[time-nuts] Re: Evaluating counter performance, seeking feedback.

[Erik Kaashoek]

Tom,
I've followed your advice and modified the HW so it can accept an 
external 10MHz reference and the SW is modified so the gating is now in 
perfect  (long term) sync with the reference. Each individual gate 
moment still depends on the arrival of the counting pulse and thus may 
vary a bit.
With the modified HW/SW a direct comparison was done between the 
tinyCount and the U6200A using the 10MHz house clock as reference while 
measuring a 11MHz input signal locked to the same house reference. Tim 
files available[1][2].
The ADEV plot of the measured 11 MHz shows the measurement error of the 
tinyCount did reduce using the eternal reference[3] as with a 1 second 
gate time the ADEV plot starts just below 2e-11 where with the internal 
reference the ADEV's at 1 second tended to start above 1e-10, even with 
the internal reference locked to an external reference using a SW 
control loop.
The frequency difference plot shows there is a small systematic error in 
the measured frequency by the tinyCount[4] of about 1e-11. This error 
depends on the difference between the measured signal and the 10MHz 
reference and is probably due to the regression algorithm used.
To be further investigated.
Erik.

[1] 
http://athome.kaashoek.com/time-nuts/11MHz%20external%20lock%20tinyGTC.tim
[2] 
http://athome.kaashoek.com/time-nuts/11MHz%20external%20lock%20U6200A.tim
[3] http://athome.kaashoek.com/time-nuts/Freq%20noise.png
[4] http://athome.kaashoek.com/time-nuts/Freq%20error.png

On 8-4-2022 21:11, Tom Van Baak wrote:
> Thanks for posting both TIM files along with the plots. Here are 
> replies to 3 of your questions:
>
> > Is this setup meaningful in assessing performance differences?
> > If not, how to improve?
>
> It's probably better to compare two different counters using the same 
> setup. So give them both the same Rb as ext REF and give them both the 
> same DUT and then collect data simultaneously (apples and apples). But 
> it sounds like you can't use or don't have an ext REF input for your 
> DIY counter? In that case, right, you have to resort to the unusual 
> arrangement that you're using (apples and oranges). This is one reason 
> why almost every frequency counter has an external REF input.
>
> > How can one compress or expand a TIM file to correct for the 
> difference in gate time?
> > A better approach would be to ensure gate times where identical.
>
> Right, I noticed the two TIM files don't line up. That's a problem. 
> They are off by several seconds at the beginning of the run and spot 
> on at the end of the run. I suspect some manual editing? Note this 
> doesn't affect the ADEV plots, but it messes up the phase and 
> frequency plots. It's easy to fix.
>
> It looks like you didn't input the correct sample rate when you loaded 
> the data into TimeLab. There's a box in the acquisition menu to set 
> the sample rate. Normally close enough is good enough, but when you're 
> working with simultaneous data you need to be much more precise. This 
> isn't a problem with timestamp data because the actual sample rate is 
> implicit in the timestamp. And it's not a problem for zero-dead-time 
> frequency measurements either because one of the clocks does the 
> pacing. But for traditional gated measurements, yes, sample rate may 
> be inexact, and may also vary depending on the measurement data or 
> auto trigger settings.
>
> A while ago I collected simultaneous data on a several oscillators for 
> many days. When my PC reads serial data from a counter I always prefix 
> lines with a MJD timestamp [1]. Days or years later it tells me when I 
> did the experiment. It can also be used to detect gaps in the data. It 
> also makes it easy to make x-y scatter plots using MJD as an axis. It 
> allows multiple runs to be correlated (e.g., environmental data on one 
> PC, counter data on another PC, GPS data from a third PC, etc.). But 
> most importantly it allows me to compute the actual sample rate, the 
> tau, for any data that I ever collect. For example 5 "identical 
> counters" set for 10 s gate time had actual sample rates of:
>
> 10.3475 s
> 10.3496 s
> 10.3496 s
> 10.3494 s
> 10.3475 s
>
> This doesn't matter for short runs, doesn't matter to ADEV, doesn't 
> matter for phase or frequency plots of one counter, but matters a lot 
> when you have multiple counters over an extended period of time.
>
> > How can one use the two TIM files to calculate the RMS of the 
> differences in frequency?
> > My hope is to use this RMS calculation as a single number quality 
> indicator.
>
> Perhaps explain more what you're trying to do. Remember that frequency 
> depends very much on the averaging time so you can't just use a single 
> number. ADEV works because it's always ADEV(tau). There is a special 
> case when ADEV measurements are strictly linear, usually with a slope 
> of -1 or -1/2. Then it is customary to use a single number. For 
> example 1e-9/tau for 1 ns of WPM, or 1e-6/√tau for 1 ppm of WFM.
>
> /tvb
>
> [1] see comcat1 and comcat2 in my www.leapsecond.com/tools/ directory.
>
>
> On 4/7/2022 12:44 AM, Erik Kaashoek wrote:
>> To better understand the performance of a home build counter a 
>> comparison was done with a Picotest U6200A
>> The two channel home build counter was setup to measure the frequency 
>> of the 10MHz output from a Rb on one channel and the 10MHz output 
>> from a not so good OCXO on the other channel.
>> The ratio between the two frequencies was measured with a 1 second 
>> gate time, multiplied by 1e+7 and send to Timelab.
>> The U6200A had the Rb output as 10MHz reference and the 10MHz from 
>> the OCXO into channel 1. Gate time was also set to 1 second.
>> In Timelab the data from the Counter under test and the U6200A where 
>> recorded simultaneously over a 1000 second period
>> The recorded data was saved and adjusted for the difference in start 
>> time of the measurements and loaded back into Timelab.
>> U6200A TIM file http://athome.kaashoek.com/time-nuts/U6200A.tim
>> Own counter TIM file http://athome.kaashoek.com/time-nuts/tinyGTC_2.tim
>> A first performance check was done by plotting the unwrapped linear 
>> residue of the phase of both measurements. (see: 
>> http://athome.kaashoek.com/time-nuts/tinyGTCvsU6200A_Phase.png ) . 
>> The measurements did show some differences in instantaneous phase but 
>> the differences where small and even at 980 seconds the two 
>> measurements agree rather well.
>> A second performance check was done using the frequency plot. (see 
>> http://athome.kaashoek.com/time-nuts/tinyGTCvsU6200A_freq.png ). 
>> Overall the two measured frequencies agreed with sometimes up to 
>> 1e-10 difference. The difference in gate time of the two counters was 
>> very visible as a gradual shift. As the measurements where aligned in 
>> time at the end of the measurement the time difference at the start 
>> was about 3 seconds.
>> A detailed plot of the measured frequencies over the last 100 seconds 
>> (see 
>> http://athome.kaashoek.com/time-nuts/tinyGTCvsU6200A_freq_detail.png 
>> ) showed an occasional difference between the frequency measurements 
>> of the two counters up to 2e-10
>> Questions:
>> 1: Is this setup meaningful in assessing performance differences? If 
>> not, how to improve?
>> 1: How can one compress or expand a TIM file to correct for the 
>> difference in gate time? A better approach would be to ensure gate 
>> times where identical.
>> 2: How can one use the two TIM files to calculate the RMS of the 
>> differences in frequency?  My hope is to use this RMS calculation as 
>> a single number quality indicator.
>> Erik.
>>
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