date:20161121

Hi

There are packages you can put on a fairly standard HP signal generator that 
will
do the mismatch stuff without spending all the money Sperient wants for one of 
their machines. One suggestion: If you *do* go with the HP solution, running 
the 
attenuator up and down to simulate fast fades (think urban canyon) will fry the 
generator
in about 30 days … I have empirical data ….

Bob

> On Nov 21, 2016, at 8:53 PM, Scott Stobbe  wrote:
> 
> I haven't used one personally but a spirent gps simulator would let you do
> a try it and see.
> 
> It will be interesting to see if out of the growing sdr community an open
> source gps simulator emerges.
> 
> On Mon, Nov 21, 2016 at 1:01 PM Dr. David Kirkby (Kirkby Microwave Ltd) <
> drkir...@kirkbymicrowave.co.uk> wrote:
> 
>> People state it is desirable to have a GPS antenna well clear of
>> obstructions, which I believe is to stop reflections. But there is another
>> source of reflections which I suspect could be just as problematic.
>> 
>> Whilst the input impedance of the antenna input terminal on a GPS receiver
>> is probably marked 50 Ohms, I'd be somewhat surprised if it was very close
>> to 50 Ohms. Antenna cables have an impedance, which is typically 50 +/- 2
>> Ohms, but this varies, not only between different makes/models of cables,
>> but even on the same real of cable.The output of the pre-amp is most
>> unlikely to have a 50 Ohm source impedance. In fact, the output impedance
>> might be close to 0 Ohms, as it may be driven by a voltage source, without
>> any 50 Ohm resistor.
>> 
>> Anything not immediately absorbed by the GPS receiver is going to be
>> reflected back up the coax, and could be reflected multiple times.
>> 
>> I just looked on my HP 8720D VNA, and see I can reduce the output power to
>> -70 dBm, which would should not do any damage. It will be interesting to
>> see just what the input impedance of the GPS receiver is. I'm tied up with
>> doing my accounts over the next few days, but later I will look.
>> 
>> If reflections on the antenna/cable/receiver are a problem, then
>> attenuators can improve the match, but of course they reduce the signal
>> level too. A more intelligent, but more difficult solution, is to build a
>> matching network. For that one would need a VNA to measure the impedance in
>> the first place.
>> 
>> Dr. David Kirkby Ph.D CEng MIET
>> Kirkby Microwave Ltd
>> Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
>> UK.
>> Registered in England and Wales, company number 08914892.
>> http://www.kirkbymicrowave.co.uk/
>> Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)
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>> 
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Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

2016-11-21 Thread Scott Stobbe

I haven't used one personally but a spirent gps simulator would let you do
a try it and see.

It will be interesting to see if out of the growing sdr community an open
source gps simulator emerges.

On Mon, Nov 21, 2016 at 1:01 PM Dr. David Kirkby (Kirkby Microwave Ltd) <
drkir...@kirkbymicrowave.co.uk> wrote:

> People state it is desirable to have a GPS antenna well clear of
> obstructions, which I believe is to stop reflections. But there is another
> source of reflections which I suspect could be just as problematic.
>
> Whilst the input impedance of the antenna input terminal on a GPS receiver
> is probably marked 50 Ohms, I'd be somewhat surprised if it was very close
> to 50 Ohms. Antenna cables have an impedance, which is typically 50 +/- 2
> Ohms, but this varies, not only between different makes/models of cables,
> but even on the same real of cable.The output of the pre-amp is most
> unlikely to have a 50 Ohm source impedance. In fact, the output impedance
> might be close to 0 Ohms, as it may be driven by a voltage source, without
> any 50 Ohm resistor.
>
> Anything not immediately absorbed by the GPS receiver is going to be
> reflected back up the coax, and could be reflected multiple times.
>
> I just looked on my HP 8720D VNA, and see I can reduce the output power to
> -70 dBm, which would should not do any damage. It will be interesting to
> see just what the input impedance of the GPS receiver is. I'm tied up with
> doing my accounts over the next few days, but later I will look.
>
> If reflections on the antenna/cable/receiver are a problem, then
> attenuators can improve the match, but of course they reduce the signal
> level too. A more intelligent, but more difficult solution, is to build a
> matching network. For that one would need a VNA to measure the impedance in
> the first place.
>
> Dr. David Kirkby Ph.D CEng MIET
> Kirkby Microwave Ltd
> Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
> UK.
> Registered in England and Wales, company number 08914892.
> http://www.kirkbymicrowave.co.uk/
> Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)
> ___
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Re: [time-nuts] How can I measure GPS Antenna quality?


On 11/21/16 10:10 AM, Tom Van Baak wrote:

For the equipment hobbyists usually have, the phase center is not that
important. Most antennas have a variation <5mm. Even 10mm would lead to
just a ~33ps variation.


I agree. And besides, for those of us here in Oregon/Washington, the very 
ground is moving northwest at several inches per year (plate tectonics).



Is that movement in absolute terms? or relative to the NA plate?
Where I live in SoCal (on the Pacific plate side ), we also have an 
annual uplift on the order of 1cm.



http://www.unavco.org/software/geodetic-utilities/plate-motion-calculator/plate-motion-calculator.html

For 34N, 118W, 47.67mm/yr at 297.53 degrees(CW from north) 22.04N, 
-42.28E in local coordinates


-31.57 X, 30.68 Y, 18.31 Z using WGS 84



for 34, 119W (Ventura county, some 50 miles west) 48.04mm/yr total, 
-31.12 X, 31.51 Y, 18.62 Z


A little less X, a little more Y, because it's starting to "turn the corner"


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Re: [time-nuts] Rohde & Schwarz XSD 2.5 MHz crystal gone bad?

Hi

If the part is 50 years old that pre-dates the invention of the SC cut. AFIK, 
all of the 
QK glass bottle crystals date to the “pre SC” era. The BT is about the only 
other 
alternative from that era. HP made it their “goto cut” for OCXO’s back then. 
Others
may have followed along ….

Bob
 
> On Nov 21, 2016, at 7:00 PM, Azelio Boriani  wrote:
> 
> A comparison between AT and SC cuts' temperature stability can be found here:
> 
> 
> 
> On Mon, Nov 21, 2016 at 4:21 AM, Bob Camp  wrote:
>> Hi
>> 
>> If the frequency drops as the oven warms up, you have an AT cut crystal. If
>> the frequency goes up as the oven warms up, you have a BT cut crystal. With 
>> an
>> AT or a BT, the frequency change between room and “hot” will depend a lot on
>> the details of the proper oven temperature. A frequency shift of 20 to 40 
>> ppm is not
>> at all unusual as the oven warms up. The oscillator will only tune on 
>> frequency
>> once the oven is hot.
>> 
>> http://www.aextal.com/tutorial-frequency-stability.htm
>> 
>> Regardless of which cut you have (an AT is the best guess). The oven 
>> temperature
>> would be adjusted to put the crystal at it’s minimum sensitivity point. For 
>> an AT that
>> is the lowest frequency. If the oven is not heating to the correct 
>> temperature, you will
>> probably be unable to get the oscillator on frequency.
>> 
>> If there is a circuit problem, the most likely culprit are the inductors. 
>> They form tuned
>> traps that put the circuit onto the correct overtone. This increases the 
>> circuit’s sensitivity
>> to changes in inductance. 50 year old inductors may have been made with core 
>> materials
>> that aged more than just a little bit. I have empirical data on this :)
>> 
>> As others have suggested, check the oven heater along with the oscillator 
>> circuit. The
>> issues you see might come from either one.
>> 
>> Bob
>> 
>>> On Nov 20, 2016, at 6:49 PM, Michael Ulbrich  wrote:
>>> 
>>> Hi there,
>>> 
>>> I'm new to this list and have some interest in quartz crystal and
>>> rubidium oscillators - GPS, NTP, PPS and "clock watching" in general ;-)
>>> 
>>> I snatched an R&S XSD frequency standard accompanied by an XKE frequency
>>> controller. Found the manuals on KO4BB's site (Thanks a lot for that!
>>> BTW there's a slight mix-up of pages in that some of the XKE pages have
>>> found their way into the XSD manual).
>>> 
>>> My initial hope of a quick and easy restoration project faded a bit when
>>> I looked at the XSD output frequency after heating up the oven. It was
>>> off by about -1 * 10-5 and after cranking the fine tuning for some time
>>> and having another look at the specs I realized that the frequency was
>>> too far off to be dialed in by the fine tuning which only covers about
>>> +/- 2 * 10-7.
>>> 
>>> Next step was to take apart the oven and check series capacitor,
>>> oscillator and the crystal itself. I found that even the coarse
>>> adjustment range of the cylindrical series capacitor (40 - 110 pF) would
>>> not allow to pull the crystal to it's specified frequency. When
>>> replacing the series cap by a ceramic and lowering the value to just
>>> before the oscillation breaks down (about 10 pF) I managed to set the
>>> oscillator frequency offset to  +2.5 * 10-6 at room temperature. But
>>> even this will not suffice when taking into account that the frequency
>>> will drop by a few parts in 10-5 (cf. XSD manual) when the oven heats up
>>> to its operating temperature. I also checked some of the components on
>>> the oscillator PCB which might have an influence on frequency but could
>>> not find any fault.
>>> 
>>> The crystal itself is a disk of about 30 mm diameter mounted in a sealed
>>> glass cylinder of about 38 mm dia. and 43 mm height. There is no socket
>>> just 2 bare wires.. It does not show any visual signs of damage.
>>> According to a reference given in the R&S XSD manual the crystal's
>>> construction follows a publication from A.W. Warner "Design and
>>> Performance of Ultraprecise 2.5-mc Quartz Crystal Units" in Vol 39,
>>> Issue 5 of Bell Labs Technical Journal (Sept. 1960). According to that
>>> it is an AT-cut 5th overtone design.
>>> 
>>> Now my questions:
>>> 
>>> a) Considering that this gear is about 50 years old, a "crystal gone
>>> bad" situation shouldn't be that much of a surprise, right?  But could
>>> there be any other cause of the "huge" frequency offset besides a bad
>>> crystal? I would very much appreciate any idea that I could try to get
>>> this baby back on spec.
>>> 
>>> b) if nothing else helps: Could any of you give me a hint about who
>>> might be able to supply a spare crystal? I tried my directly reachable
>>> contacts but unfortunately to no avail so far. Please consider that
>>> similar crystals might also have found their way into other
>>> manufacturer's constructions from that era - Sulzer, Racal, HP ... you
>>> name it ...
>>> 
>>> Many thanks in advance!
>

Re: [time-nuts] How can I measure GPS Antenna quality?


