Re: [time-nuts] Noise Floor

[John Miles]

> Is there a _done_ solution to expand a Timepod to
> ~ 100 MHz, narrow-bandish is OK. I'm willing to solder,
> but I'm not ready for yet another development project.
> Not now.

A pair of double-balanced mixers driven by independent signal generators works 
well.  The mixer RF inputs are driven by a splitter, while the IF outputs go to 
channels 0 and 2.  Add gain/loss as needed.  

Independent bandpass filters at each mixer RF port can be helpful, since 
common-mode noise on the unwanted sideband can cause artifacts.  It will 
usually (but not always) be fairly obvious when you have problems with this.  

> When I've flushed my project pipeline, I might consider
> something serious with JESD-204B ADCs and such.
> That would require a larger FPGA with GTX transceivers
> and maybe a Beaglebone as a controller.

Some of the newer parts are pretty tempting, but note that the need for 
wideband acquisition is driven by offset frequency range rather than carrier 
frequency range.  Most noise measurement applications won't need 100 MHz-1 GHz 
of acquisition bandwidth, unlike the wireless base stations and high-end signal 
analyzers that the ADC manufacturers are targeting. 

-- john



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Re: [time-nuts] Noise Floor

[Bob kb8tq]

Hi

> On Mar 24, 2020, at 7:31 PM, jimlux  wrote:
> 
> On 3/24/20 2:17 PM, John Miles wrote:
 It would be interesting to know what ADC was used and if there's an
 SDR-board out there that uses the same ADC.
>>> 
>>> Uh.. I remember John telling me what ADC it was, but I forgot, sorry.
>> It uses four AD9265s.  The TimePod used four LTC2216s, but the AD9265s
>> support higher clock rates with less power consumption, and both of those
>> attributes were important this time around.
> 
> And they work in space, should one care about that.. They also have nice low 
> clock input noise.
> (i.e. the SNR isn't too degraded from the ideal, compared to some other fast 
> ADCs).
> 
> 
>>> I am pretty sure I could design something like the PhaseStation as well.
>>> The working principle is easy and can be explained on a napkin in 5
>> minutes.
>>> But getting it to this remarkable perfomrance? Not without a lot of
>>> trial and error. And even then, I wouldn't be sure.
>> I don't think there's any way to avoid the trial-and-error part unless you
>> have the luxury of an unlimited ceiling for both the R&D budget and the
>> target retail price, and maybe not even then.  One reason it took longer
>> than expected to ship the 53100A was that a lot of lessons that I *thought*
>> had been adequately learned on the 5330A/3120A project didn't pay off when
>> different ADCs were used, and when the carrier and offset frequency
>> requirements grew by 6x and 10x respectively.
>> With the TimePod, for example, noise and spur performance weren't strongly
>> influenced by ADC clock distribution.  On PhaseStation, that particular
>> "unlearned lesson" cost me a respin.
> 
> That is a real lesson - It's one that people doing their first low noise 
> systems get burned by (do NOT run your clock signal from your quiet 
> oscillator through the FPGA, and yes, that fancy fast clock driver might 
> actually degrade performance because it has a 1GHz BW and so does the clock 
> input on the ADC...)
> 
> And then, getting the data reliably out of the ADC into the FPGA, and 
> synchronized across multiple channels. That particular part doesn't guarantee 
> the startup state of the internal pipeline.
> 
> 
> 
>> Keeping parts and manufacturing costs under control was also more difficult
>> than anticipated.  Another lesson that wasn't learned soon enough was that a
>> design with four or five internal PCBs ends up being much more expensive
>> than one that uses only two, even if the total board area is similar.  We
>> had to increase the price twice to maintain standard T&M industry margins,
>> and (having just come back from visiting Said and Giovanni at Jackson Labs)
>> that's about to happen again.  The original vision of a four-figure price
>> tag was unrealistic, and that's definitely a lesson for next time.
> 
> is that because of connectors and interfaces?
> Or because of "unit test" time and process?
> 
> I know that for space stuff, all on one board or in one box is cheaper than 
> breaking it up, particularly for RF systems. Because if it's individual 
> widgets, everyone wants individual test data at the widget level, before you 
> combine them into superwidget assemblies.


