
> damped, what value would be? Any guesses? BTW, I really like that  
> plot of Tom's that tracks the oven and then gets better from the GPS...
>
> Dick Moore
>
>   
Richard

As always, the problem is how do you know that the time constant you are
using is anywhere near optimum?


Bruce

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Re: [time-nuts] GPSDO TC

[Magnus Danielson]

Dick,

Richard Moore skrev:
> On Jan 8, 2009, at 2:58 AM, time-nuts-requ...@febo.com wrote:
> 
>> Message: 6
>> Date: Thu, 08 Jan 2009 11:51:50 +0100
>> From: Magnus Danielson 
>> Subject: Re: [time-nuts] GPSDO time constant
>> To: Tom Van Baak ,  Discussion of precise time and
>>  frequency measurement 
>> For ThunderBolt owners it is pretty straightforward to adjust the  
>> TC and
>> damping, which is very nice. Use this oppertunity!
> 
> So, Magnus (and Tom), what damping factor do you suggest for a TBolt?  
> I'm running a verrry long TC now. If 1.2 is not actually critically  
> damped, what value would be? Any guesses? BTW, I really like that  
> plot of Tom's that tracks the oven and then gets better from the GPS...

Assuming that damping factors match classical analysis of damping, then 
the square root of 2 is the answer... 1.414 or there abouts.

I would be more conservative than that. I would consider damping factors 
such as 3-4 or so. I have no support from measurements on GPSDOs but it 
is reasonable that the rise of gain at and near the PLL frequency we see 
for other systems will occur and result in similar effects even here.
This gain raises the noise floor and amount of gain is directly coupled 
to the damping factor. It's just standard PLL stuff all over again. The 
only difference is that we view the result in ADEV or MDEV views.

Cheers,
Magnus

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Re: [time-nuts] GPSDO TC

[Richard Moore]

On Jan 8, 2009, at 2:58 AM, time-nuts-requ...@febo.com wrote:

> Message: 6
> Date: Thu, 08 Jan 2009 11:51:50 +0100
> From: Magnus Danielson 
> Subject: Re: [time-nuts] GPSDO time constant
> To: Tom Van Baak ,   Discussion of precise time and
>   frequency measurement 
> >>>
> For ThunderBolt owners it is pretty straightforward to adjust the  
> TC and
> damping, which is very nice. Use this oppertunity!

So, Magnus (and Tom), what damping factor do you suggest for a TBolt?  
I'm running a verrry long TC now. If 1.2 is not actually critically  
damped, what value would be? Any guesses? BTW, I really like that  
plot of Tom's that tracks the oven and then gets better from the GPS...

Dick Moore

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Re: [time-nuts] Standards sought for immunity of shielded cablelinks to power-frequency ground loops

[Bruce Griffiths]

Scott Newell wrote:
> At 01:59 PM 1/8/2009 , Poul-Henning Kamp wrote:
>   
>> In message <49665a6d.2030...@xtra.co.nz>, Bruce Griffiths writes:
>>
>> 
>>> I have been unable to find a reference to triax consisting of 3
>>> conductors within a shield, however such confusion is understandable
>>> given the confusion over quadrax:-
>>>   
>> I have only ever seen it used for very old 3-electrode condenser
>> microphones.
>> 
>
> Doesn't Keithley use triax connectors on some of their high-impedance
> instrumentation?
>
>   
Scott

They use the concentic conductor definition/interpretation of the term
triax.
I was seeking an example of the 3 conductor within a shield interpretation.

See the links on quadrax posted earlier for an idea of the pervasive
rampant confusion.

Bruce

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[time-nuts] GPSDO time constant

[Mark Sims]

In the world of precision scales you can often beat the many of the 
manufactures specs by an order of magnitude by adjusting and calibrating the 
scale in the exact location and orientation where it will be used (and not 
moving it or afterwards).  Some of  the adjustments involve moving rather 
crudely threaded mechanical adjustments the equivalent of a few wavelengths of 
light.  There is no way to actually make the adjustments other than trial and 
error...  move it enough times and it will eventually wind up in the right 
place...  and hysteresis and backlash are a bitch...

The alignment spec for the color monitor in  the HP16500 logic analyzers says 
to face the unit to the west when adjusting it (but they don't say which end to 
point west).  Also some of the adjustments are on the bottom of the monitor and 
others are on the side.  You usually make the adjustments with the unit on its 
side...  but nobody ever runs the unit in that orientation.


I've heard that the xtal oscillator cal procedure for some HP test
equipment says that the instrument should be in the same position as when
it's operating.  For heavy rack equipment that means you lay on a
mechanics creeper when making the adjustment rather than flipping the
instrument 90 degrees on a bench.


_
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http://windowslive.com/explore?ocid=TXT_TAGLM_WL_t1_allup_explore_012009
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Re: [time-nuts] Standards sought for immunity of shielded cablelinks to power-frequency ground loops

[Scott Newell]

At 01:59 PM 1/8/2009 , Poul-Henning Kamp wrote:
>In message <49665a6d.2030...@xtra.co.nz>, Bruce Griffiths writes:
>
>>I have been unable to find a reference to triax consisting of 3
>>conductors within a shield, however such confusion is understandable
>>given the confusion over quadrax:-
>
>I have only ever seen it used for very old 3-electrode condenser
>microphones.

Doesn't Keithley use triax connectors on some of their high-impedance
instrumentation?

-- 
newell   N5TNL


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Re: [time-nuts] GPSDO time constant

[Lux, James P]

Google "Leaf blower hovercraft" and you'll get dozens of useful hits.
Here's a real old link: http://amasci.com/amateur/hovercft.html

> -Original Message-
>James,
>
>Do you have any web sites that show such a contration using leaf
>blowers ?
>
>thanks,
>
>BillWB6BNQ

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Re: [time-nuts] TB PS/Antenna recommendations

[James R. Gorr]

Here is some information on different power-supplies for the tbolt and their 
performance:

http://www.leapsecond.com/pages/tbolt/noise.htm

My tbolt is a little different, it uses: +12/+5/-7 volts.

The only thing I had on hand was a box for of wall-warts, so I grabbed three 
out and configured them as such:

http://n3toy.net/images/power_supply.png

Not sure of the quality of power, but it works. 