On 11/21/16 2:58 PM, Attila Kinali wrote:

On Mon, 21 Nov 2016 08:22:50 -0800
jimlux  wrote:


I'm not sure about whether an anechoic (which is really "hypoechoic")
chamber is going to get you the data you need.  Calibrating the chamber
to the needed level of accuracy might be harder than doing field
measurements.

[...]

  sin(2 degrees) is 0.034, or -30dB.  So a spurious reflection that is 3
cm different path length (modulo wavelength) and 30 dB down will give
you a 1mm phase center error.  0.1 mm is -50dB.


Interesting. I haven't done the math, so I didn't think about that.
Yes, the reflections in the chamber would probably limit the resolution.

Now I wonder how the calibration data for mass produced geodetic antenna
are collected. I very much doubt they put them outside for a couple
of days to measure them exactly.


That's exactly what they probably do (put them outside) - assuming they 
have an individual cal at all.


A *good* antenna design is one where if the mechanical assembly is 
within manufacturing tolerances, it will have the same performance as 
all the others made to the same tolerance.



ftp://www.ngs.noaa.gov/pub/abilich/calibPapers/Goerres2006.pdf

Note that the residuals after cal were biggest at 0 and 90 elevation, 
and best in the mid elevations..










Attila Kinali



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Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?


On 11/21/16 3:11 PM, Attila Kinali wrote:

On Mon, 21 Nov 2016 14:36:49 -0500
Bob Camp  wrote:


The reflection issue ahead of the antenna is a reflection of the signal from
a single satellite. The multipath
reflection makes that satellite appear to be further away than it really is.
In the case that the reflected
signal  is *stronger* than the desired signal, the multipath reflection
“captures” the receiver and the net
solution is messed up.


Even a weaker reflected signal can cause significant change of the
correlation peak and thus of the apparent distance of the satellite.


Only if the multipath is less than a chip away.. if it's more than a 
chip away, it doesn't change the timing of the correlation peak for the 
primary path.


Actually, since the "timing"of the recovered code is averaged over many 
chips/code periods, "close by" multipath might have an effect because it 
affects the "shape" of the peak - it's not nice and triangular.


1 chip = 1 microsecond = 300 meters.


So multipath <300 meters away (probably a specular reflection off 
something) - here the geometry helps as you average over multiple code 
periods - the multipath timing, relative to the true path, is not fixed 
-  the reflected path may just disappear and reappear (specular 
reflections from something "far-ish" away - 10 meters)


That's why, if you can get your receiver antenna up high above the 
surroundings, multipath is less of a problem: the antenna has poor gain 
below the horizon, and the enforced distance between antenna and 
"nearest possible reflector" is greater, which makes the temporal 
movement of the spurious signal that much bigger.



A pathological case would be an antenna next to a vertical wall.  There 
will likely be two signals with identical strength and very small 
differential time that doesn't change very much.




The multipath error envolope diagrams are usually for multipath to
direct path ratios of between 1:2 to 1:10 (mostly depending on what
the author wants to show or how much he wants to cheat).



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Re: [time-nuts] Rohde & Schwarz XSD 2.5 MHz crystal gone bad?

Hi

Thanks for the pictures !!!

Another possible thing to watch for: It may take a *long* time for the oven to 
stabilize at the correct temperature. 

Bob

> On Nov 21, 2016, at 6:31 PM, Michael Ulbrich  wrote:
> 
> Hi all,
> 
> thanks a lot for your helpful suggestions and pointers to sources of
> additional information.
> 
> I also got a few replies off list and one list member may actually have
> a spare crystal!
> 
> Will report to the list as this project (hopefully) progresses.
> 
> BTW what is the "attachment policy" of this list? I will try to attach 2
> - small - images of the crystal (side and top view) and see if they get
> through somehow.
> 
> Thanks again ... Michael
> 
> 
> 
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Re: [time-nuts] Rohde & Schwarz XSD 2.5 MHz crystal gone bad?

2016-11-21 Thread Azelio Boriani

A comparison between AT and SC cuts' temperature stability can be found here:



On Mon, Nov 21, 2016 at 4:21 AM, Bob Camp  wrote:
> Hi
>
> If the frequency drops as the oven warms up, you have an AT cut crystal. If
> the frequency goes up as the oven warms up, you have a BT cut crystal. With an
> AT or a BT, the frequency change between room and “hot” will depend a lot on
> the details of the proper oven temperature. A frequency shift of 20 to 40 ppm 
> is not
> at all unusual as the oven warms up. The oscillator will only tune on 
> frequency
> once the oven is hot.
>
> http://www.aextal.com/tutorial-frequency-stability.htm
>
> Regardless of which cut you have (an AT is the best guess). The oven 
> temperature
> would be adjusted to put the crystal at it’s minimum sensitivity point. For 
> an AT that
> is the lowest frequency. If the oven is not heating to the correct 
> temperature, you will
> probably be unable to get the oscillator on frequency.
>
> If there is a circuit problem, the most likely culprit are the inductors. 
> They form tuned
> traps that put the circuit onto the correct overtone. This increases the 
> circuit’s sensitivity
> to changes in inductance. 50 year old inductors may have been made with core 
> materials
> that aged more than just a little bit. I have empirical data on this :)
>
> As others have suggested, check the oven heater along with the oscillator 
> circuit. The
> issues you see might come from either one.
>
> Bob
>
>> On Nov 20, 2016, at 6:49 PM, Michael Ulbrich  wrote:
>>
>> Hi there,
>>
>> I'm new to this list and have some interest in quartz crystal and
>> rubidium oscillators - GPS, NTP, PPS and "clock watching" in general ;-)
>>
>> I snatched an R&S XSD frequency standard accompanied by an XKE frequency
>> controller. Found the manuals on KO4BB's site (Thanks a lot for that!
>> BTW there's a slight mix-up of pages in that some of the XKE pages have
>> found their way into the XSD manual).
>>
>> My initial hope of a quick and easy restoration project faded a bit when
>> I looked at the XSD output frequency after heating up the oven. It was
>> off by about -1 * 10-5 and after cranking the fine tuning for some time
>> and having another look at the specs I realized that the frequency was
>> too far off to be dialed in by the fine tuning which only covers about
>> +/- 2 * 10-7.
>>
>> Next step was to take apart the oven and check series capacitor,
>> oscillator and the crystal itself. I found that even the coarse
>> adjustment range of the cylindrical series capacitor (40 - 110 pF) would
>> not allow to pull the crystal to it's specified frequency. When
>> replacing the series cap by a ceramic and lowering the value to just
>> before the oscillation breaks down (about 10 pF) I managed to set the
>> oscillator frequency offset to  +2.5 * 10-6 at room temperature. But
>> even this will not suffice when taking into account that the frequency
>> will drop by a few parts in 10-5 (cf. XSD manual) when the oven heats up
>> to its operating temperature. I also checked some of the components on
>> the oscillator PCB which might have an influence on frequency but could
>> not find any fault.
>>
>> The crystal itself is a disk of about 30 mm diameter mounted in a sealed
>> glass cylinder of about 38 mm dia. and 43 mm height. There is no socket
>> just 2 bare wires.. It does not show any visual signs of damage.
>> According to a reference given in the R&S XSD manual the crystal's
>> construction follows a publication from A.W. Warner "Design and
>> Performance of Ultraprecise 2.5-mc Quartz Crystal Units" in Vol 39,
>> Issue 5 of Bell Labs Technical Journal (Sept. 1960). According to that
>> it is an AT-cut 5th overtone design.
>>
>> Now my questions:
>>
>> a) Considering that this gear is about 50 years old, a "crystal gone
>> bad" situation shouldn't be that much of a surprise, right?  But could
>> there be any other cause of the "huge" frequency offset besides a bad
>> crystal? I would very much appreciate any idea that I could try to get
>> this baby back on spec.
>>
>> b) if nothing else helps: Could any of you give me a hint about who
>> might be able to supply a spare crystal? I tried my directly reachable
>> contacts but unfortunately to no avail so far. Please consider that
>> similar crystals might also have found their way into other
>> manufacturer's constructions from that era - Sulzer, Racal, HP ... you
>> name it ...
>>
>> Many thanks in advance!
>>
>> Best regards ... Michael U.
>> ___
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>
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Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

2016-11-21 Thread Hal Murray


p...@phk.freebsd.dk said:
> I think the installation manual for Trimbles timing products say you can use
> either 75 or 50 Ohm cable... 

I think they suggest using RG-6, the classic cable TV and/or satellite dish 
cable.  It's widely available at low cost.

The loss due to impedance mismatch is much less than the gain from the 
reduced attenuation when compared to equivalent size 50 ohm cable.


-- 
These are my opinions.  I hate spam.



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Re: [time-nuts] 53132 replacement fan

2016-11-21 Thread Charles Steinmetz


Adrian wrote:


This one's pretty quiet. But the airflow is a bit lower.