If you are building an oscillator, you run into the same thing. Part of the 
issue
is simply that each board has some sort of testing / minimum charge. Split 
the board into quarters and the price goes up significantly. As soon as you 
set it up for assembly, you hit the same sort of cost issues. It then goes into
some sort of test process … cost goes up yet again ….

Bob

> 
> 
> 
>> -- john, KE5FX
>> Miles Design LLC / Jackson Labs Technologies, Inc.
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Re: [time-nuts] Noise Floor

[John Miles]

> And then, getting the data reliably out of the ADC into the FPGA, and
> synchronized across multiple channels. That particular part doesn't
> guarantee the startup state of the internal pipeline.

There's a sync pin for use with the clock divider, but AFAIK it does nothing
when the divider isn't used (which it isn't).  Fortunately, fixed delay
between channels isn't a big concern, as long as it stays put.  With a 1:1
clock, the time from clock in to DCO out is always consistent.  

> is that because of connectors and interfaces?
> Or because of "unit test" time and process?

Mostly the former, as we test the system as a whole.  Multiple boards cost
more at the PCB fab and cost more to stuff, and of course the interconnects
are both an expense on the BOM and a liability during assembly. The front
panel and input PCB alone have two dozen SMA jacks, so assembly training and
documentation isn't a trivial matter.

Then there's the additional mounting hardware and assembly steps associated
with multiple boards.  And while we test at the system level as noted, the
test script has to do its best to narrow down failures at the subassembly
level, rather than simply reporting go/no-go conditions.  Lots of little
things that add up to real costs. 
 
> I know that for space stuff, all on one board or in one box is cheaper
> than breaking it up, particularly for RF systems. Because if it's
> individual widgets, everyone wants individual test data at the widget
> level, before you combine them into superwidget assemblies.

Yes, and I'm sure the cost of the interconnects and mounting hardware is
insane in that business!

-- john



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Re: [time-nuts] Mains frequency data for Europe

[Azelio Boriani]

The entsoe.eu site is full of 404... the only site I can find with
frequency data is this (for the Belgian grid):


What happened to the grid frequency in Europe in Jan 2019 and Apr 2019:



On Wed, Mar 25, 2020 at 12:52 AM Pieter-Tjerk de Boer
 wrote:
>
> Hello Attila,
>
> In principle I have that data (I've been logging the grid's phase since 2008),
> but I'll need to do a bit of work to extract it to a usable format.
> Please contact me off-list.
>
> Regards,
>   Pieter-Tjerk
>
>
> On Tue, Mar 24, 2020 at 04:30:25PM +0100, Attila Kinali wrote:
> > Hi,
> >
> > Does someone here have recordings of the mains frequency of the west
> > European Grid from the last weeks and from the same time last year?
> > There is something I've been told which I would like to fact-check.
> > I wasn't able to find anything online.
> >
> > Thanks in advance
> >
> >   Attila Kinali
> > --
> > Science is made up of so many things that appear obvious
> > after they are explained. -- Pardot Kynes
> >
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> > and follow the instructions there.
>
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[time-nuts] Symmetricom analog network clocks.

[djl]

Some years ago, symmetricom sold out some analog network clocks. Last 
winter, both of mine froze up and would not adjust. Being naturally 
curious, I opened one up, and found the motor and wheels that drive the 
hands. Knowing that this type of motor has problems, I simply unplugged 
it and manually forced the going train and the motor on each clock to 
turn in both directions(use your thumb.) After all, it was no good as 
is. I guess the grease was redistributed, as both clocks now function 
properly. They do have to be set using the software sort of often, but 
sure look classy :-). No wonder we got them so cheap!

73, Don

--
Dr. Don Latham  AJ7LL
PO Box 404, Frenchtown, MT, 59834
VOX: 406-626-4304


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Re: [time-nuts] Noise Floor

[jimlux]

On 3/24/20 2:17 PM, John Miles wrote:

It would be interesting to know what ADC was used and if there's an
SDR-board out there that uses the same ADC.


Uh.. I remember John telling me what ADC it was, but I forgot, sorry.


It uses four AD9265s.  The TimePod used four LTC2216s, but the AD9265s
support higher clock rates with less power consumption, and both of those
attributes were important this time around.