Jamie




--- On Wed, 1/7/09, Richard Moore  wrote:

> From: Richard Moore 
> Subject: Re: [time-nuts] TBolt antenna
> To: time-nuts@febo.com
> Date: Wednesday, January 7, 2009, 8:54 PM
> On Jan 7, 2009, at 2:43 PM, time-nuts-requ...@febo.com
> wrote:
> >
> > Message: 6
> > Date: Wed, 7 Jan 2009 17:08:22 -0500
> > From: "Ronald Held"
> 
> > Subject: [time-nuts] TB PS/Antenna recommendations
> > To: time-nuts@febo.com
> > Message-ID:
> >
>   <9a86fb0e0901071408r4a205518mc346d7694f9af...@mail.gmail.com>
> > Content-Type: text/plain; charset=ISO-8859-1
> >
> > I have not following all of discussions for a while,
> so perhaps I
> > missed out on some of this. What do people suggest for
> a power supply
> > and  from where? The harder request is for an indoor
> antenna probable
> > placed up against the window. Reliable dealers or
> stores would be very
> > helpful.
> >  Ronald
> >
> 
> I'm using a fairly crappy triple-output open-frame
> Autec supply from  
> Marlin P. Jones that was $13. I'm not recommending it,
> but it works.  
> My antenna is an active patch type by Hawk that runs off
> +5V from the  
> TBolt. It has a magnetic base for top of vehicle use.
> I've got it  
> feeding two GPSDOs thru a splitter and it's working
> great. I got it  
> from Synergy for $35 if I remember correctly -- been a
> while. I've  
> got it duct taped to a 2x4 sticking our from under the eave
> of my  
> lab. Most any active antenna will likely work OK outdoors,
> but I  
> don't know about indoors -- think you'll want some
> higher gain version.
> 
> Dick Moore
> 
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Re: [time-nuts] 10 MHz Band-Pass Filter Needed

[Bruce Griffiths]

John Ackermann N8UR wrote:
> Dan Rae wrote:
>   
>> Richard W. Solomon wrote:
>> 
>>> The GPSDO I want to use has an output rich in harmonics. In some
>>> cases that is good, but Murphy rules and in the application I have
>>> today, it is not good.
>>>
>>> I need a 10 MHz Band-Pass Filter, Bandwidth is not critical, something
>>> small with SMA connectors would be ideal, but I can live with BNC.
>>>
>>> Anyone have such a beast or know where I can get one ? I checked 
>>> Mini-Circuits
>>> and choked on the price !!
>>>
>>>   
>>>   
>> Dick, it is really easy to build one.  The wonderful (free!) filter 
>> design program ELSIE will give you all the help you need.  I would have 
>> thought an hour or so with a couple of toroids would do the trick.  Even 
>> just a Low Pass filter would usually do to turn a square wave into a sine...
>> 
>
> It's actually much better to use an LPF than a bandpass filter if you 
> don't have subharmonic energy to deal with.  An LPF with a cutoff midway 
> between the fundamental and the 2nd harmonic will show much less tempco 
> (in the form of phase shift over temperature) than a bandpass filter.
>
> John
>
>   
John

Another approach is to use relatively high Q traps/bandstop filters to
eliminate/minimise the unwanted harmonics together with a low pass filter.
The high Q traps contribute little phase shift and phase shift tempco at
the fundamental.

Bruce

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Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops

[Poul-Henning Kamp]

In message <49665a6d.2030...@xtra.co.nz>, Bruce Griffiths writes:

>I have been unable to find a reference to triax consisting of 3
>conductors within a shield, however such confusion is understandable
>given the confusion over quadrax:-

I have only ever seen it used for very old 3-electrode condenser
microphones.

-- 
Poul-Henning Kamp   | UNIX since Zilog Zeus 3.20
p...@freebsd.org | TCP/IP since RFC 956
FreeBSD committer   | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

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Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops

[Bruce Griffiths]

Poul-Henning Kamp wrote:
> In message 
>  y.com>, Joseph M Gwinn writes:
>
>   
>>> That's technically speaking not triax, that's double shield.  Triax
>>> would have the conductors and one shield.
>>>   
>> No, I think that's twinax: . 
>>
>> Triax is a center plus two concentric shields: 
>> .
>> 
>
> Sorry, I fumbled what I wrote there.  I would say wiki is wrong
> here, the usage I am used to is:
>   coax: single conductor + shield
>   twinax: twisted pair + shield
>   triax: the wires + shield
>   

Poul-Henning

I have been unable to find a reference to triax consisting of 3
conductors within a shield, however such confusion is understandable
given the confusion over quadrax:-

concentric conductors for triax, quadrax, quintax etc:

Cable Shielding for Electromagnetic Compatibility By Anatoly Tsaliovich

http://www.lemo.com/techlibrary/glossary.jsp?catID=003


Alternate definitions for quadrax (but not triax):

http://www.picwire.com/technical/Coax%20vs%20Triax.pdf

http://www.ecnmag.com/ethernet-cable-suits-aerospace.aspx?menuid=336


Triax consisting of a central conductor surrounded by 2 concentric
conductors is widely used to interconnect televsion cameras.

Bruce

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Re: [time-nuts] GPSDO time constant

[Chris Kuethe]

On Thu, Jan 8, 2009 at 11:36 AM, WB6BNQ  wrote:
>   Do you have any web sites that show such a contration using leaf
>   blowers ?

mythbusters


-- 
GDB has a 'break' feature; why doesn't it have 'fix' too?

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Re: [time-nuts] GPSDO time constant

[WB6BNQ]

   "Lux, James P" wrote:

 When I worked in the special effects business, we used to use these
 all the time to move heavy stuff around. It actually doesn't
 require a "real flat" floor (1 cm grooves aren't a big issue).
 Think about them as small hovercraft. It also doesn't take much air
 pressure to lift things (large area * small pressure).  For
 instance, folks build small hovercraft using electric leaf blowers
 as the pressurization fan to support a disk some 1.2m in diameter
 which will easily support a couple people.

 The lift pads we used were about 30cm in diameter. 1 psi (7kPa)
 lifts about 50kg. Moving around 1 ton things with 4 pads wasn't
 unusual.

 The rougher the floor, the more airflow you need.

   James,

   Do you have any web sites that show such a contration using leaf
   blowers ?

   thanks,

   BillWB6BNQ
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Re: [time-nuts] 10 MHz Band-Pass Filter Needed

[Richard W. Solomon]

Looks like I can beat that price, the SCLF or SXLP devices are $8.95 each.