This one moves a little more air than the original (7.7 cfm) and is 
somewhat quieter than the original (by 2dB):




If you want to improve cooling, this one moves much more air (nearly 11 
cfm) with not too much more noise than the original (3.5dB):




Best regards,

Charles


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Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

On Mon, 21 Nov 2016 14:36:49 -0500
Bob Camp  wrote:

> The reflection issue ahead of the antenna is a reflection of the signal from 
> a single satellite. The multipath 
> reflection makes that satellite appear to be further away than it really is. 
> In the case that the reflected 
> signal  is *stronger* than the desired signal, the multipath reflection 
> “captures” the receiver and the net
> solution is messed up. 

Even a weaker reflected signal can cause significant change of the
correlation peak and thus of the apparent distance of the satellite.
The multipath error envolope diagrams are usually for multipath to
direct path ratios of between 1:2 to 1:10 (mostly depending on what
the author wants to show or how much he wants to cheat).

> In the case of a mismatched cable, there is no “single satellite” issue. 
> Everything is impacted by the mismatch. 
> Even if the mismatch is pretty bad, the “primary” wave is the one that will 
> dominate at the receiver end. The 
> reflections will always be lower in amplitude. That effectively guarantees 
> that you don’t have a multipath 
> issue from the coax. 

As above, weaker signals can still cause quite a bit of change in the
correlation peak, but in this case it will not matter because the
reflection acts the same on all signals. I.e. the net result is a small
time offset (but no position offset). Unfortunately, there is one big
assumption in here that does not hold true: for all signals to be affected
the same way by the reflection, the receiver must be exactly linear.
But we know that many of the components in the signal path of the receiver
are distinctly non-linear functions. So there is a slight change of the
position (and thus time) due to reflections in the cable. But, as we are
usually dealing with minute differences in impedance, the reflected signals
are heavily attenuated. Assuming we have a nominally 50Ω

-- 
Malek's Law:
Any simple idea will be worded in the most complicated way.
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Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?


Sorry... I pressed the wrong button while editing the Mail and cut short...

Continuing where I left off

> In the case of a mismatched cable, there is no “single satellite” issue. 
> Everything is impacted by the mismatch. 
> Even if the mismatch is pretty bad, the “primary” wave is the one that will 
> dominate at the receiver end. The 
> reflections will always be lower in amplitude. That effectively guarantees 
> that you don’t have a multipath 
> issue from the coax. 

As above, weaker signals can still cause quite a bit of change in the
correlation peak, but in this case it will not matter because the
reflection acts the same on all signals. I.e. the net result is a small
time offset (but no position offset). Unfortunately, there is one big
assumption in here that does not hold true: for all signals to be affected
the same way by the reflection, the receiver must be exactly linear.
But we know that many of the components in the signal path of the receiver
are distinctly non-linear functions. So there is a slight change of the
position (and thus time) due to reflections in the cable. But, as we are
usually dealing with minute differences in impedance, the reflected signals
are heavily attenuated. Assuming we have a nominally 50Ω system and have
a variation of +/-2Ω, then the worst case 48Ω vs 52Ω would give a reflection
loss of 27dB. Even assuming the back reflection has no loss (which isn't
true) would mean that the cable reflection "multipath" is almost 30dB dampend.
More likely to be something in the order of 54dB (2*27dB). Even with a 50Ω
to 75Ω impedance missmatch the reflection loss is 14dB and the multipath
should be in the order of 28dB. So wy below the signal and thus distortion
should be very much limited and receiver non-linearities should be negligible.

Attila Kinali


-- 
Malek's Law:
Any simple idea will be worded in the most complicated way.
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Re: [time-nuts] How phase stable is rg59 or alternate coax

2016-11-21 Thread Mark Spencer

At one point I contemplated running Andrews "Heliax" for my GPS antenna.   Part 
of the rationale was due to the data presented in page 2 of the following paper.

http://ivs.nict.go.jp/mirror/meetings/v2c_wm1/phase_stability.pdf

I subsequently decided to stay with my existing run of plenum rated RG58.  The 
bulk of my cable run is indoors where the temperature is fairly stable.

Regards
Mark Spencer



> On Nov 21, 2016, at 12:59 PM, Scott Stobbe  wrote:
> 
> When I first took a look at some of the coax datasheets I couldn't find
> anything. I was able to find the following paper "phase stability of
> typical navy radio frequency coaxial cables"
> http://www.dtic.mil/dtic/tr/fulltext/u2/628682.pdf I attached the table
> from the last page. They estimate RG59 to have a tempCo of -330 PPM/degC
> for electrical length. They also estimated RG-58 at -480 PPM/degC.
> 
>> On Mon, Nov 21, 2016 at 2:44 PM, John Ackermann N8UR  wrote:
>> 
>> I can't find the data right now, but will keep digging.  There's also a
>> short paper from the early 2000s from Haystack on their measurement of
>> LMR400 in an environmental chamber.  They came to the same conclusion, but
>> I can't find that paper either. :-
> 
> 
> John, many thanks for the Haystack tip! That is a wonderful paper, I
> believe the one you are quoting is "Dispersion and temperature effects in
> coax cables" http://www.haystack.mit.edu/tech/vlbi/mark5/mark5_memos/067.pdf
> 
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Re: [time-nuts] How can I measure GPS Antenna quality?

On Mon, 21 Nov 2016 08:22:50 -0800
jimlux  wrote:

> I'm not sure about whether an anechoic (which is really "hypoechoic") 
> chamber is going to get you the data you need.  Calibrating the chamber 
> to the needed level of accuracy might be harder than doing field 
> measurements.
[...]
>   sin(2 degrees) is 0.034, or -30dB.  So a spurious reflection that is 3 
> cm different path length (modulo wavelength) and 30 dB down will give 
> you a 1mm phase center error.  0.1 mm is -50dB.

Interesting. I haven't done the math, so I didn't think about that.
Yes, the reflections in the chamber would probably limit the resolution.

Now I wonder how the calibration data for mass produced geodetic antenna
are collected. I very much doubt they put them outside for a couple
of days to measure them exactly.

Attila Kinali

-- 
Malek's Law:
Any simple idea will be worded in the most complicated way.
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Re: [time-nuts] Question about AD9832 "I out Full Scale" (what does it mean?)

2016-11-21 Thread Richard (Rick) Karlquist


Well, here is response from ADI:

Quote:
If I remember correctly, this part implements a differential DAC current 
source, but only one of the outputs is bonded externally.
In this case, the current settle by Rset should  be divided by 2..., 
which explain your results.

I'll update the DS as soon as this has been confirmed.
Unquote...

This sounded reasonable when I first read it, but then I realized
that what he was talking about would explain a factor of 2 error
in output VOLTAGE, but not current.  So I think he confused himself.

It should be interesting to see if he can "confirm" it.

Stay tuned.

Rick

On 11/18/2016 1:27 AM, David G. McGaw wrote:

There is something wrong with the example.  The output is single-ended,
so using info from the AD9832 data sheet with Rset=3.9K and Rload=300
ohms as shown in the EVB schematic, it should go from 0 to 3.88mA and 0
to 1.16V.  Figure 12 shows only half this, including only about .3V DC
bias instead of .58V.

I apologize.  As a former Analog Devices applications engineer (Digital
Audio Group), I find this data sheet and user guide poorly written.
They have a lot of digital and almost no analog information.

David N1HAC


On 11/17/16 11:09 PM, Richard (Rick) Karlquist wrote:

Trying to figure out what "Iout Full Scale" means on the AD9832.
Some time nuts may have used this one.

On page 7 of this doc:

http://www.analog.com/media/en/technical-documentation/user-guides/UG-313.pdf


It shows the AD9832 output as 572 mV peak to peak
across 300 ohms.  This works out to 1.9 mA peak to
peak current through the resistor.  But Rset on the
board is 3.9K, which is supposed to give a value
for so-called "Iout Full Scale" of 3.878 mA.

I would have thought (just guessing) that peak to
peak output current would be equal to Iout Full
Scale, but it appears to be only half of that.

Can anyone clarify this?

Rick
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Re: [time-nuts] 53132 replacement fan

Hi

The bigger issue when you replace the fan:

Do the best job you possibly can cleaning out the power supply. Also check the 
soldering and the rest
of the workmanship on the power supply pc board. It’s the weak link in the 
counter. You don’t have to
give it much of a look to figure out HP bought the whole thing supply as a unit 
from “somebody else”. 
Whoever that vendor was … probably the low bidder.

Bob

> On Nov 21, 2016, at 4:45 PM, Tom Knox  wrote:
> 
> Hi All;
> 
> Mouser has more then 200 of the EFB0412MD OEM fans in stock. 10.93 in singles.
> 
> I have so many fans running in my lab I do not worry much about a little more 
> noise. But a noisy fan is a major concern because of overheating if the fan 
> fails, and although I usually go for an OEM parts in some cases with product 
> that generate a lot of heat (especially product where I have seen heat 
> contribute to failure) I will go with a more powerful fan.
> 
> Cheers;
> 
> Thomas Knox
> 1-303-554-0307
> tom.k...@nist.gov and act...@hotmail.com
> 
> 
> 
> From: time-nuts  on behalf of John Ackermann N8UR 
> 
> Sent: Monday, November 21, 2016 11:37 AM
> To: time-nuts@febo.com
> Subject: Re: [time-nuts] 53132 replacement fan
> 
> Thanks, all for the tips.  Glad to know it's a standard size so there's
> plenty of choice.
> 
> John
> 
> 
> On 11/21/2016 12:57 PM, jimlux wrote:
>> On 11/21/16 6:39 AM, Charles Steinmetz wrote:
>>> Tom wrote:
>>> 
 EFB0412MD
 Airflow 7.17 CFM
 6300 RPM
 Noise 24 dBA
 
 FBK04F12U
 Same exact form factor.
 Air Flow 9.2 CFM
 9500 RPM
 Noise 42 dB(A)
>>> 
>>> Note the 18dB greater noise (that's a HUGE difference).  Even with bad
>>> bearings in the original fan, it is probably considerably quieter (by
>>> 10dB or more) than the proposed replacement.  On the other hand, the
>>> replacement moves 28% more air, which may be a good thing.
>>> 
>> 
>> That's a 40mm fan, which is a standard size, I'll bet you can find a
>> slower turning/quieter fan.
>> 
>> ___
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> 
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Re: [time-nuts] Need for a document comparing time interval counters

2016-11-21 Thread Richard (Rick) Karlquist




On 11/21/2016 1:10 PM, Bob Camp wrote:

Hi

With both counters running on the same external standard (and no internal 
OCXO), the 53230
beats the 53132 both on frequency and time. It also has slightly better 
isolation of the 10 MHz
internals so the “dead zone” at 10 MHz is not quite as bad. I have no idea how 
either one
works with the OCXO option … I’ve never seen either one with an OCXO.