And they work in space, should one care about that.. They also have nice 
low clock input noise.
(i.e. the SNR isn't too degraded from the ideal, compared to some other 
fast ADCs).




I am pretty sure I could design something like the PhaseStation as well.
The working principle is easy and can be explained on a napkin in 5

minutes.

But getting it to this remarkable perfomrance? Not without a lot of
trial and error. And even then, I wouldn't be sure.


I don't think there's any way to avoid the trial-and-error part unless you
have the luxury of an unlimited ceiling for both the R&D budget and the
target retail price, and maybe not even then.  One reason it took longer
than expected to ship the 53100A was that a lot of lessons that I *thought*
had been adequately learned on the 5330A/3120A project didn't pay off when
different ADCs were used, and when the carrier and offset frequency
requirements grew by 6x and 10x respectively.

With the TimePod, for example, noise and spur performance weren't strongly
influenced by ADC clock distribution.  On PhaseStation, that particular
"unlearned lesson" cost me a respin.


That is a real lesson - It's one that people doing their first low noise 
systems get burned by (do NOT run your clock signal from your quiet 
oscillator through the FPGA, and yes, that fancy fast clock driver might 
actually degrade performance because it has a 1GHz BW and so does the 
clock input on the ADC...)


And then, getting the data reliably out of the ADC into the FPGA, and 
synchronized across multiple channels. That particular part doesn't 
guarantee the startup state of the internal pipeline.






Keeping parts and manufacturing costs under control was also more difficult
than anticipated.  Another lesson that wasn't learned soon enough was that a
design with four or five internal PCBs ends up being much more expensive
than one that uses only two, even if the total board area is similar.  We
had to increase the price twice to maintain standard T&M industry margins,
and (having just come back from visiting Said and Giovanni at Jackson Labs)
that's about to happen again.  The original vision of a four-figure price
tag was unrealistic, and that's definitely a lesson for next time.


is that because of connectors and interfaces?
Or because of "unit test" time and process?

I know that for space stuff, all on one board or in one box is cheaper 
than breaking it up, particularly for RF systems. Because if it's 
individual widgets, everyone wants individual test data at the widget 
level, before you combine them into superwidget assemblies.






-- john, KE5FX
Miles Design LLC / Jackson Labs Technologies, Inc.



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Re: [time-nuts] Mains frequency data for Europe

[Pieter-Tjerk de Boer]

Hello Attila,

In principle I have that data (I've been logging the grid's phase since 2008),
but I'll need to do a bit of work to extract it to a usable format.
Please contact me off-list.

Regards,
  Pieter-Tjerk


On Tue, Mar 24, 2020 at 04:30:25PM +0100, Attila Kinali wrote:
> Hi,
> 
> Does someone here have recordings of the mains frequency of the west
> European Grid from the last weeks and from the same time last year?
> There is something I've been told which I would like to fact-check.
> I wasn't able to find anything online.
> 
> Thanks in advance
> 
>   Attila Kinali
> -- 
> Science is made up of so many things that appear obvious 
> after they are explained. -- Pardot Kynes
> 
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> time-nuts mailing list -- time-nuts@lists.febo.com
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> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
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Re: [time-nuts] Noise Floor

[Gerhard Hoffmann]

Am 24.03.20 um 20:06 schrieb Attila Kinali:


If you want an SDR like system to play with, try the


Red Pitaya?

You get 2 ADC channels 125MHz/14Bit or 128.x MHz/16 Bit,
2 DAC channels, 2 ARM CPUs, Network etc and it boots Linux when
you plug in an USB power supply. It has existing apps such as scope,
vector network analyzer, Bode plotter, ham radio etc - and the
software as well as the web server that presents the results to
your browser is open.
It's fun to watch tuning S21 of a filter in Firefox. :-)


With 2 boards one could do the whole cross correlation thing,
with at least the 2 ADC pairs completely isolated.
<  https://www.redpitaya.com/Catalog   >
Prices are reasonable.





While I'm at it:

Is there a _done_ solution to expand a Timepod to
~ 100 MHz, narrow-bandish is OK. I'm willing to solder,
but I'm not ready for yet another development project.
Not now.