73, Dick, W1KSZ

-Original Message-
>From: "Lux, James P" 
>Sent: Jan 8, 2009 2:11 PM
>To: Discussion of precise time and frequency measurement 
>Subject: Re: [time-nuts] 10 MHz Band-Pass Filter Needed
>
>
>
>James Lux, P.E.
>Task Manager, SOMD Software Defined Radios
>Flight Communications Systems Section
>Jet Propulsion Laboratory
>4800 Oak Grove Drive, Mail Stop 161-213
>Pasadena, CA, 91109
>+1(818)354-2075 phone
>+1(818)393-6875 fax
>
>> -Original Message-
>> From: time-nuts-boun...@febo.com
>> [mailto:time-nuts-boun...@febo.com] On Behalf Of John Ackermann N8UR
>> Sent: Thursday, January 08, 2009 10:30 AM
>> To: dan...@verizon.net; Discussion of precise time and
>> frequency measurement
>> Subject: Re: [time-nuts] 10 MHz Band-Pass Filter Needed
>>
>> Dan Rae wrote:
>> > Richard W. Solomon wrote:
>> >> The GPSDO I want to use has an output rich in harmonics. In some
>> >> cases that is good, but Murphy rules and in the application I have
>> >> today, it is not good.
>> >>
>> >> I need a 10 MHz Band-Pass Filter, Bandwidth is not critical,
>> >> something small with SMA connectors would be ideal, but I
>> can live with BNC.
>> >>
>> >> Anyone have such a beast or know where I can get one ? I checked
>> >> Mini-Circuits and choked on the price !!
>
>You mean the ever popular BBP-10.7 for $41?
>Hard to beat that price for something comparable.
>
>
>> >>
>> >>
>> >
>> > Dick, it is really easy to build one.  The wonderful (free!) filter
>> > design program ELSIE will give you all the help you need.  I would
>> > have thought an hour or so with a couple of toroids would do the
>> > trick.  Even just a Low Pass filter would usually do to
>> turn a square wave into a sine...
>>
>> It's actually much better to use an LPF than a bandpass
>> filter if you don't have subharmonic energy to deal with.  An
>> LPF with a cutoff midway between the fundamental and the 2nd
>> harmonic will show much less tempco (in the form of phase
>> shift over temperature) than a bandpass filter.
>
>You might even be able to use some wonky little filter feedthrough with a 
>suitable cutoff frequency.
>
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Re: [time-nuts] GPSDO time constant

[Lux, James P]

> -Original Message-
> From: time-nuts-boun...@febo.com
> [mailto:time-nuts-boun...@febo.com] On Behalf Of Magnus Danielson
> Sent: Thursday, January 08, 2009 11:19 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] GPSDO time constant
>
> Poul-Henning Kamp skrev:
> > In message <4965e869.4090...@rubidium.dyndns.org>, Magnus
> Danielson writes:
> >
> >> Ericsson created a rack system in which the bottom of the rack was
> >> actually forming a closed container with the floor.
> >
> > We had that at one place I worked, except it was two small
> "dogs" you
> > attached front and back a standard 19" rack.
>
> I never seen it myself, so you have more correct info. I go
> back from something told to me way back in time, so details
> got a bit fuzzy.
>
> > Trouble was that you had to wash the floor first, or the dust would
> > blow all over the place.
>
> You also have some rather high requirements on the floor
> quality, it needs to be fairly flat as well.
>
When I worked in the special effects business, we used to use these all the 
time to move heavy stuff around. It actually doesn't require a "real flat" 
floor (1 cm grooves aren't a big issue).  Think about them as small hovercraft. 
It also doesn't take much air pressure to lift things (large area * small 
pressure).  For instance, folks build small hovercraft using electric leaf 
blowers as the pressurization fan to support a disk some 1.2m in diameter which 
will easily support a couple people.

The lift pads we used were about 30cm in diameter. 1 psi (7kPa) lifts about 
50kg. Moving around 1 ton things with 4 pads wasn't unusual.

The rougher the floor, the more airflow you need.

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Re: [time-nuts] GPSDO time constant

[Magnus Danielson]

Lux, James P skrev:
> 
>> -Original Message-
>> From: time-nuts-boun...@febo.com
>> [mailto:time-nuts-boun...@febo.com] On Behalf Of Magnus Danielson
>> Sent: Thursday, January 08, 2009 9:51 AM
>> To: Discussion of precise time and frequency measurement
>> Subject: Re: [time-nuts] GPSDO time constant
>>
>> Hi Brooke,
> 
>> So you didn't check out your HP gravity field wrapping mirror?
>> Those are SO handy. :)
>>
>> Cheers,
>> Magnus
> 
> 
> I think that division got sold off when it became Agilent.

I beleive ILM picked them up.

Cheers,
Magnus

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Re: [time-nuts] GPSDO time constant

[Magnus Danielson]

Poul-Henning Kamp skrev:
> In message <4965e869.4090...@rubidium.dyndns.org>, Magnus Danielson writes:
> 
>> Ericsson created a rack system in which the bottom of the rack 
>> was actually forming a closed container with the floor. 
> 
> We had that at one place I worked, except it was two small "dogs" you
> attached front and back a standard 19" rack.

I never seen it myself, so you have more correct info. I go back from 
something told to me way back in time, so details got a bit fuzzy.

> Trouble was that you had to wash the floor first, or the dust would
> blow all over the place.

You also have some rather high requirements on the floor quality, it 
needs to be fairly flat as well.

Cheers,
Magnus

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Re: [time-nuts] 10 MHz Band-Pass Filter Needed

[Lux, James P]

James Lux, P.E.
Task Manager, SOMD Software Defined Radios
Flight Communications Systems Section
Jet Propulsion Laboratory
4800 Oak Grove Drive, Mail Stop 161-213
Pasadena, CA, 91109
+1(818)354-2075 phone
+1(818)393-6875 fax

> -Original Message-
> From: time-nuts-boun...@febo.com
> [mailto:time-nuts-boun...@febo.com] On Behalf Of John Ackermann N8UR
> Sent: Thursday, January 08, 2009 10:30 AM
> To: dan...@verizon.net; Discussion of precise time and
> frequency measurement
> Subject: Re: [time-nuts] 10 MHz Band-Pass Filter Needed
>
> Dan Rae wrote:
> > Richard W. Solomon wrote:
> >> The GPSDO I want to use has an output rich in harmonics. In some
> >> cases that is good, but Murphy rules and in the application I have
> >> today, it is not good.
> >>
> >> I need a 10 MHz Band-Pass Filter, Bandwidth is not critical,
> >> something small with SMA connectors would be ideal, but I
> can live with BNC.
> >>
> >> Anyone have such a beast or know where I can get one ? I checked
> >> Mini-Circuits and choked on the price !!

You mean the ever popular BBP-10.7 for $41?
Hard to beat that price for something comparable.


> >>
> >>
> >
> > Dick, it is really easy to build one.  The wonderful (free!) filter
> > design program ELSIE will give you all the help you need.  I would
> > have thought an hour or so with a couple of toroids would do the
> > trick.  Even just a Low Pass filter would usually do to
> turn a square wave into a sine...
>
> It's actually much better to use an LPF than a bandpass
> filter if you don't have subharmonic energy to deal with.  An
> LPF with a cutoff midway between the fundamental and the 2nd
> harmonic will show much less tempco (in the form of phase
> shift over temperature) than a bandpass filter.

You might even be able to use some wonky little filter feedthrough with a 
suitable cutoff frequency.

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Re: [time-nuts] GPSDO time constant

[Lux, James P]

> -Original Message-
> From: time-nuts-boun...@febo.com
> [mailto:time-nuts-boun...@febo.com] On Behalf Of Magnus Danielson
> Sent: Thursday, January 08, 2009 9:51 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] GPSDO time constant
>
> Hi Brooke,

> So you didn't check out your HP gravity field wrapping mirror?
> Those are SO handy. :)
>
> Cheers,
> Magnus


I think that division got sold off when it became Agilent.