Bob



I remember when the 10 MHz section of the 53132 was designed.
The designers had little knowledge of how to do that correctly.
It wasn't their fault; it was the fault of the manager who
assigned the task to them.

Rick
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Re: [time-nuts] Need for a document comparing time interval counters

Hi

With both counters running on the same external standard (and no internal 
OCXO), the 53230
beats the 53132 both on frequency and time. It also has slightly better 
isolation of the 10 MHz
internals so the “dead zone” at 10 MHz is not quite as bad. I have no idea how 
either one 
works with the OCXO option … I’ve never seen either one with an OCXO. 

Bob

> On Nov 21, 2016, at 8:22 AM, Richard (Rick) Karlquist  
> wrote:
> 
> I had a 53230 the last few years I worked for Agilent.
> The oven oscillator in it is inferior to a 10811.
> Its only claim to fame IMHO is that it can measure
> Allan Deviation.  Turns out that we really needed
> Hadamard, but it doesn't do that.  It is very expensive.
> The old Santa Clara Division with the counter expertise
> has now been gone nearly 20 years.  The counter line
> has been offshored.  The expertise over there is
> a complete unknown.
> 
> Rick
> 
> On 11/19/2016 5:55 PM, Dr. David Kirkby wrote:
>> If anyone has the time & inclination, a document comparing different time 
>> interval counters would be useful to budding time nuts. There are quite a 
>> few different models at prices that are affordable to many.
>> 
>> I have had a HP 5370B and a Stanford Research  SR620, but have neither now. 
>> I regretted selling the HP, but then after buying the SR620, I swapped that 
>> and a 4.2 GHz HP signal generator for an HP 4291B 1.8 GHz impedance and 
>> material analyzer.
>> 
>> I was looking for something cheap,  but see a used Keysight 53230A on eBay 
>> for considerably more than a new one from Keysight. It had a "make offer" so 
>> I could not resist pointing out a new one is much less, and making a 
>> redicously low offer of $1000. I doubt it will be accepted,  but one never 
>> knows.  I once got a current Ketsight product for 2% of the current price,  
>> so anything is possible.
>> 
>> But a  comparison of TI counters,  and a discussion of the important 
>> specifications would be njce.
>> 
>> 
>> .Dave.
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Re: [time-nuts] How phase stable is rg59 or alternate coax

2016-11-21 Thread Scott Stobbe

When I first took a look at some of the coax datasheets I couldn't find
anything. I was able to find the following paper "phase stability of
typical navy radio frequency coaxial cables"
http://www.dtic.mil/dtic/tr/fulltext/u2/628682.pdf I attached the table
from the last page. They estimate RG59 to have a tempCo of -330 PPM/degC
for electrical length. They also estimated RG-58 at -480 PPM/degC.

On Mon, Nov 21, 2016 at 2:44 PM, John Ackermann N8UR  wrote:
>
> I can't find the data right now, but will keep digging.  There's also a
> short paper from the early 2000s from Haystack on their measurement of
> LMR400 in an environmental chamber.  They came to the same conclusion, but
> I can't find that paper either. :-

John, many thanks for the Haystack tip! That is a wonderful paper, I
believe the one you are quoting is "Dispersion and temperature effects in
coax cables" http://www.haystack.mit.edu/tech/vlbi/mark5/mark5_memos/067.pdf
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Re: [time-nuts] 53132 replacement fan

2016-11-21 Thread Tom Knox

Hi All;

Mouser has more then 200 of the EFB0412MD OEM fans in stock. 10.93 in singles.

I have so many fans running in my lab I do not worry much about a little more 
noise. But a noisy fan is a major concern because of overheating if the fan 
fails, and although I usually go for an OEM parts in some cases with product 
that generate a lot of heat (especially product where I have seen heat 
contribute to failure) I will go with a more powerful fan.

Cheers;

Thomas Knox
1-303-554-0307
tom.k...@nist.gov and act...@hotmail.com

From: time-nuts  on behalf of John Ackermann N8UR 

Sent: Monday, November 21, 2016 11:37 AM
To: time-nuts@febo.com
Subject: Re: [time-nuts] 53132 replacement fan

Thanks, all for the tips.  Glad to know it's a standard size so there's
plenty of choice.

John

On 11/21/2016 12:57 PM, jimlux wrote:
> On 11/21/16 6:39 AM, Charles Steinmetz wrote:
>> Tom wrote:
>>
>>> EFB0412MD
>>> Airflow 7.17 CFM
>>> 6300 RPM
>>> Noise 24 dBA
>>>
>>> FBK04F12U
>>> Same exact form factor.
>>> Air Flow 9.2 CFM
>>> 9500 RPM
>>> Noise 42 dB(A)
>>
>> Note the 18dB greater noise (that's a HUGE difference).  Even with bad
>> bearings in the original fan, it is probably considerably quieter (by
>> 10dB or more) than the proposed replacement.  On the other hand, the
>> replacement moves 28% more air, which may be a good thing.
>>
>
> That's a 40mm fan, which is a standard size, I'll bet you can find a
> slower turning/quieter fan.
>
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> and follow the instructions there.
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Re: [time-nuts] 53132 replacement fan

2016-11-21 Thread Adrian Godwin

This one's pretty quiet. But the airflow is a bit lower.

http://www.frozencpu.com/products/9367/fan-587/Sunon_MagLev-Vapo_40mm_x_20mm_Fan_w_TAC_Sensor_Wire_-_Bare_Wire_HA40201V4--C99.html?tl=c15s560b53&id=mrFX8Rvo

On Mon, Nov 21, 2016 at 5:57 PM, jimlux  wrote:

> On 11/21/16 6:39 AM, Charles Steinmetz wrote:
>
>> Tom wrote:
>>
>> EFB0412MD
>>> Airflow 7.17 CFM
>>> 6300 RPM
>>> Noise 24 dBA
>>>
>>> FBK04F12U
>>> Same exact form factor.
>>> Air Flow 9.2 CFM
>>> 9500 RPM
>>> Noise 42 dB(A)
>>>
>>
>> Note the 18dB greater noise (that's a HUGE difference).  Even with bad
>> bearings in the original fan, it is probably considerably quieter (by
>> 10dB or more) than the proposed replacement.  On the other hand, the
>> replacement moves 28% more air, which may be a good thing.
>>
>>
> That's a 40mm fan, which is a standard size, I'll bet you can find a
> slower turning/quieter fan.
>
>
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Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

2016-11-21 Thread EB4APL

I believe that reflections inside the cable (that is, after the antenna) 
are very different from reflections before the antenna. GPS receivers do 
their calculations based in the different arrival times of the 
satellites signals to the antenna center, so delays caused from 
different paths caused by reflections on external objects really 
matters. Reflections inside the cable, where phase relationships between 
the signals is already fixed, does not matter, or at least, not very much.


I don mean to disregard matching, which can cause big signal level 
losses if it is very bad.  BTW, GPSDO's like the popular Thunderbolt, 
are meant to use 75 Ohm cable and connectors but 50 Ohm cable can be 
used without much difference.   The PPS phase with respect to UTC, witch 
is affected by the cable delay, must be adjusted with the corresponding 
parameter. I think that a cable delay measurement could be more useful 
in this regard.


Anyway, I can be wrong and if it is the case, I would like comments from 
the experts here.


Best regards,

Ignacio, EB4APL


El 21/11/2016 a las 14:45, Dr. David Kirkby (Kirkby Microwave Ltd) escribió:

People state it is desirable to have a GPS antenna well clear of
obstructions, which I believe is to stop reflections. But there is another
source of reflections which I suspect could be just as problematic.

Whilst the input impedance of the antenna input terminal on a GPS receiver
is probably marked 50 Ohms, I'd be somewhat surprised if it was very close
to 50 Ohms. Antenna cables have an impedance, which is typically 50 +/- 2
Ohms, but this varies, not only between different makes/models of cables,
but even on the same real of cable.The output of the pre-amp is most
unlikely to have a 50 Ohm source impedance. In fact, the output impedance
might be close to 0 Ohms, as it may be driven by a voltage source, without
any 50 Ohm resistor.

Anything not immediately absorbed by the GPS receiver is going to be
reflected back up the coax, and could be reflected multiple times.

I just looked on my HP 8720D VNA, and see I can reduce the output power to
-70 dBm, which would should not do any damage. It will be interesting to
see just what the input impedance of the GPS receiver is. I'm tied up with
doing my accounts over the next few days, but later I will look.

If reflections on the antenna/cable/receiver are a problem, then
attenuators can improve the match, but of course they reduce the signal
level too. A more intelligent, but more difficult solution, is to build a
matching network. For that one would need a VNA to measure the impedance in
the first place.