When I've flushed my project pipeline, I might consider
something serious with JESD-204B ADCs and such.
That would require a larger FPGA with GTX transceivers
and maybe a Beaglebone as a controller.


cheers, Gerhard



 The bad part of Zurich is where the degenerates
throw DARK chocolate at you.

Does Zuerich have a good part? That's news! No one threw
chocolate at me. Driving in the inner city of Berlin in
the rushhour is the pure relaxation in comparison.
Never ever again!  =8-(   )







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Re: [time-nuts] Noise Floor

[John Miles]

> > It would be interesting to know what ADC was used and if there's an
> > SDR-board out there that uses the same ADC.
> 
> Uh.. I remember John telling me what ADC it was, but I forgot, sorry.

It uses four AD9265s.  The TimePod used four LTC2216s, but the AD9265s
support higher clock rates with less power consumption, and both of those
attributes were important this time around.

> I am pretty sure I could design something like the PhaseStation as well.
> The working principle is easy and can be explained on a napkin in 5
minutes.
> But getting it to this remarkable perfomrance? Not without a lot of
> trial and error. And even then, I wouldn't be sure.

I don't think there's any way to avoid the trial-and-error part unless you
have the luxury of an unlimited ceiling for both the R&D budget and the
target retail price, and maybe not even then.  One reason it took longer
than expected to ship the 53100A was that a lot of lessons that I *thought*
had been adequately learned on the 5330A/3120A project didn't pay off when
different ADCs were used, and when the carrier and offset frequency
requirements grew by 6x and 10x respectively.  

With the TimePod, for example, noise and spur performance weren't strongly
influenced by ADC clock distribution.  On PhaseStation, that particular
"unlearned lesson" cost me a respin.

Keeping parts and manufacturing costs under control was also more difficult
than anticipated.  Another lesson that wasn't learned soon enough was that a
design with four or five internal PCBs ends up being much more expensive
than one that uses only two, even if the total board area is similar.  We
had to increase the price twice to maintain standard T&M industry margins,
and (having just come back from visiting Said and Giovanni at Jackson Labs)
that's about to happen again.  The original vision of a four-figure price
tag was unrealistic, and that's definitely a lesson for next time.

-- john, KE5FX
Miles Design LLC / Jackson Labs Technologies, Inc.



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Re: [time-nuts] Noise Floor

[Attila Kinali]

On Tue, 24 Mar 2020 18:26:28 +0200
Anders Wallin  wrote:

> It would be interesting to know what ADC was used and if there's an
> SDR-board out there that uses the same ADC.
> Our experience with the Ettus radios is that although they have 2 channels
> there is a lot of cross-talk between rx1 and rx2 channels, which makes a
> diy SDR-based phase-meter challenging.

BTW: I forgot to mention:

If you want an SDR like system to play with, try the ADC/FPGA board
that Nicholas designed as part of his master thesis at CERN (in
the group that brought us white rabbit) [1]. Design files are all
online, including his thesis itself that should contain all the
design decisions (if not just ask). To the best of my knowledge
the design should be as low noise as the ADC can get to. You will
not get the isolation the Timepod or PhaseStation has, due to using
a 4-channel ADC, but it should be good for quite a few things.
Cost for the small run (5 boards) they did was IIRC 600CHF per board.
I don't remember whether that included the MicroZedBoard or not.

Attila Kinali


[1] https://www.ohwr.org/project/r19-tdc-del-a/wikis/home

-- 
The bad part of Zurich is where the degenerates
throw DARK chocolate at you.

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Re: [time-nuts] weird Raspberry PPS+GPS NTP server behaviour

[David J Taylor via time-nuts]

Folks,

Thanks for all the help.  I've put up a simple Web page showing the program 
and its results here:


 https://www.satsignal.eu/ntp/Raspberry-Pi-ntpheat.html

A good gain on that Raspberry Pi in a centrally heated room and nearer the 
radiator than it should be.


Cheers,
David
--
SatSignal Software - Quality software for you
Web: http://www.satsignal.eu
Email: david-tay...@blueyonder.co.uk
Twitter: @gm8arv 



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Re: [time-nuts] Noise Floor

[Bob kb8tq]

Hi

Since the unit is still in warranty, I’m a bit hesitant to crack it open and 
start
poking around :).