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Re: [time-nuts] 10 MHz Band-Pass Filter Needed (REVISED)

[Richard W. Solomon]

Another thought came to me, a PC Mount device would also do as I
could mount it inside the GPSDO box and eliminate another set of
connectors.
Building one is not viable as the space I have is restricted.

73, Dick, W1LSZ

-Original Message-
>From: "Richard W. Solomon" 
>Sent: Jan 8, 2009 1:11 PM
>To: time-nuts@febo.com
>Subject: [time-nuts] 10 MHz Band-Pass Filter Needed
>
>The GPSDO I want to use has an output rich in harmonics. In some
>cases that is good, but Murphy rules and in the application I have
>today, it is not good.
>
>I need a 10 MHz Band-Pass Filter, Bandwidth is not critical, something
>small with SMA connectors would be ideal, but I can live with BNC.
>
>Anyone have such a beast or know where I can get one ? I checked Mini-Circuits
>and choked on the price !!
>
>Thanks, Dick, W1KSZ
>
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Re: [time-nuts] GPSDO time constant

[John Ackermann N8UR]

I mentioned in another post that I picked up a couple of militarized 
FTS-4100 "fly-away" Cs units.  These have an option installed that adds 
an accelerometer to the OCXO; it's supposed to reduce G sensitivity by 
at least an order of magnitude.

John


saidj...@aol.com wrote:
> Hi Magnus,
>  
> all depends - in an aircraft you have 6g+ turn-overs :)
>  
> We make a version of our FireFly-II GPSDO with ultra-low-g sensitivity and  
> ruggedization, that one you can run in a back-pack while doing skating-tricks 
> on  a ramp and you won't see much change in frequency. A bit more pricey on 
> that  OCXO of course.
>  
> Most "standard" oscillators will have about 1-2ppb change after a  turn-over. 
> I have seen some that actually change the Crystal temperature when  turned 
> over, so you can see the initial frequency change due to gravity, then  you 
> see 
> the operating current change, and the frequency slowly drift away as the  
> temperature of the crystal is changed. Bad. Very bad.
>  
> bye,
> Said
>  
>  
> In a message dated 1/8/2009 09:29:41 Pacific Standard Time,  
> bro...@pacific.net writes:
> 
> Magnus  Danielson wrote:
> . . .
>> Most labs would have issues with a 2g  turnover. :)
>>
>> Flipping oscillators that runs is evil and should  be avoided at all
>> times. Should be in the rulebook for  time-nuts.
>>
>> Cheers,
>>  Magnus
> 
> 
> 
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Re: [time-nuts] 10 MHz Band-Pass Filter Needed

[John Ackermann N8UR]

Dan Rae wrote:
> Richard W. Solomon wrote:
>> The GPSDO I want to use has an output rich in harmonics. In some
>> cases that is good, but Murphy rules and in the application I have
>> today, it is not good.
>>
>> I need a 10 MHz Band-Pass Filter, Bandwidth is not critical, something
>> small with SMA connectors would be ideal, but I can live with BNC.
>>
>> Anyone have such a beast or know where I can get one ? I checked 
>> Mini-Circuits
>> and choked on the price !!
>>
>>   
> 
> Dick, it is really easy to build one.  The wonderful (free!) filter 
> design program ELSIE will give you all the help you need.  I would have 
> thought an hour or so with a couple of toroids would do the trick.  Even 
> just a Low Pass filter would usually do to turn a square wave into a sine...

It's actually much better to use an LPF than a bandpass filter if you 
don't have subharmonic energy to deal with.  An LPF with a cutoff midway 
between the fundamental and the 2nd harmonic will show much less tempco 
(in the form of phase shift over temperature) than a bandpass filter.

John

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Re: [time-nuts] GPSDO time constant

[SAIDJACK]

Hi Magnus,
 
all depends - in an aircraft you have 6g+ turn-overs :)
 
We make a version of our FireFly-II GPSDO with ultra-low-g sensitivity and  
ruggedization, that one you can run in a back-pack while doing skating-tricks 
on  a ramp and you won't see much change in frequency. A bit more pricey on 
that  OCXO of course.
 
Most "standard" oscillators will have about 1-2ppb change after a  turn-over. 
I have seen some that actually change the Crystal temperature when  turned 
over, so you can see the initial frequency change due to gravity, then  you see 
the operating current change, and the frequency slowly drift away as the  
temperature of the crystal is changed. Bad. Very bad.
 
bye,
Said
 
 
In a message dated 1/8/2009 09:29:41 Pacific Standard Time,  
bro...@pacific.net writes:

Magnus  Danielson wrote:
. . .
> Most labs would have issues with a 2g  turnover. :)
>
> Flipping oscillators that runs is evil and should  be avoided at all
> times. Should be in the rulebook for  time-nuts.
>
> Cheers,
>  Magnus



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Re: [time-nuts] 10 MHz Band-Pass Filter Needed

[Dan Rae]

Richard W. Solomon wrote:
> The GPSDO I want to use has an output rich in harmonics. In some
> cases that is good, but Murphy rules and in the application I have
> today, it is not good.
>
> I need a 10 MHz Band-Pass Filter, Bandwidth is not critical, something
> small with SMA connectors would be ideal, but I can live with BNC.
>
> Anyone have such a beast or know where I can get one ? I checked Mini-Circuits
> and choked on the price !!
>
>   

Dick, it is really easy to build one.  The wonderful (free!) filter 
design program ELSIE will give you all the help you need.  I would have 
thought an hour or so with a couple of toroids would do the trick.  Even 
just a Low Pass filter would usually do to turn a square wave into a sine...

Dan

ac6ao / g3ncr


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[time-nuts] 10 MHz Band-Pass Filter Needed

[Richard W. Solomon]

The GPSDO I want to use has an output rich in harmonics. In some
cases that is good, but Murphy rules and in the application I have
today, it is not good.

I need a 10 MHz Band-Pass Filter, Bandwidth is not critical, something
small with SMA connectors would be ideal, but I can live with BNC.

Anyone have such a beast or know where I can get one ? I checked Mini-Circuits
and choked on the price !!

Thanks, Dick, W1KSZ

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Re: [time-nuts] GPSDO time constant

[Magnus Danielson]

Hi Brooke,

bro...@pacific.net skrev:
> Hi Magnus:
> 
> I think some labs don't think about 90 degree turnovers, for example:
> 
> I've heard that the xtal oscillator cal procedure for some HP test
> equipment says that the instrument should be in the same position as when
> it's operating.  For heavy rack equipment that means you lay on a
> mechanics creeper when making the adjustment rather than flipping the
> instrument 90 degrees on a bench.