Dr. David Kirkby Ph.D CEng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
UK.
Registered in England and Wales, company number 08914892.
http://www.kirkbymicrowave.co.uk/
Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)



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Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

2016-11-21 Thread Poul-Henning Kamp


In message 
, "Dr. 
David Kirkby (Kirkby Microwave Ltd)" writes:

>Whilst the input impedance of the antenna input terminal on a GPS receiver
>is probably marked 50 Ohms, I'd be somewhat surprised if it was very close
>to 50 Ohms. Antenna cables have an impedance, which is typically 50 +/- 2
>Ohms, but this varies, not only between different makes/models of cables,

I think the installation manual for Trimbles timing products say you
can use either 75 or 50 Ohm cable...


-- 
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] Do reflections up/down the antenna cable cause a problem with GPS?

2016-11-21 Thread EB4APL

I believe that reflections within the cable doesn't matter regarding  
the GPS measurements, unlike the reflections coming from outside the 
antenna. The measurements are made from the differences in the arrival 
times of the different satellite signals to the antenna and delays after 
that basically does not affect them. Well, the cable delay affects to 
the phase of the PPS but this is accounted for in a parameter.


Anyway, I don't mean that a good matching is something that can be 
totally disregarded and if the mismatch is very large it will affect to 
the signal lever at the receiver input but not to the measurement 
mechanism. BTW some GPSDO's, the Thunderbolt being an example, are 
supposed to be feed with 75 Ohm antenna cable with F connectors.


I may be be totally wrong, but in that case I'll appreciate comments 
from more knowledgeable people.


Best regards,

Ignacio, EB4APL



El 21/11/2016 a las 14:45, Dr. David Kirkby (Kirkby Microwave Ltd) escribió:

People state it is desirable to have a GPS antenna well clear of
obstructions, which I believe is to stop reflections. But there is another
source of reflections which I suspect could be just as problematic.

Whilst the input impedance of the antenna input terminal on a GPS receiver
is probably marked 50 Ohms, I'd be somewhat surprised if it was very close
to 50 Ohms. Antenna cables have an impedance, which is typically 50 +/- 2
Ohms, but this varies, not only between different makes/models of cables,
but even on the same real of cable.The output of the pre-amp is most
unlikely to have a 50 Ohm source impedance. In fact, the output impedance
might be close to 0 Ohms, as it may be driven by a voltage source, without
any 50 Ohm resistor.

Anything not immediately absorbed by the GPS receiver is going to be
reflected back up the coax, and could be reflected multiple times.

I just looked on my HP 8720D VNA, and see I can reduce the output power to
-70 dBm, which would should not do any damage. It will be interesting to
see just what the input impedance of the GPS receiver is. I'm tied up with
doing my accounts over the next few days, but later I will look.

If reflections on the antenna/cable/receiver are a problem, then
attenuators can improve the match, but of course they reduce the signal
level too. A more intelligent, but more difficult solution, is to build a
matching network. For that one would need a VNA to measure the impedance in
the first place.

Dr. David Kirkby Ph.D CEng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
UK.
Registered in England and Wales, company number 08914892.
http://www.kirkbymicrowave.co.uk/
Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)
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Re: [time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

Hi

Based on extensive testing of the line mismatch issue, the answer turns out to 
be “it does not matter”.

The reflection issue ahead of the antenna is a reflection of the signal from a 
single satellite. The multipath 
reflection makes that satellite appear to be further away than it really is. In 
the case that the reflected 
signal  is *stronger* than the desired signal, the multipath reflection 
“captures” the receiver and the net
solution is messed up. 

In the case of a mismatched cable, there is no “single satellite” issue. 
Everything is impacted by the mismatch. 
Even if the mismatch is pretty bad, the “primary” wave is the one that will 
dominate at the receiver end. The 
reflections will always be lower in amplitude. That effectively guarantees that 
you don’t have a multipath 
issue from the coax. 

Yes, there is more to it than this simple explanation. The conclusion is still 
correct. There is no significant impact switching 
coax from 50 ohms to 75 ohms and having both ends of the cable at an impedance 
that is not equal to the cable’s
characteristic impedance. 

Bob

> On Nov 21, 2016, at 8:45 AM, Dr. David Kirkby (Kirkby Microwave Ltd) 
>  wrote:
> 
> People state it is desirable to have a GPS antenna well clear of
> obstructions, which I believe is to stop reflections. But there is another
> source of reflections which I suspect could be just as problematic.
> 
> Whilst the input impedance of the antenna input terminal on a GPS receiver
> is probably marked 50 Ohms, I'd be somewhat surprised if it was very close
> to 50 Ohms. Antenna cables have an impedance, which is typically 50 +/- 2
> Ohms, but this varies, not only between different makes/models of cables,
> but even on the same real of cable.The output of the pre-amp is most
> unlikely to have a 50 Ohm source impedance. In fact, the output impedance
> might be close to 0 Ohms, as it may be driven by a voltage source, without
> any 50 Ohm resistor.
> 
> Anything not immediately absorbed by the GPS receiver is going to be
> reflected back up the coax, and could be reflected multiple times.
> 
> I just looked on my HP 8720D VNA, and see I can reduce the output power to
> -70 dBm, which would should not do any damage. It will be interesting to
> see just what the input impedance of the GPS receiver is. I'm tied up with
> doing my accounts over the next few days, but later I will look.
> 
> If reflections on the antenna/cable/receiver are a problem, then
> attenuators can improve the match, but of course they reduce the signal
> level too. A more intelligent, but more difficult solution, is to build a
> matching network. For that one would need a VNA to measure the impedance in
> the first place.
> 
> Dr. David Kirkby Ph.D CEng MIET
> Kirkby Microwave Ltd
> Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
> UK.
> Registered in England and Wales, company number 08914892.
> http://www.kirkbymicrowave.co.uk/
> Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)
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Re: [time-nuts] How phase stable is rg59 or alternate coax

2016-11-21 Thread John Ackermann N8UR

Several years ago I measured the delay of about 80 feet of LMR400 
feeding a GPS antenna, much of which was lying on a black shingle roof 
in the Georgia sun.  I checked in early afternoon when the sun was 
beating, and in the wee hours of the morning, to get the greatest 
temperature delta.  My recollection is that the tempco was surprisingly 
small -- maybe a couple of nanoseconds.  It was much less than the other 
elements of the GPS timing error budget.


I can't find the data right now, but will keep digging.  There's also a 
short paper from the early 2000s from Haystack on their measurement of 
LMR400 in an environmental chamber.  They came to the same conclusion, 
but I can't find that paper either. :-(


I don't know how much different RG58 results would be.

John


On 11/21/2016 09:38 AM, Scott Stobbe wrote:

If you had 30 ft of rg59 outdoors seeing maybe 10 degC swings everyday,
would the propagation time be stable to ps? ns?

On Sun, Nov 20, 2016 at 7:04 PM Hal Murray  wrote:



Is that even a sensible question?  Is there a better way to phrase it?


The problem I'm trying to avoid is that the weather and the satellite
geometry change over time so I can't just collect data for X hours, switch
to
the other antenna or move the antenna to another location, collect more
data,
then compare the two chunks of data.

The best I can think of would be to setup a reference system so I can
collect
data from  2 antennas and 2 receivers at the same time.  It would probably
require some preliminary work to calibrate the receivers.  I think I can do
that by swapping the antenna cables.


If I gave you a pile of data, how would you compute a quality number?  Can
I
just sum up the S/N slots for each visible/working satellite?


--
These are my opinions.  I hate spam.



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Re: [time-nuts] How can I measure GPS Antenna quality?

2016-11-21 Thread Tom Van Baak

> For the equipment hobbyists usually have, the phase center is not that
> important. Most antennas have a variation <5mm. Even 10mm would lead to
> just a ~33ps variation.

I agree. And besides, for those of us here in Oregon/Washington, the very 
ground is moving northwest at several inches per year (plate tectonics).

http://leapsecond.com/pages/quake/

/tvb
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Re: [time-nuts] 53132 replacement fan

2016-11-21 Thread Charles Steinmetz


Tom wrote:


EFB0412MD
Airflow 7.17 CFM
6300 RPM
Noise 24 dBA

FBK04F12U
Same exact form factor.
Air Flow 9.2 CFM
9500 RPM
Noise 42 dB(A)


Note the 18dB greater noise (that's a HUGE difference).  Even with bad 
bearings in the original fan, it is probably considerably quieter (by 
10dB or more) than the proposed replacement.  On the other hand, the 
replacement moves 28% more air, which may be a good thing.


Best regards,

Charles


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Re: [time-nuts] How phase stable is rg59 or alternate coax


On 11/21/16 6:38 AM, Scott Stobbe wrote:

If you had 30 ft of rg59 outdoors seeing maybe 10 degC swings everyday,
would the propagation time be stable to ps? ns?


Figure it's copper, so 16 ppm/deg C.  velocity factor is about 2/3, so 
30 ft is about 45 nanoseconds.  about 1ps/degree


Really, you also need to look at the effect on the propagation velocity 
of the radial expansion of the coax, too, and the change in epsilon.



You can look up a "phase vs frequency" curve for most coax which factors 
all of this in.





On Sun, Nov 20, 2016 at 7:04 PM Hal Murray  wrote:



Is that even a sensible question?  Is there a better way to phrase it?


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Re: [time-nuts] 53132 replacement fan

2016-11-21 Thread John Ackermann N8UR

Thanks, all for the tips.  Glad to know it's a standard size so there's 
plenty of choice.


John


On 11/21/2016 12:57 PM, jimlux wrote:

On 11/21/16 6:39 AM, Charles Steinmetz wrote:

Tom wrote:


EFB0412MD
Airflow 7.17 CFM
6300 RPM
Noise 24 dBA

FBK04F12U
Same exact form factor.
Air Flow 9.2 CFM
9500 RPM
Noise 42 dB(A)


Note the 18dB greater noise (that's a HUGE difference).  Even with bad
bearings in the original fan, it is probably considerably quieter (by
10dB or more) than the proposed replacement.  On the other hand, the
replacement moves 28% more air, which may be a good thing.