While it’s not 100% obvious, the ADC clock comes out the back and then 
returns to the unit via a SMA jumper. At least in theory you *could* supply
your own clock to the unit. How well that would work …. no idea. 

The internal ADC clock also is under program control !! You can move it around
from TimeLab if you think you might have a spur issue. Pretty neat feature. 
I always wondered what I’d do if I had to test something that had a harmonic
at / near the clock in the TimePod. I never had a problem so it’s very much
a “who knows?” sort of thing to me. 

ADC wise, there are at least 4 channels into the device. You can do a four 
device cross comparison. How well that works … no idea. Fiddling with it 
*is* on the list. 

Isolation channel to channel is very good. I had issues early on with (phrase 
edited out) cables and questionable fixture wring. Channel to channel isolation
issues inside the box… not so much. At 10 MHz it was below the noise (at -130 
dbc) when I last looked. 

The three corner hat math works, but with the “normal” limitations. Comparing 
one ultra stable device (say a 5065a)  to two not so stable parts (say a pair 
of 
10811’s at 10K seconds) can result in strange plots.  Not the fault of the 
device 
(what ever you are using). It’s the limitation of the math involved. When one 
is “decades” more stable it falls apart. 

Just as with the good old 3048 ( and the TimePod), you can edit the FFT 
segment table. If you are after spurs in a specific region or have other 
application
specific needs, that’s a nice feature. It’s one thing  that kept the 3048’s 
going long 
after newer gear had mostly replaced them. 

Lots of fun 

Bob





> On Mar 24, 2020, at 12:26 PM, Anders Wallin  
> wrote:
> 
> Thanks for posting these!
> The user-manual provides some details on the internals:
> http://www.miles.io/PhaseStation_53100A_user_manual.pdf
> Figure 6, page 35, would indicate that there is no analog mixing stage,
> just direct ADC.
> Maybe it also shows the ADCs clocked at (max) 125 MHz, with some HPF and
> under-sampling going on in order to measure a 200 MHz carrier!?
> 
> It would be interesting to know what ADC was used and if there's an
> SDR-board out there that uses the same ADC.
> Our experience with the Ettus radios is that although they have 2 channels
> there is a lot of cross-talk between rx1 and rx2 channels, which makes a
> diy SDR-based phase-meter challenging.
> 
> Another paper to look at is the R&S one
> https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_application/pdfs/FSWP_Paper_EFTF2016_V02.pdf
> 
> I can try to dig out my old comparison-data [1] and include this new data,
> if Bob makes the results (or raw data) available as numbers somewhere.
> 
> Anders
> [1]
> http://www.anderswallin.net/2015/12/spectrum-analyzer-or-phase-noise-probe-for-phase-noise-measurement/
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Re: [time-nuts] Noise Floor

[jimlux]

On 3/24/20 9:26 AM, Anders Wallin wrote:


Our experience with the Ettus radios is that although they have 2 channels
there is a lot of cross-talk between rx1 and rx2 channels, which makes a
diy SDR-based phase-meter challenging.



The early USRPs in particular (I don't have as much experience with the 
new little ones) are really intended for use as an educational tool - 
they don't have particularly high performance in many ways - spurious 
radiated emissions are a big problem (the internal LO leaks and gets 
into the input of your receiver) - cross talk - lack of an easy way to 
time tag data precisely or to have deterministic timing between Tx and Rx.




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Re: [time-nuts] Noise Floor

[Attila Kinali]

On Tue, 24 Mar 2020 18:26:28 +0200
Anders Wallin  wrote:

> The user-manual provides some details on the internals:
> http://www.miles.io/PhaseStation_53100A_user_manual.pdf
> Figure 6, page 35, would indicate that there is no analog mixing stage,
> just direct ADC.

I was in the lucky position that John gave me a quick tour of the
hardware last year at IFCS. The design is basically the same
as in the TimePod (see the schematics in the back of the TimePod manual),
but with everything that John had learned from the TimePod applied.

There are a few details and these details make up quite a bit of
the better performance.

> Maybe it also shows the ADCs clocked at (max) 125 MHz, with some HPF and
> under-sampling going on in order to measure a 200 MHz carrier!?