So you didn't check out your HP gravity field wrapping mirror?
Those are SO handy. :)

Cheers,
Magnus

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Re: [time-nuts] GPSDO time constant

[brooke]

Hi Magnus:

I think some labs don't think about 90 degree turnovers, for example:

I've heard that the xtal oscillator cal procedure for some HP test
equipment says that the instrument should be in the same position as when
it's operating.  For heavy rack equipment that means you lay on a
mechanics creeper when making the adjustment rather than flipping the
instrument 90 degrees on a bench.

Have Fun,

Brooke Clarke
http://www.prc68.com

Magnus Danielson wrote:
 . . .
> Most labs would have issues with a 2g turnover. :)
>
> Flipping oscillators that runs is evil and should be avoided at all
> times. Should be in the rulebook for time-nuts.
>
> Cheers,
> Magnus



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Re: [time-nuts] Motorola M12+T - On board oscillator phase noise specand/or part number

[John Ackermann N8UR]

Stephan Sandenbergh wrote:
> Hi Tom,
> 
> Thanks - the data is rather useful. Do you perhaps know what the high freq
> phase noise looks like?  I'd like to lock the GPS LO to the output of my
> GPSDO. However, the integrated jitter will probably determine if it could be
> done, by something as simple as filtering the output of a fractional N
> (implemented inside a FPGA) or maybe a dedicated fractional N IC with
> low-noise VCO (such as those from Analog Devices). I guess the latter is
> more realistic.
> 
> I'd guess the stock TCXO onboard the M12+T would have a noise floor below
> say -140dBc/Hz.

I have a plot that shows the phase noise of two Tbolts and two Z3801As 
at http://www.febo.com/pages/gpsdo_comparison/ -- scroll down the page a 
bit to find the phase noise info.

John

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Re: [time-nuts] Motorola M12+T - On board oscillator phase noise specand/or part number

[Stephan Sandenbergh]

Hi Tom,

Thanks - the data is rather useful. Do you perhaps know what the high freq
phase noise looks like?  I'd like to lock the GPS LO to the output of my
GPSDO. However, the integrated jitter will probably determine if it could be
done, by something as simple as filtering the output of a fractional N
(implemented inside a FPGA) or maybe a dedicated fractional N IC with
low-noise VCO (such as those from Analog Devices). I guess the latter is
more realistic.

I'd guess the stock TCXO onboard the M12+T would have a noise floor below
say -140dBc/Hz.

Regards,

Stephan.

2009/1/2 Tom Van Baak 

> > Hi All,
> >
> > Has anyone been able to figure out the part number and/or measured the
> > phase noise of the quartz oscillator on board the Motorola M12+T?
> >
> > This will be an interesting figure to see.
> >
> > Regards,
> >
> > Stephan.
>
> This is an ADEV+MDEV plot for the 100 Hz output of an
> M12+ in free-run mode (no gps lock).
>
> http://www.leapsecond.com/pages/m12-adev/m12-free.gif
>
> If you want more data let me know.
>
> /tvb
>
>
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Re: [time-nuts] GPSDO time constant

[Poul-Henning Kamp]

In message <4965e869.4090...@rubidium.dyndns.org>, Magnus Danielson writes:

>Ericsson created a rack system in which the bottom of the rack 
>was actually forming a closed container with the floor. 

We had that at one place I worked, except it was two small "dogs" you
attached front and back a standard 19" rack.

Trouble was that you had to wash the floor first, or the dust would
blow all over the place.

-- 
Poul-Henning Kamp   | UNIX since Zilog Zeus 3.20
p...@freebsd.org | TCP/IP since RFC 956
FreeBSD committer   | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

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Re: [time-nuts] GPSDO time constant

[Magnus Danielson]

Poul-Henning Kamp skrev:
> In message <4965dc62.9070...@rubidium.dyndns.org>, Magnus Danielson writes:
> 
>>> In a lab setting, it makes sense to increase over the default
>>> time-constant, just remember that it impacts your loops ability
>>> to cope with for instance a 2g turnover.
>>>
>> Most labs would have issues with a 2g turnover. :)
>>
>> Flipping oscillators that runs is evil and should be avoided at all 
>> times. Should be in the rulebook for time-nuts.
> 
> Correct, but it is perfectly normal behaviour for telecom techs
> who remodel installations while running, so the products must
> cope with it.
> 

True. Very true. The only thing which to some degree prohibits that is 
to make them monolithic. You are not entierly safe even with monolithic 
racks. Ericsson created a rack system in which the bottom of the rack 
was actually forming a closed container with the floor. Hook up 
compressed air to the nossle and the rack was floating, and hand-pushing 
the rack into position was no trouble, pull the plugg and it would sit 
tight on the floor again. That way they could shift in their racks while 
running, making cut-over time go down to zero.

Cheers,
Magnus

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Re: [time-nuts] GPSDO time constant

[Poul-Henning Kamp]

In message <4965dc62.9070...@rubidium.dyndns.org>, Magnus Danielson writes:

>> In a lab setting, it makes sense to increase over the default
>> time-constant, just remember that it impacts your loops ability
>> to cope with for instance a 2g turnover.
>> 
>Most labs would have issues with a 2g turnover. :)
>
>Flipping oscillators that runs is evil and should be avoided at all 
>times. Should be in the rulebook for time-nuts.

Correct, but it is perfectly normal behaviour for telecom techs
who remodel installations while running, so the products must
cope with it.

-- 
Poul-Henning Kamp   | UNIX since Zilog Zeus 3.20
p...@freebsd.org | TCP/IP since RFC 956
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Never attribute to malice what can adequately be explained by incompetence.

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Re: [time-nuts] GPSDO time constant

[Poul-Henning Kamp]

In message <4965dece.3060...@xtra.co.nz>, Bruce Griffiths writes:
>Hej Magnus
>
>Magnus Danielson wrote:
>
>How exactly can one measure the resultant performance or even select the
>optimum time constant and damping factor if one doesn't have a quieter
>reference?
>Can one really resort to some sort of N cornered hat?

I experimented with that, and you can actually get pretty far with
simpler standalone means such as periodograms of zero crossings.

My theory behind this is that the noise (of all kinds) is basically
gaussian or other symmetric distributions.

Consequently, the offset between your oscillator and the reference
should also have a symmetric distribution of sign, subject to well
known random "paradoxes" like the fact that repeatedly rolling 6
with dice doesn't prove you cheat, you might just be very very
lucky.

I have not fully developed this lead, and would welcome others
to experiement and improve on it, it takes more patience and
stability than my lap can muster when it must also work for my
salary.

The way I autotune the timeconstant of the PLL in NTPns is based
on this lack of zero-crossings of the offset from the reference input:

The first sign that a PLL has been torqued to hard is that the
reference input is not able to steer the oscillator adequately and
you can detect this very early, by monitoring how long runs you have
where the offset from the reference stays the same sign: the runs
on one side will get longer and more frequent than on the other
side.

You can also tell that the timeconstant is too short, because
the runs are not long enough, indicating that the PLL follows
the reference signal more aggresively than it need.