That's a 40mm fan, which is a standard size, I'll bet you can find a
slower turning/quieter fan.

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Re: [time-nuts] How can I measure GPS Antenna quality?

On 11/21/16 8:38 AM, Bob Camp wrote:

On Nov 21, 2016, at 9:54 AM, Attila Kinali wrote:

On Sun, 20 Nov 2016 14:13:58 -0800
Hal Murray wrote:

If I gave you a pile of data, how would you compute a quality number? Can I
just sum up the S/N slots for each visible/working satellite?

If “sum the S/N” gives you a difference you should immediately ask why.

Normal receiving antennas are a gain = directivity sort of beast. There is
not a lot you can do about that. For a GPS antenna, you want to be able to
receive over a hemisphere. You don’t really know in advance where the antenna
will be used, so that’s how it’s done.

Early on the designs had more gain straight up than at the horizon. That’s a
bad thing. If anything, you want more gain at the horizon.

the satellites have a pattern that is "edge weighted" so that the
received power on the ground is roughly constant into an isotropic
antenna - that is, extra gain when the satellite is on the horizon is
built into the transmit side of the link.

http://www.lockheedmartin.com/content/dam/lockheed/data/space/photo/gps/gpspubs/GPS%20Block%20IIR%20and%20IIR-M%20Antenna%20Panel%20Pattern,%20Marquis,%20Feb2014%20-%20publically%20releasable%20data.pdf

actually shows that the "peak" of the pattern is at around 45 degrees
elevation (from the user)

And slide 32 shows that the variation (on the newer satellites) is on
the order of 1-2 dB.

Signals are strong

from straight overhead (short path, less atmosphere) and weak(er) at the
horizon. They could easily give you a better “sum the S/N” number while
actually performing worse in a location with sat’s mostly overhead. A “straight
up”
antenna might be wonderful at a location on the equator. It’s probably a
disaster
at a location on the arctic circle.

That might be. Also, there's a difference between "optimum combining for
timing" vs "optimum timing for position/velocity". I don't know that
the geometry makes as much difference for timing, but it sure makes a
difference for position. That might actually be a rationale for higher
transmit antenna gain at 45 degree user elevation: those are the
satellites you want for a good fix: the one overhead isn't as useful.

A lot of GPS design decisions were based on use cases and requirements
from 30 or more years ago: where were users likely to be, how
sophisticated receivers were (e.g. a "pick the 4 strongest signals and
hope they give good DOP geometry), what kind of antennas were easy (quad
helix)

The real answer for signal to noise will always be location dependent. If I’m in
an urban canyon the only “sky” may be straight up. If I have a lot of
terrestrial
broadband noise close to the horizon, again straight up might be the answer.
If my antenna is on top of a pole and I have a clean view 360 degrees around and
down to < 5 degrees elevation, a straight up antenna is very much what I do
*not* want.

Even more complex: If I have a bunch of transmitters at a wide range of
frequencies running at the
same site as the GPS, I may want *really* good filtering ahead of the preamp.
Those
filters likely will have a temperature sensitivity.The filters create loss
ahead
of the preamp so the noise figure (and thus S/N) take a hit. I get something I
desperately need
and trade it off against degraded performance in other areas.

This is very much the case.. we had a high performance survey quality
antenna/preamp that in one location was unusable because of interference
from close by signals (cellphones, I think).

The receiver had very little front end filtering, presumably for the
stability reasons you mention above.

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Re: [time-nuts] How can I measure GPS Antenna quality?

2016-11-21 Thread David

When I was doing VHF and UHF direction finding antenna design, I would
drive out to the highest readily accessible hilltop for testing.  Once
I came up with a low sidelobe design, I started picking up things like
lamp posts, trees, and bushes in the parking lot, aircraft over LAX
and John Wayne airports 50+ miles away, etc. which limited testing
performance.

While a perfect test environment is handy for design, a GPS antenna is
going to be subject to all kinds of environmental limitations so I
would accept field testing which includes considerations like
multipath, temperature variation, and a generally hostile RF
environment.

On Mon, 21 Nov 2016 08:22:50 -0800, you wrote:

>I'm not sure about whether an anechoic (which is really "hypoechoic") 
>chamber is going to get you the data you need.  Calibrating the chamber 
>to the needed level of accuracy might be harder than doing field 
>measurements.
>
>It might just be because there's a ton of analysis software out there, 
>but the folks who really, really care about 0.1 mm shifts in phase 
>center seem to use field data in a well characterized site, and 
>accumulate it for a number of days.
>
>The GPS antenna folks at JPL, when they're testing a spacecraft antenna 
>for things like precision orbit determination (a basic choke ring sort 
>of thing) go out with the antenna and a test receiver on a cart in a 
>parking lot.
>
>Looking at it in terms of numbers:
>1mm is 1/150 wavelength, or about 2-3 degrees of phase.
>
>  sin(2 degrees) is 0.034, or -30dB.  So a spurious reflection that is 3 
>cm different path length (modulo wavelength) and 30 dB down will give 
>you a 1mm phase center error.  0.1 mm is -50dB.
>
>Now, it's true that if you had a good spherical near field range, with 
>time gating, you can probably get rid of the reflections from the 
>chamber (and, in fact, you can do the measurements in a regular lab, or 
>your garage). But even there, it's tricky, because the probe calibration 
>has to be very good, and the structure supporting the scanning probe 
>also has to be accounted for.  You might be able to do it by doing 
>transmit/receive measurements on something like a spherical target of 
>appropriate size.
>
>I've done measurements on what was essentially an interferometer with a 
>2 meter baseline, in a conventional chamber on a conventional pedestal 
>(JPL Mesa 60 ft chamber  http://mesa.jpl.nasa.gov/60_Foot_Chamber/). 
>You could easily see -40dB specular reflections as the array rotated. 
>(and you could also see things like the ladder on the positioner behind 
>the antenna we accidentally left in there, even though it was behind the 
>horn antennas in the array)
>
>I think a good test using satellites and a very well characterized 
>comparison antenna in a open air test site is probably the easiest, and 
>most accurate, way to do it.
>Arranging your test on a post well above the terrain, and making sure 
>that the surface is flat is easy.
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Re: [time-nuts] How phase stable is rg59 or alternate coax

2016-11-21 Thread Poul-Henning Kamp


In message 
, Scott 
Stobbe writes:

>If you had 30 ft of rg59 outdoors seeing maybe 10 degC swings everyday,
>would the propagation time be stable to ps? ns?

ps ?

No way - *ever*

ns ?

Probably, but it depends a lot on the exact materials and manufacturing.

-- 
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] 53132 replacement fan


On 11/21/16 6:39 AM, Charles Steinmetz wrote:

Tom wrote:


EFB0412MD
Airflow 7.17 CFM
6300 RPM
Noise 24 dBA

FBK04F12U
Same exact form factor.
Air Flow 9.2 CFM
9500 RPM
Noise 42 dB(A)


Note the 18dB greater noise (that's a HUGE difference).  Even with bad
bearings in the original fan, it is probably considerably quieter (by
10dB or more) than the proposed replacement.  On the other hand, the
replacement moves 28% more air, which may be a good thing.



That's a 40mm fan, which is a standard size, I'll bet you can find a 
slower turning/quieter fan.


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Re: [time-nuts] How can I measure GPS Antenna quality?

2016-11-21 Thread Dr. David Kirkby (Kirkby Microwave Ltd)

Hi

> On Nov 21, 2016, at 9:54 AM, Attila Kinali  wrote:
> 
> On Sun, 20 Nov 2016 14:13:58 -0800
> Hal Murray  wrote:
> 
>> If I gave you a pile of data, how would you compute a quality number?  Can I 
>> just sum up the S/N slots for each visible/working satellite?

If “sum the S/N” gives you a difference you should immediately ask why. 

Normal receiving antennas are a gain = directivity sort of beast. There is 
not a lot you can do about that. For a GPS antenna, you want to be able to 
receive over a hemisphere. You don’t really know in advance where the antenna
will be used, so that’s how it’s done. 

Early on the designs had more gain straight up than at the horizon. That’s a 
bad thing. If anything, you want more gain at the horizon. Signals are strong 
from straight overhead (short path, less atmosphere) and weak(er) at the 
horizon. They could easily give you a better “sum the S/N” number while
actually performing worse in a location with sat’s mostly overhead. A “straight 
up”
antenna might be wonderful at a location on the equator. It’s probably a 
disaster
at a location on the arctic circle. 

The real answer for signal to noise will always be location dependent. If I’m 
in 
an urban canyon the only “sky” may be straight up. If I have a lot of 
terrestrial 
broadband noise close to the horizon, again straight up might be the answer. 
If my antenna is on top of a pole and I have a clean view 360 degrees around 
and 
down to < 5 degrees elevation, a straight up antenna is very much what I do 
*not* want. 

Even more complex: If I have a bunch of transmitters at a wide range of 
frequencies running at the 
same site as the GPS, I may want *really* good filtering ahead of the preamp. 
Those
filters likely will have a temperature sensitivity.The filters create  loss 
ahead
of the preamp so the noise figure (and thus S/N) take a hit. I get something I 
desperately need 
and trade it off against degraded performance in other areas. 

Lots of variables. 