One of the big changes is that the sampling clock can be changed.
This allows more flexibility in measurements, especially not having
any frequencies at which measurement performance would drop, because
of being too close to integer fraction of the sampling frequency.
Also the mandatory low-pass filter that the TimePod had has been
replaced by a set of filters (or no filter) to make it even more flexible.

> It would be interesting to know what ADC was used and if there's an
> SDR-board out there that uses the same ADC.

Uh.. I remember John telling me what ADC it was, but I forgot, sorry.

> Our experience with the Ettus radios is that although they have 2 channels
> there is a lot of cross-talk between rx1 and rx2 channels, which makes a
> diy SDR-based phase-meter challenging.

Yes. This is the key point here. The Ettus SDRs are just that: radios.
They don't need to have high isolation, because it doesn't matter for
what they are doing. Neither do they need to keep the two signal paths
closely matched. The PhaseStation and the TimePod are measurement
instruments and John designed them as such. When you see the inards
of the PhaseStation, you can tell that.

I am pretty sure I could design something like the PhaseStation as well.
The working principle is easy and can be explained on a napkin in 5 minutes.
But getting it to this remarkable perfomrance? Not without a lot of 
trial and error. And even then, I wouldn't be sure.

Jupp! I need one of those! And I need to learn how exactly they work!
For SCIENCE! :-D

Attila Kinali
-- 
Science is made up of so many things that appear obvious 
after they are explained. -- Pardot Kynes

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Re: [time-nuts] Noise Floor

[Bob kb8tq]

Hi


> On Mar 24, 2020, at 10:01 AM, Attila Kinali  wrote:
> 
> Hoi Bob,
> 
> I see you got your hands on a PhaseStation. Color me jealous! :-)

If you recall all my grumbles, back in a bit I mentioned my pile of 
ever more broken test gear. Keeping it all in repair had become so 
time consuming that nothing else was getting done. A secondary
issue was the sort of bench space things like a three corner setup 
with HP 3048 era gear takes up …..

> 
> The noise floor data is impressive! For reference: Expensive DMTD
> systems for metrological applications are usually at 1e-13 @ 1s
> and a lot more expensive.
> 
> I see I have to pester John more on how he designed the PhaseStation.
> 
> One intersting thing to note is, that the noise floor does not have
> an exactly 1/τ slope. Which suggests that some additional effect
> of higher order is affecting the measurement. This can be seen from
> the phase data, which shows a quite prominent kink around 50ks and
> is (almost?) linear before and after. It would be interesting to
> know what caused this.

(which is why I included the phase data ….)

The mixer was hooked to the inputs with some pretty good / short microwave
cables. They both are as identical as Pasternack Enterprises could make
them. The guys at Mini Circuits did their best to deliver a splitter (via 
eBay).  
None the less, they are by no means perfect. My guess is that the bump is an 
artifact of temperature change / mechanical relaxation  on either the cables or 
the splitter. 

I ran similar “floor” data on a TimePod back when I had one at work. My 
vague recollection is that it did not go quite as far before “ripples” set in. 
Since that was with different cables / splitter / environment it’s not clear
how they would compare directly. On the TimePod data run I could fairly 
easily map the ripples to the changes in room temperature. 


> 
> On Tue, 24 Mar 2020 08:03:31 -0400
> Bob kb8tq  wrote:
> 
>> One of the nice things about devices that work like a DMTD is that measuring 
>> the floor is a matter of driving the two inputs through a power splitter. 
>> With a single mixer setup (which *is* much easier to build) the floor is not 
>> as simple to estimate. The same is true of some (but not all)  counter based 
>> setups. 
> 
> Be careful here. DMTD and DMTD-like systems have a dependence of the noise
> floor on the relative phase of the input signals. With the lowest noise floor
> being at when both signals have the same phase. To trully assess the noise
> floor, you have to shift the relative phases through 2π, while making sure
> that the phase shift, however you implement it, does not degrade the signal.
> And because you are shifting the singal, that the short term noise on the
> signal is lower than the noise floor of the measurement system (in laser
> systems is called the correlation length).

Yup, but at least you *have* a way to do it…..

Bob


> 
> 
>   Attila Kinali
> -- 
>   The bad part of Zurich is where the degenerates
>throw DARK chocolate at you.
> 
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[time-nuts] Mains frequency data for Europe

[Attila Kinali]

Hi,

Does someone here have recordings of the mains frequency of the west
European Grid from the last weeks and from the same time last year?
There is something I've been told which I would like to fact-check.
I wasn't able to find anything online.