Look in the main/pllmath.c file of NTPns to see my current
implementation. (http://phk.freebsd.dk/phkrel/).

One of my NTP servers use the same GPS PPS to steer the PRS10 Rb
and for the NTP data feed, so the NTPns should obviously end up
with a zero frequency error, since the PRS10 takes care of that.

Consequently the PLL keeps stretching the timeconstant of the
software PLL, until it had identified a 2e-18 rounding error
in the frequency estimation code in the kernel.

The third order code on the other hand, does not seem to do me
much good yet, but maybe I simply don't have good enough kit for
that to be dominant.

The experiements I would propose requires that you can get hold
of the reference/oscillator difference signal somehow, and
then what you want to do is simply record runs of these values
for various timeconstants and damping factors.

The run them through a statistical package, or possibly even
the DIEHARD tests, and see how different timeconstants score,
the ones where the offset looks most random are optimal.

Have fun :-)

Poul-Henning

-- 
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Re: [time-nuts] GPSDO time constant

[Magnus Danielson]

Hej Bruce,

>>> The good news is that if you, the time-nut, have the gear and
>>> the time to measure the stability of the OCXO in your GPSDO,
>>> and know your environment well, then you can probably safely
>>> lengthen the TC and achieve much better mid-term stability out
>>> of your GPSDO as shown in the plot above.
>>> 
>> Agreed. But also recall that long-term effects like frequency drift is 
>> pretty easy to measure with a GPS. It would not take too much research 
>> to figure out some suitable drift-TC relationship such that you could 
>> either just look the charts to find a good match or even apply them in 
>> real time steering.
>>
>> For ThunderBolt owners it is pretty straightforward to adjust the TC and 
>> damping, which is very nice. Use this oppertunity!
> 
> How exactly can one measure the resultant performance or even select the
> optimum time constant and damping factor if one doesn't have a quieter
> reference?
> Can one really resort to some sort of N cornered hat?

If time constants is allowed to be limited by frequency drift 
(regardless of its source) then you can use a long term drift estimate 
to control the time constant of the control loop. Essentially being an 
adaptive filter.

It is still a gross oversimplification, but may still be a handy one.

Cheers,
Magnus

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Re: [time-nuts] GPSDO time constant

[Bruce Griffiths]

Hej Magnus

Magnus Danielson wrote:
> Tom Van Baak skrev:
>   
>> Here are ADEV plots and interesting results from a recent
>> experiment on varying the time-constant of a GPSDO:
>>
>> http://www.leapsecond.com/pages/tbolt-tc/
>>
>> There was a thread recently where Warren suggested that the
>> loop time constant (TC) of GPSDO was less than ideal. He is
>> correct. There are a couple of reasons for this, if I may guess.
>> 
>
> I particular liked that you tweaked the damping constant. If it is not 
> scaled off the normal charts (it doesn't look like it) I think it is 
> rather low damping factor and this infact does not supprise me at least 
> to produce a definitive bump. Keeping it at 1.2 is still not "critically 
> damped" but rather under-damped.
>
> I would love to see a few variants of measure at say tau = 100s but for 
> various damping coefficients. By the looks of it, the bump can be 
> attributed almost completely to resonance in the PLL loop, which is 
> certainly not what we would like to see... adding to the noise response.
>
>   
>> 1) Some GPSDO, like the surplus SmartClock designs from HP,
>> were designed to meet spec even when S/A was still in effect.
>> With the much greater wander in civilian GPS timing during
>> those years, the TC needed to be less than what you can get
>> away with today.
>>
>> 2) If you are a manufacturer and have a GPSDO spec to meet,
>> you need to make sure the TC is valid for all OCXO that you ship,
>> not just the average one. The way to do this is to be conservative
>> and to ship the units with a TC that is short enough that even the
>> parts on the lower end of the bell curve still meet your spec.
>>
>> The other alternative is to individually measure (days, weeks?)
>> every OCXO and individually burn a default TC into each unit
>> shipped.
>> 
>
> With the end result being that noise specs would still vary between 
> units. Also, if one unit was good at fab and gets pounded during 
> shipping doesn't help either.
>
>   
>> 3) Most commercial GPSDO need to work over a fairly wide
>> temperature range. This might require a tighter TC. If the user
>> has a more controlled environment they can probably tolerate a
>> longer TC that what the manufacturer dare ship as a default.
>>
>> 4) A GPSDO should still work reliably in the face of phase or
>> frequency jumps in the OCXO. Although the timing of these is
>> not predictable, their typical magnitude is probably something
>> that the designer can learn. The TC needs to be short enough
>> so that the GPSDO gracefully handles these jumps. If one is
>> too aggressive the GPSDO will wander out of spec instead of
>> more closely tracking GPS.
>> 
>
> All these 4 points really argue against the principle of choosing one TC 
> to fit them all. Using suitable heuristics to adapt TC to conditions and 
> recent history runtime will provide a much more dynamic fashion in which 
> units would adapt to their performance and their environment.
>
>   
>> 5) I'm not sure it's possible to optimize for both time stability
>> and frequency stability at the same time. A long TC will help
>> avoid too sudden frequency changes in the GPSDO; a short
>> TC will help the 1pps stay close to UTC. I suppose a GPSDO
>> might be optimized more for one application than the other;
>> this would affect the choice of default TC.
>> 
>
> Actually no, not really.
>
> If we remove the issues of hanging bridge, which the ThunderBolt 
> effectively has since it steers it's timing clock, then another reason 
> for shifting PPS is due to changing symmetry and when removing or adding 
> a satellite from the chosen constellation (by tracking set or TRAIM 
> selective set) the apparent receiver time will jump. In the meanwhile it 
> may glide as the symmetry changes. Multipath can also aid in shifting 
> position. So a shorter time constant does not render a closer rendition 
> of UTC but rather a quicker follow of apparent time position of the 
> receiver, which isn't quite the same thing. Thus, a longer time 
> constants acts like an averaging of apparent time position.
>
> Another factor which plauges most single frequency receivers is that 
> they can't correct for actual ionospheric delay. They run according to a 
> parameterized model. There is a deviation between the actual and 
> apparent delay and it is changing over time.
>
> Thus, using Rubidium or Caesium to steer local clock will allow for even 
> greater time constants and thus greater averaging potential, as 1000 s 
> is still kind of "short".
>
> The conclusion is that the GPS receivers we use have many inherrent 
> non-static error sources
>
>   
>> 6) Most OCXO demonstrate much better drift rates after they
>> have been in operation for weeks or months. The drift rate has
>> a some impact on the choice of TC. It's probably not a good
>> business model to ship a GPSDO with a TC optimized for how
>> the unit might eventually run a year from now. It has to work
>> out of the

Re: [time-nuts] GPSDO time constant

[Magnus Danielson]

Poul-Henning Kamp skrev:
> In message , "Tom Van Baak" writes:
>> Here are ADEV plots and interesting results from a recent
>> experiment on varying the time-constant of a GPSDO:
>>
>> http://www.leapsecond.com/pages/tbolt-tc/
> 
> Tom, part of the reason for the sub-optimally short default time
> constant is to be able to cope with worst-case specs of the
> oscillator.
> 
> Even though they are pretty good, shifts in voltage, temperature
> and orientation does affect them.
> 
> A very good example of this effect is the NTPD PLL, which
> uncritically belives otherwise unplausible good news, and
> lengthens the poll-period to 20 minutes and then refuses
> to accept that it did it wrong, until i thas seen three
> or four (= one hour) samples saying so, after which it
> breaks lock and starts over.