Bob

> 
> There are multiple issues. As already mentioned, SNR is only a part of the
> picture. What you are looking for is an uniform gain pattern over most
> of the hemisphere, with a sharp decrease at low elevations. Then the
> left vs right polarization ratio should be as high as possible over
> the whole hemisphere (most antennas only have good polarization ratio
> at the zenith and behave like a linear polarized antenna at low elevations).
> 
> Additionally to this comes the phase center stability. Ie that the phase
> center is a fixed location, independent of azimuth and elevation. And this
> is probably the hardest to measure.
> 
> Absolute (and probably the most precise) measures of these properties are
> done in an anechoic chambers with a rotating antenna mount. 
> 
> The second way to do it, is to use a "known good" reference antenna and
> use this as a comparison with a short (3-15m) baseline between reference
> and antenna under test. For additional fancyness and to get better results
> one can add the antenna onto robotic arm (like on the picture in [1]) and
> get a more complete picture of the antenna. In this setup you want to have
> an as fancy receiver as possible. At the minimum it needs to support carrier
> phase data. The better receivers allow you to connect two antennas to the
> same receiver and do a direct phase/amplitude comparison of the signals.
> 
> For the equipment hobbyists usually have, the phase center is not that
> important. Most antennas have a variation <5mm. Even 10mm would lead to
> just a ~33ps variation. Ie for the normal GPSDO that has a loop time constant
> in the 100s to 1000s seconds and is using "normal" receivers, this is
> completely drowned in the noise of the receiver's PPS output. Having good
> sky view and as little multipath as possible is much more important.
> 
> 
> 
>   Attila Kinali
> 
> [1] https://www.ife.uni-hannover.de/aoa-dm-t_absolute.html
> 
> -- 
> It is upon moral qualities that a society is ultimately founded. All 
> the prosperity and technological sophistication in the world is of no 
> use without that foundation.
> -- Miss Matheson, The Diamond Age, Neil Stephenson
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Re: [time-nuts] Inductor core material aging (was: Rohde & Schwarz XSD 2.5 MHz crystal gone bad?)

2016-11-21 Thread Graham / KE9H

The core of the inductors are commonly powdered iron (of various alloys),
held together with a plastic binder.
The early plastics shrank and changed shape with time, changing the
inductance over decades.
Later binders are much more stable.
Not much you can do about it, other than change the inductors.

--- Graham

==

On Mon, Nov 21, 2016 at 10:14 AM, Bob Camp  wrote:

> Hi
>
> > On Nov 21, 2016, at 9:23 AM, Attila Kinali  wrote:
> >
> > Hoi Bob,
> >
> > On Sun, 20 Nov 2016 22:21:29 -0500
> > Bob Camp  wrote:
> >
> >> 50 year old inductors may have been made with core materials
> >> that aged more than just a little bit. I have empirical data on this :)
> >
> > Do you have that data available somewhere? I would be very much
> > interested in it.
> >
>
> We had a few thousand oscillators that had to be replaced due to coil
> issues
> in the 1970’s. We were not the only ones with the problem. Other outfits
> had very
> similar experiences. Eventually the coil guys took a look at the materials
> and
> “changed something”. After that the coils did not have as much aging when
> run at 100C forever and ever.
>
> Bob
>
> >   Attila Kinali
> >
> > --
> > It is upon moral qualities that a society is ultimately founded. All
> > the prosperity and technological sophistication in the world is of no
> > use without that foundation.
> > -- Miss Matheson, The Diamond Age, Neil Stephenson
> > ___
> > time-nuts mailing list -- time-nuts@febo.com
> > 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] How phase stable is rg59 or alternate coax

2016-11-21 Thread Scott Stobbe

If you had 30 ft of rg59 outdoors seeing maybe 10 degC swings everyday,
would the propagation time be stable to ps? ns?

On Sun, Nov 20, 2016 at 7:04 PM Hal Murray  wrote:

>
> Is that even a sensible question?  Is there a better way to phrase it?
>
>
> The problem I'm trying to avoid is that the weather and the satellite
> geometry change over time so I can't just collect data for X hours, switch
> to
> the other antenna or move the antenna to another location, collect more
> data,
> then compare the two chunks of data.
>
> The best I can think of would be to setup a reference system so I can
> collect
> data from  2 antennas and 2 receivers at the same time.  It would probably
> require some preliminary work to calibrate the receivers.  I think I can do
> that by swapping the antenna cables.
>
>
> If I gave you a pile of data, how would you compute a quality number?  Can
> I
> just sum up the S/N slots for each visible/working satellite?
>
>
> --
> These are my opinions.  I hate spam.
>
>
>
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[time-nuts] Do reflections up/down the antenna cable cause a problem with GPS?

People state it is desirable to have a GPS antenna well clear of
obstructions, which I believe is to stop reflections. But there is another
source of reflections which I suspect could be just as problematic.

Whilst the input impedance of the antenna input terminal on a GPS receiver
is probably marked 50 Ohms, I'd be somewhat surprised if it was very close
to 50 Ohms. Antenna cables have an impedance, which is typically 50 +/- 2
Ohms, but this varies, not only between different makes/models of cables,
but even on the same real of cable.The output of the pre-amp is most
unlikely to have a 50 Ohm source impedance. In fact, the output impedance
might be close to 0 Ohms, as it may be driven by a voltage source, without
any 50 Ohm resistor.

Anything not immediately absorbed by the GPS receiver is going to be
reflected back up the coax, and could be reflected multiple times.

I just looked on my HP 8720D VNA, and see I can reduce the output power to
-70 dBm, which would should not do any damage. It will be interesting to
see just what the input impedance of the GPS receiver is. I'm tied up with
doing my accounts over the next few days, but later I will look.

If reflections on the antenna/cable/receiver are a problem, then
attenuators can improve the match, but of course they reduce the signal
level too. A more intelligent, but more difficult solution, is to build a
matching network. For that one would need a VNA to measure the impedance in
the first place.

Dr. David Kirkby Ph.D CEng MIET
Kirkby Microwave Ltd
Registered office: Stokes Hall Lodge, Burnham Rd, Althorne, Essex, CM3 6DT,
UK.
Registered in England and Wales, company number 08914892.
http://www.kirkbymicrowave.co.uk/
Tel: 07910 441670 / +44 7910 441670 (0900 to 2100 GMT only please)
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Re: [time-nuts] Inductor core material aging (was: Rohde & Schwarz XSD 2.5 MHz crystal gone bad?)

Hi

> On Nov 21, 2016, at 9:23 AM, Attila Kinali  wrote:
> 
> Hoi Bob,
> 
> On Sun, 20 Nov 2016 22:21:29 -0500
> Bob Camp  wrote:
> 
>> 50 year old inductors may have been made with core materials 
>> that aged more than just a little bit. I have empirical data on this :)
> 
> Do you have that data available somewhere? I would be very much
> interested in it.
> 

We had a few thousand oscillators that had to be replaced due to coil issues
in the 1970’s. We were not the only ones with the problem. Other outfits had 
very
similar experiences. Eventually the coil guys took a look at the materials and 
“changed something”. After that the coils did not have as much aging when 
run at 100C forever and ever. 

Bob

>   Attila Kinali
> 
> -- 
> It is upon moral qualities that a society is ultimately founded. All 
> the prosperity and technological sophistication in the world is of no 
> use without that foundation.
> -- Miss Matheson, The Diamond Age, Neil Stephenson
> ___
> time-nuts mailing list -- time-nuts@febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
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Re: [time-nuts] How can I measure GPS Antenna quality?


On 11/21/16 6:54 AM, Attila Kinali wrote:

On Sun, 20 Nov 2016 14:13:58 -0800
Hal Murray  wrote:


If I gave you a pile of data, how would you compute a quality number?  Can I
just sum up the S/N slots for each visible/working satellite?


There are multiple issues. As already mentioned, SNR is only a part of the
picture. What you are looking for is an uniform gain pattern over most
of the hemisphere, with a sharp decrease at low elevations. Then the
left vs right polarization ratio should be as high as possible over
the whole hemisphere (most antennas only have good polarization ratio
at the zenith and behave like a linear polarized antenna at low elevations).

Additionally to this comes the phase center stability. Ie that the phase
center is a fixed location, independent of azimuth and elevation. And this
is probably the hardest to measure.

Absolute (and probably the most precise) measures of these properties are
done in an anechoic chambers with a rotating antenna mount.



I'm not sure about whether an anechoic (which is really "hypoechoic") 
chamber is going to get you the data you need.  Calibrating the chamber 
to the needed level of accuracy might be harder than doing field 
measurements.


It might just be because there's a ton of analysis software out there, 
but the folks who really, really care about 0.1 mm shifts in phase 
center seem to use field data in a well characterized site, and 
accumulate it for a number of days.


The GPS antenna folks at JPL, when they're testing a spacecraft antenna 
for things like precision orbit determination (a basic choke ring sort 
of thing) go out with the antenna and a test receiver on a cart in a 
parking lot.



Looking at it in terms of numbers:
1mm is 1/150 wavelength, or about 2-3 degrees of phase.

 sin(2 degrees) is 0.034, or -30dB.  So a spurious reflection that is 3 
cm different path length (modulo wavelength) and 30 dB down will give 
you a 1mm phase center error.  0.1 mm is -50dB.



Now, it's true that if you had a good spherical near field range, with 
time gating, you can probably get rid of the reflections from the 
chamber (and, in fact, you can do the measurements in a regular lab, or 
your garage). But even there, it's tricky, because the probe calibration 
has to be very good, and the structure supporting the scanning probe 
also has to be accounted for.  You might be able to do it by doing 
transmit/receive measurements on something like a spherical target of 
appropriate size.


I've done measurements on what was essentially an interferometer with a 
2 meter baseline, in a conventional chamber on a conventional pedestal 
(JPL Mesa 60 ft chamber  http://mesa.jpl.nasa.gov/60_Foot_Chamber/). 
You could easily see -40dB specular reflections as the array rotated. 
(and you could also see things like the ladder on the positioner behind 
the antenna we accidentally left in there, even though it was behind the 
horn antennas in the array)



I think a good test using satellites and a very well characterized 
comparison antenna in a open air test site is probably the easiest, and 
most accurate, way to do it.
Arranging your test on a post well above the terrain, and making sure 
that the surface is flat is easy.






