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

[Tom Van Baak]

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.
>