[time-nuts] Re: Testing frequency pulling on a DYI counter

[Tom Van Baak]

 > the measured frequency the regression looses some accuracy.

Yes, the hp/Agilent/Keysight 53132A does that too. Here's the footnote 
from the user manual:

http://leapsecond.com/pages/53132/53132-reduced-resolution.gif

And it's not just at 10 MHz. Any fraction or multiple within 600 ppb is 
affected too. What impressed me is that the hp firmware engineers 
specifically detected this condition and reduced the number of digits 
displayed accordingly. This avoids the user from getting a false sense 
of precision.

 > Tom Van Baak warned me against using fractional relations in
 > a counter but otherwise it is impossible to measure a 10 MHz
 > input signal with any accuracy without a HW time to digital
 > as the interpolation no longer works.

Right, which is probably why many high-end commercial counters use 
interpolators. But you aren't, and that's ok because your design spec is 
on the order of 9 digits. Keep it simple. Later if you design a 10 or 11 
or 12 digit counter you'll have to resort to using h/w interpolation as 
well. Even the Lars GPSDO uses a crude interpolator; it's not that 
difficult. Many threads in the time-nuts archive on the topic.

----

Your slow sweeping experiments reminded me of a great example I once ran 
into. So I wrote it up with photos and plots:

http://leapsecond.com/pages/racal/
http://leapsecond.com/pages/racal/stdev.htm

Even if you aren't building a frequency counter, this strange event is 
quite interesting. One of the plots is attached; the rest are in the URL 
above.

/tvb


On 8/4/2022 11:04 AM, Erik Kaashoek via time-nuts wrote:
> Bob, Magnus,
>
> Using a second counter (my famous Picotest U6200A) locked to the 
> reference output of the DIY counter and measuring the output of the 
> signal generator and also set to gate of 10 s it is confirmed that the 
> frequency pulling (if any) is below 1E-11 (not more digits on the 
> display of the U6200A)
> Generator is set to 10.000,000,000,2 MHz and is measured as such by 
> the U6200A
> As there seems to be no frequency pulling I went back to the 
> simulation of the linear regression algorithm and discovered that when 
> there is a integer  divide/multiply relation between the internal 
> reference and the measured frequency the regression looses some accuracy.
> For sure if the reference is close to an integer multiple of the 
> measured frequency (10 Mhz measured -> 200 MHz reference) the 
> regression collapses completely in accuracy. I hoped that by creating 
> a fractional relation this collapse would not happen at 10 MHz but is 
> still there, although much smaller. For this test I'm using a "div 3 
> times 64 e.g. 213.333,333,333,333... MHz" internal reference frequency 
> derived from the external 10MHz reference. Ton van Baak warned me 
> against using fractional relations in a counter but otherwise it is 
> impossible to measure a 10 MHz input signal with any accuracy without 
> a HW time to digital as the interpolation no longer works. I can 
> switch dynamically to 200 MHz or 245 MHz reference and these produce 
> much much worse results.
> I realize this test only measures if the TCXO used as reference in the 
> DIY counter does not show frequency pulling but it does not show if 
> the PLL used to convert the 10MHz to 213.333333333... MHz for the 
> internal counters shows any frequency pulling.
> NOt
> Erik.
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