Isn't this more due to surrounding (obviously flawed) heuristics rather 
than the PLL?

Interesting never the less.

> In a lab setting, it makes sense to increase over the default
> time-constant, just remember that it impacts your loops ability
> to cope with for instance a 2g turnover.
> 
Most labs would have issues with a 2g turnover. :)

Flipping oscillators that runs is evil and should be avoided at all 
times. Should be in the rulebook for time-nuts.

Cheers,
Magnus

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Re: [time-nuts] GPSDO time constant

[Magnus Danielson]

Tom Van Baak skrev:
> Here are ADEV plots and interesting results from a recent
> experiment on varying the time-constant of a GPSDO:
> 
> http://www.leapsecond.com/pages/tbolt-tc/
> 
> There was a thread recently where Warren suggested that the
> loop time constant (TC) of GPSDO was less than ideal. He is
> correct. There are a couple of reasons for this, if I may guess.

I particular liked that you tweaked the damping constant. If it is not 
scaled off the normal charts (it doesn't look like it) I think it is 
rather low damping factor and this infact does not supprise me at least 
to produce a definitive bump. Keeping it at 1.2 is still not "critically 
damped" but rather under-damped.

I would love to see a few variants of measure at say tau = 100s but for 
various damping coefficients. By the looks of it, the bump can be 
attributed almost completely to resonance in the PLL loop, which is 
certainly not what we would like to see... adding to the noise response.

> 1) Some GPSDO, like the surplus SmartClock designs from HP,
> were designed to meet spec even when S/A was still in effect.
> With the much greater wander in civilian GPS timing during
> those years, the TC needed to be less than what you can get
> away with today.
> 
> 2) If you are a manufacturer and have a GPSDO spec to meet,
> you need to make sure the TC is valid for all OCXO that you ship,
> not just the average one. The way to do this is to be conservative
> and to ship the units with a TC that is short enough that even the
> parts on the lower end of the bell curve still meet your spec.
> 
> The other alternative is to individually measure (days, weeks?)
> every OCXO and individually burn a default TC into each unit
> shipped.

With the end result being that noise specs would still vary between 
units. Also, if one unit was good at fab and gets pounded during 
shipping doesn't help either.

> 3) Most commercial GPSDO need to work over a fairly wide
> temperature range. This might require a tighter TC. If the user
> has a more controlled environment they can probably tolerate a
> longer TC that what the manufacturer dare ship as a default.
> 
> 4) A GPSDO should still work reliably in the face of phase or
> frequency jumps in the OCXO. Although the timing of these is
> not predictable, their typical magnitude is probably something
> that the designer can learn. The TC needs to be short enough
> so that the GPSDO gracefully handles these jumps. If one is
> too aggressive the GPSDO will wander out of spec instead of
> more closely tracking GPS.

All these 4 points really argue against the principle of choosing one TC 
to fit them all. Using suitable heuristics to adapt TC to conditions and 
recent history runtime will provide a much more dynamic fashion in which 
units would adapt to their performance and their environment.

> 5) I'm not sure it's possible to optimize for both time stability
> and frequency stability at the same time. A long TC will help
> avoid too sudden frequency changes in the GPSDO; a short
> TC will help the 1pps stay close to UTC. I suppose a GPSDO
> might be optimized more for one application than the other;
> this would affect the choice of default TC.

Actually no, not really.

If we remove the issues of hanging bridge, which the ThunderBolt 
effectively has since it steers it's timing clock, then another reason 
for shifting PPS is due to changing symmetry and when removing or adding 
a satellite from the chosen constellation (by tracking set or TRAIM 
selective set) the apparent receiver time will jump. In the meanwhile it 
may glide as the symmetry changes. Multipath can also aid in shifting 
position. So a shorter time constant does not render a closer rendition 
of UTC but rather a quicker follow of apparent time position of the 
receiver, which isn't quite the same thing. Thus, a longer time 
constants acts like an averaging of apparent time position.

Another factor which plauges most single frequency receivers is that 
they can't correct for actual ionospheric delay. They run according to a 
parameterized model. There is a deviation between the actual and 
apparent delay and it is changing over time.

Thus, using Rubidium or Caesium to steer local clock will allow for even 
greater time constants and thus greater averaging potential, as 1000 s 
is still kind of "short".

The conclusion is that the GPS receivers we use have many inherrent 
non-static error sources

> 6) Most OCXO demonstrate much better drift rates after they
> have been in operation for weeks or months. The drift rate has
> a some impact on the choice of TC. It's probably not a good
> business model to ship a GPSDO with a TC optimized for how
> the unit might eventually run a year from now. It has to work
> out of the box. So this cause the default TC to be set shorter
> than ideal.
> 
> 7) There may also be a SV, sky-view, or latitude dependence.
> Someone enjoying all 32 SV today, with a clear 360 degree
> view of the sky at mid-lati

Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops

[Bruce Griffiths]

Poul-Henning Kamp wrote:
> In message 
>  y.com>, Joseph M Gwinn writes:
>
>   
>>> That's technically speaking not triax, that's double shield.  Triax
>>> would have the conductors and one shield.
>>>   
>> No, I think that's twinax: . 
>>
>> Triax is a center plus two concentric shields: 
>> .
>> 
>
> Sorry, I fumbled what I wrote there.  I would say wiki is wrong
> here, the usage I am used to is:
>   coax: single conductor + shield
>   twinax: twisted pair + shield
>   triax: the wires + shield
>   
>   
>>> (Who once lost all ethernet interfaces, the access control system
>>> and a few minor computers when a moron first created and then cut
>>> a 600+ A ground loop).
>>>   
>> Was there a big bang?  What was the source of the 600 amps?
>> 
>
> They replaced the separation transformer with a UPS, and they
> connected the two sides ground together at the UPS.
>
> Unfortunately the grounding on our secondary side was much better
> than the power companys grounding on the primary side, which was the
> entire point of having the the transformer in the first place.
>
> Yes, there were a significant bang and his two-hand wire-cutter was
> recategorized from "tool" to "industrial art".
>
>   
Similarly for Quadraxial cable there are 2 interpretations:

1) an inner conductor surrounded by 3 coaxial tubular conductors all
insulated from each other.