The second way to do it, is to use a "known good" reference antenna and
use this as a comparison with a short (3-15m) baseline between reference
and antenna under test. For additional fancyness and to get better results
one can add the antenna onto robotic arm (like on the picture in [1]) and
get a more complete picture of the antenna. In this setup you want to have
an as fancy receiver as possible. At the minimum it needs to support carrier
phase data. The better receivers allow you to connect two antennas to the
same receiver and do a direct phase/amplitude comparison of the signals.

For the equipment hobbyists usually have, the phase center is not that
important. Most antennas have a variation <5mm. Even 10mm would lead to
just a ~33ps variation. Ie for the normal GPSDO that has a loop time constant
in the 100s to 1000s seconds and is using "normal" receivers, this is
completely drowned in the noise of the receiver's PPS output. Having good
sky view and as little multipath as possible is much more important.



Attila Kinali

[1] https://www.ife.uni-hannover.de/aoa-dm-t_absolute.html



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Re: [time-nuts] 53132 replacement fan

2016-11-21 Thread Max


On 21-Nov-16 6:14 AM, John Ackermann N8UR wrote:
Does anyone have a part number for the 53132 fan (or equivalent)?  
Mine is getting pretty noisy.


Thanks!
John
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Just replaced the fan in my 53131 and I believe that its the same.  You 
can use any 40mm square by 20mm thick
fan . Any current draw over 500ma means that it is grunty enough for the 
job.
You need to carefully remove the power supply assembly after removing 
the 3 scews.  Easy.


regards


---
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https://www.avast.com/antivirus

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Re: [time-nuts] How can I measure GPS Antenna quality?


On 11/21/16 4:15 AM, Bob Camp wrote:

Hi

At spectrum analyzer bandwidths, the GPS signals out of the antenna are
more than 20 db below the noise floor. You can’t see them with an analyzer.
You need to run things into the equivalent of a receiver to turn it into 
anything
you can see above the noise.

What you will see on an analyzer is the noise out of the preamp. That will at
least tell you that the output stage on the amp is working. It will tell you 
very
little about the input stage to the amp and very little about the antenna on
the other side of the preamp.

One thing you want to know about a GPS antenna system is the
stability of it’s phase center as things change (like sat angles). If the phase
center moves, your solutions change between satellite readings. Another
thing you want to know is how it rejects multipath. Neither one is easy
to measure in a direct way.



The GPS folks do things like look at the correlation peak vs timing 
offset - you need the raw bits to do this, and you run a cross 
correlation against the spreading code.  A "good" antenna and location 
should show a nice triangular peak.  A "bad" antenna and/or location 
will show multiple peaks (corresponding to the multiple paths).


I think it would be fairly easy to collect the data - there's several 
"filter+threshold" widgets out there that could generate your data 
stream.  Then, doing the correlation just requires some fairly simple 
software: matlab could do it in a few lines, once you have some code (or 
a file) with the spreading sequence of interest.


Our receivers at JPL sample at 38.something MHz, so that the GPS 
frequency aliases to somewhat to the side of zero (so that even with max 
negative doppler, the signal is still positive frequency)






A more formal method of testing would be to use a proper antenna test setup.
Those normally are indoor systems that rotate things to test the antenna. Even
there, the systems are only so good.


I haven't looked at whether a near field range could do it, but most 
indoor regular ranges don't have good enough multipath suppression for 
this kind of thing. The absorber on the walls is probably good to 30-40 
dB, but even then, there will be "something" that gives you a noticeable 
reflection.



Another proposed solution is to run the antenna

in the real world on a robotic arm and rotate it while in use. Again there are 
limitations
to the process.



re: robotic arm

Or the way they do it for geodetic antennas - a grad student goes out 
and manually rotates the antenna to a new orientation.


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Re: [time-nuts] How can I measure GPS Antenna quality?

On Sun, 20 Nov 2016 14:13:58 -0800
Hal Murray  wrote:

> If I gave you a pile of data, how would you compute a quality number?  Can I 
> just sum up the S/N slots for each visible/working satellite?

There are multiple issues. As already mentioned, SNR is only a part of the
picture. What you are looking for is an uniform gain pattern over most
of the hemisphere, with a sharp decrease at low elevations. Then the
left vs right polarization ratio should be as high as possible over
the whole hemisphere (most antennas only have good polarization ratio
at the zenith and behave like a linear polarized antenna at low elevations).

Additionally to this comes the phase center stability. Ie that the phase
center is a fixed location, independent of azimuth and elevation. And this
is probably the hardest to measure.

Absolute (and probably the most precise) measures of these properties are
done in an anechoic chambers with a rotating antenna mount. 

The second way to do it, is to use a "known good" reference antenna and
use this as a comparison with a short (3-15m) baseline between reference
and antenna under test. For additional fancyness and to get better results
one can add the antenna onto robotic arm (like on the picture in [1]) and
get a more complete picture of the antenna. In this setup you want to have
an as fancy receiver as possible. At the minimum it needs to support carrier
phase data. The better receivers allow you to connect two antennas to the
same receiver and do a direct phase/amplitude comparison of the signals.

For the equipment hobbyists usually have, the phase center is not that
important. Most antennas have a variation <5mm. Even 10mm would lead to
just a ~33ps variation. Ie for the normal GPSDO that has a loop time constant
in the 100s to 1000s seconds and is using "normal" receivers, this is
completely drowned in the noise of the receiver's PPS output. Having good
sky view and as little multipath as possible is much more important.

Attila Kinali

[1] https://www.ife.uni-hannover.de/aoa-dm-t_absolute.html

-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
 -- Miss Matheson, The Diamond Age, Neil Stephenson
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[time-nuts] Inductor core material aging (was: Rohde & Schwarz XSD 2.5 MHz crystal gone bad?)

Hoi Bob,

On Sun, 20 Nov 2016 22:21:29 -0500
Bob Camp  wrote:

> 50 year old inductors may have been made with core materials 
> that aged more than just a little bit. I have empirical data on this :)

Do you have that data available somewhere? I would be very much
interested in it.

Attila Kinali

-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
 -- Miss Matheson, The Diamond Age, Neil Stephenson
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Re: [time-nuts] How can I measure GPS Antenna quality?

2016-11-21 Thread Poul-Henning Kamp

In message <2974f356-3729-448e-a428-dc4d340ff...@n1k.org>, Bob Camp writes:

>If I put up a handful of antennas on the back porch, I can indeed hook them
>up to various receivers and cables.

I actually had a chance to do that once:

http://phk.freebsd.dk/raga/

I was very careful to space the antennas exactly 300mm apart, but that
didn't inspire them to align in any way.

In all fairness, those were "timing" receivers so nobody cared where
their phase-center was during production...

-- 
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] Need for a document comparing time interval counters

2016-11-21 Thread Richard (Rick) Karlquist


I had a 53230 the last few years I worked for Agilent.
The oven oscillator in it is inferior to a 10811.
Its only claim to fame IMHO is that it can measure
Allan Deviation.  Turns out that we really needed
Hadamard, but it doesn't do that.  It is very expensive.
The old Santa Clara Division with the counter expertise
has now been gone nearly 20 years.  The counter line
has been offshored.  The expertise over there is
a complete unknown.

Rick

On 11/19/2016 5:55 PM, Dr. David Kirkby wrote:

If anyone has the time & inclination, a document comparing different time 
interval counters would be useful to budding time nuts. There are quite a few 
different models at prices that are affordable to many.

I have had a HP 5370B and a Stanford Research  SR620, but have neither now. I 
regretted selling the HP, but then after buying the SR620, I swapped that and a 
4.2 GHz HP signal generator for an HP 4291B 1.8 GHz impedance and material 
analyzer.

I was looking for something cheap,  but see a used Keysight 53230A on eBay for 
considerably more than a new one from Keysight. It had a "make offer" so I 
could not resist pointing out a new one is much less, and making a redicously low offer 
of $1000. I doubt it will be accepted,  but one never knows.  I once got a current 
Ketsight product for 2% of the current price,  so anything is possible.

But a  comparison of TI counters,  and a discussion of the important 
specifications would be njce.


.Dave.
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Re: [time-nuts] How can I measure GPS Antenna quality?


On 11/20/16 7:41 PM, Mark Sims wrote:

When I was developing the precision survey code for Lady Heather,  I
used a lot of antennas.  My definition of antenna quality boiled down
to how well the results of a 48 hour survey compared to the cm level
survey point that I had for my antenna position (from a Ashtech Z12
receiver / matched choke ring antenna).



This is similar to the UNAVCO evaluaton approach
http://kb.unavco.org/kb/article/unavco-resources-gnss-antennas-458.html

"Antenna phase center variations can be characterized by mean phase 
center offsets and by phase and amplitude patterns for L1, L2 and L3 
(ionosphere free combination) tracking as a function of azimuth and 
angle. Mean offsets are defined as the average phase center locations 
relative to a physical reference point on the antennas (typically the 
base of the antenna preamplifier as used in RINEX files). The patterns 
are defined as the azimuth and elevation dependence to be added to the 
average phase center offsets. The sum of the mean phase offset and 
pattern gives the signal path delay for a given satellite elevation and 
azimuth. Precise knowledge of these phase patterns is essential for 
mixing antennas of different design where uncorrected effects can be as 
large as 10 cm in the vertical and 1 cm in the horizontal baseline 
components. The effect is more subtle for antennas of the same design. 
Here the problems arise over long baselines where the same satellite is 
observed at different relative directions and therefore experiences 
different delays at each site which can introduce solution scale errors. 
In addition, there is the issue of consistency of the phase patterns and 
offsets for each individual antenna of the same design."



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Re: [time-nuts] How can I measure GPS Antenna quality?