Thanks in advance

Attila Kinali
-- 
Science is made up of so many things that appear obvious 
after they are explained. -- Pardot Kynes

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Re: [time-nuts] Noise Floor

[Anders Wallin]

Thanks for posting these!
The user-manual provides some details on the internals:
http://www.miles.io/PhaseStation_53100A_user_manual.pdf
Figure 6, page 35, would indicate that there is no analog mixing stage,
just direct ADC.
Maybe it also shows the ADCs clocked at (max) 125 MHz, with some HPF and
under-sampling going on in order to measure a 200 MHz carrier!?

It would be interesting to know what ADC was used and if there's an
SDR-board out there that uses the same ADC.
Our experience with the Ettus radios is that although they have 2 channels
there is a lot of cross-talk between rx1 and rx2 channels, which makes a
diy SDR-based phase-meter challenging.

Another paper to look at is the R&S one
https://scdn.rohde-schwarz.com/ur/pws/dl_downloads/dl_application/pdfs/FSWP_Paper_EFTF2016_V02.pdf

I can try to dig out my old comparison-data [1] and include this new data,
if Bob makes the results (or raw data) available as numbers somewhere.

Anders
[1]
http://www.anderswallin.net/2015/12/spectrum-analyzer-or-phase-noise-probe-for-phase-noise-measurement/
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Re: [time-nuts] Noise Floor

[Attila Kinali]

Hoi Bob,

I see you got your hands on a PhaseStation. Color me jealous! :-)

The noise floor data is impressive! For reference: Expensive DMTD
systems for metrological applications are usually at 1e-13 @ 1s
and a lot more expensive.

I see I have to pester John more on how he designed the PhaseStation.

One intersting thing to note is, that the noise floor does not have
an exactly 1/τ slope. Which suggests that some additional effect
of higher order is affecting the measurement. This can be seen from
the phase data, which shows a quite prominent kink around 50ks and
is (almost?) linear before and after. It would be interesting to
know what caused this.

On Tue, 24 Mar 2020 08:03:31 -0400
Bob kb8tq  wrote:

> One of the nice things about devices that work like a DMTD is that measuring 
> the floor is a matter of driving the two inputs through a power splitter. 
> With a single mixer setup (which *is* much easier to build) the floor is not 
> as simple to estimate. The same is true of some (but not all)  counter based 
> setups. 

Be careful here. DMTD and DMTD-like systems have a dependence of the noise
floor on the relative phase of the input signals. With the lowest noise floor
being at when both signals have the same phase. To trully assess the noise
floor, you have to shift the relative phases through 2π, while making sure
that the phase shift, however you implement it, does not degrade the signal.
And because you are shifting the singal, that the short term noise on the
signal is lower than the noise floor of the measurement system (in laser
systems is called the correlation length).


Attila Kinali
-- 
The bad part of Zurich is where the degenerates
throw DARK chocolate at you.

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[time-nuts] Noise Floor

[Bob kb8tq]

Hi

Sorry about dumping the text from the previous post !!! Obviously the mailer on 
that
machine needs a bit of attention.

So here’s what it *should* have said:

When we are doing typical measurements on our time / frequency standards, much 
of it involves measuring noise. In many ways it is closer to a noise figure 
measurement than to a lot of other lab work. The “noise floor” of the 
measurement setup is an important partameter. Resolution might be 10,00X better 
than the noise floor. If so, resolution not a key parameter (at least not to 
me). 
 
When setting up a lab, one of the first things to do is to get a working 
measurement setup and to characterize that setup.  This often involves a lot of 
yelling, screaming, and kicking the dog. More often than not things like bum 
connectors, faulty cables, and bad grounding are what takes the most time to 
sort out. Weeks / months of work to get a setup going properly are *not* at all 
unusual.

One of the nice things about devices that work like a DMTD is that measuring 
the floor is a matter of driving the two inputs through a power splitter. With 
a single mixer setup (which *is* much easier to build) the floor is not as 
simple to estimate. The same is true of some (but not all)  counter based 
setups. 
 
Bob
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