2) 2 twisted pairs with an outer tubular shield used in some high speed
network cabling.

Both meanings are in common use.

Quintaxial cable seems only to be mentioned in texts on cable shielding.
In which it consists of a central conductor surrounded by 4 coaxial
tubular screens all of which are insulated from each other.

Bruce

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Re: [time-nuts] GPSDO time constant

[Poul-Henning Kamp]

In message , "Tom Van Baak" writes:
>Here are ADEV plots and interesting results from a recent
>experiment on varying the time-constant of a GPSDO:
>
>http://www.leapsecond.com/pages/tbolt-tc/

Tom, part of the reason for the sub-optimally short default time
constant is to be able to cope with worst-case specs of the
oscillator.

Even though they are pretty good, shifts in voltage, temperature
and orientation does affect them.

A very good example of this effect is the NTPD PLL, which
uncritically belives otherwise unplausible good news, and
lengthens the poll-period to 20 minutes and then refuses
to accept that it did it wrong, until i thas seen three
or four (= one hour) samples saying so, after which it
breaks lock and starts over.

In a lab setting, it makes sense to increase over the default
time-constant, just remember that it impacts your loops ability
to cope with for instance a 2g turnover.

-- 
Poul-Henning Kamp   | UNIX since Zilog Zeus 3.20
p...@freebsd.org | TCP/IP since RFC 956
FreeBSD committer   | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

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Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops

[Poul-Henning Kamp]

In message <49657762.5060...@rubidium.dyndns.org>, Magnus Danielson writes:


>> Was there a big bang?  What was the source of the 600 amps?
>
>I think there (with some delay) was some awfull scream of dispare.
>The cost of Ethernet interfaces where much more significant back then.

The most expensive one we lost was in a UNISYS 2200, where three
microprocessors worked together to limit bandwidth to 100 kB/s.
I belive the sticker prices as $15k.

-- 
Poul-Henning Kamp   | UNIX since Zilog Zeus 3.20
p...@freebsd.org | TCP/IP since RFC 956
FreeBSD committer   | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

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Re: [time-nuts] Standards sought for immunity of shielded cable links to power-frequency ground loops

[Poul-Henning Kamp]

In message , Joseph M Gwinn writes:

>> That's technically speaking not triax, that's double shield.  Triax
>> would have the conductors and one shield.
>
>No, I think that's twinax: . 
>
>Triax is a center plus two concentric shields: 
>.

Sorry, I fumbled what I wrote there.  I would say wiki is wrong
here, the usage I am used to is:
coax: single conductor + shield
twinax: twisted pair + shield
triax: the wires + shield

>> (Who once lost all ethernet interfaces, the access control system
>> and a few minor computers when a moron first created and then cut
>> a 600+ A ground loop).
>
>Was there a big bang?  What was the source of the 600 amps?

They replaced the separation transformer with a UPS, and they
connected the two sides ground together at the UPS.

Unfortunately the grounding on our secondary side was much better
than the power companys grounding on the primary side, which was the
entire point of having the the transformer in the first place.

Yes, there were a significant bang and his two-hand wire-cutter was
recategorized from "tool" to "industrial art".

-- 
Poul-Henning Kamp   | UNIX since Zilog Zeus 3.20
p...@freebsd.org | TCP/IP since RFC 956
FreeBSD committer   | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

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Re: [time-nuts] GPSDO time constant

[Neville Michie]

Thanks Tom,
that answered questions I had been wondering about for some time.
I think that was one of the best posts that I have read.

It also reinforces my plans to build a passive temperature control  
for my TB,
putting it in an insulated aluminium box with a tiny cooling fan  
regulating its self-
heating temperature to about 40C.

great post!

Neville Michie


On 08/01/2009, at 7:02 PM, Tom Van Baak wrote:

> Here are ADEV plots and interesting results from a recent
> experiment on varying the time-constant of a GPSDO:
>
> http://www.leapsecond.com/pages/tbolt-tc/
>
>
> /tvb
>
>
>
> ___
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> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/ 
> time-nuts
> and follow the instructions there.


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[time-nuts] GPSDO time constant

[Tom Van Baak]

Here are ADEV plots and interesting results from a recent
experiment on varying the time-constant of a GPSDO:

http://www.leapsecond.com/pages/tbolt-tc/

There was a thread recently where Warren suggested that the
loop time constant (TC) of GPSDO was less than ideal. He is
correct. There are a couple of reasons for this, if I may guess.

1) Some GPSDO, like the surplus SmartClock designs from HP,
were designed to meet spec even when S/A was still in effect.
With the much greater wander in civilian GPS timing during
those years, the TC needed to be less than what you can get
away with today.

2) If you are a manufacturer and have a GPSDO spec to meet,
you need to make sure the TC is valid for all OCXO that you ship,
not just the average one. The way to do this is to be conservative
and to ship the units with a TC that is short enough that even the
parts on the lower end of the bell curve still meet your spec.

The other alternative is to individually measure (days, weeks?)
every OCXO and individually burn a default TC into each unit
shipped.

3) Most commercial GPSDO need to work over a fairly wide
temperature range. This might require a tighter TC. If the user
has a more controlled environment they can probably tolerate a
longer TC that what the manufacturer dare ship as a default.

4) A GPSDO should still work reliably in the face of phase or
frequency jumps in the OCXO. Although the timing of these is
not predictable, their typical magnitude is probably something
that the designer can learn. The TC needs to be short enough
so that the GPSDO gracefully handles these jumps. If one is
too aggressive the GPSDO will wander out of spec instead of
more closely tracking GPS.

5) I'm not sure it's possible to optimize for both time stability
and frequency stability at the same time. A long TC will help
avoid too sudden frequency changes in the GPSDO; a short
TC will help the 1pps stay close to UTC. I suppose a GPSDO
might be optimized more for one application than the other;
this would affect the choice of default TC.

6) Most OCXO demonstrate much better drift rates after they
have been in operation for weeks or months. The drift rate has
a some impact on the choice of TC. It's probably not a good
business model to ship a GPSDO with a TC optimized for how
the unit might eventually run a year from now. It has to work
out of the box. So this cause the default TC to be set shorter
than ideal.

7) There may also be a SV, sky-view, or latitude dependence.
Someone enjoying all 32 SV today, with a clear 360 degree
view of the sky at mid-latitude will probably enjoy slightly better
performance than someone a few years ago when there were
less operational SV in orbit, or with mountain, forest, building
obstructions, or at extreme latitudes. If you have much better
than average reception you could probably move the TC out
further.

So for these reasons (more like guesses), it would not surprise
me if most GPSDO have the TC set on the low side. Let me
know if you have additional info on this topic.

The good news is that if you, the time-nut, have the gear and
the time to measure the stability of the OCXO in your GPSDO,
and know your environment well, then you can probably safely
lengthen the TC and achieve much better mid-term stability out
of your GPSDO as shown in the plot above.

/tvb



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