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

[Bob kb8tq]

Hi

Frequency pulling on an oscillator is indeed a very real thing. The closer
the injected signal is to the oscillator output, the more likely some form 
of lockup is to occur. It’s not at all a bad way to measure / check the output
isolation of a design. Doing the measurement is (unfortunately) a bit tricky. 

For a counter, the “big deal” is typically the reference. It’s always there and
you can’t shut it off ( at least if you want a useful measurement …). There are
a lot of counters that go a bit deaf as you get very close to the reference 
frequency. Some of it is software based ( not enough data in enough buckets).
Some of it is RF isolation ( one signal masks the other ). Just what goes wrong
with this or that one is never very easy to work out.  

With most of them “close” means deltas in the < 1x10^-8 range. A useful sweep
might be over +/- 5x10^-8 or less. Step size could easily be below 1x10^-11.
(= the steps are down in the noise of the sources ….). Run length could be pretty 
long to average out noise issues. Running <= a second per step is probably a bit 
fast, minutes per step is getting a bit crazy. Yes, this is something you automate
and let run overnight. 

One very typical way to try to spot this is with two sources. One is stable and
acts as the reference. The other is allowed to very slowly drift / tune across the
other. Rb’s are not a bad way to do this since they are likely to have the parts
per trillion sort of tuning steps you are looking for. As you plot what should be
a nice linear frequency change, you look for flat spots / jumps / nonsense in the
output data plot. 

If you want to extend this to multiple inputs, that could be done. Multiple sources
all being tuned in this or that pattern probably get the job done best time wise. 
Cost wise “best" might be a very different thing. They all need to be quite stable 
in order to keep random movement in the sources from masking the desired data.
That tends to drive up the cost a bit. 

One could play with this or that synthesis approach. There is no rule that says it 
can’t work. There is a risk of various spurs / artifacts getting in the way. There’s 
also the issues with tuning word length to get really fine grain resolution. My guess
is that a couple ( = 2 to 4 ) of cheap telecom Rb’s still beat a built from scratch gizmo 
cost wise. 

Looking at your plot, it would be nice to get the “random stuff” down to < 100 ppt. 
Then you would have a better idea if the 500 to 600 ppt items are problems or not. 
The alternative might be to do a *lot* of runs and look for things that show up often. 

Crazy !!

Bob



> On Aug 3, 2022, at 10:58 AM, Erik Kaashoek via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> A well know technique for measuring phase pulling is measuring the phase of two signals 0.01 Hz or less apart.
> To test if this pulling has a relevant impact on frequency measurement the ratio between two 10 MHz output signals from a signal generator was measured. One output signal was changed in frequency in steps of 0.01 Hz between 9999999 Hz and 10000001 Hz [1]
> The plot shows the error at all measured frequencies differences (+/- 1Hz in steps of 0.01 Hz) is below the accuracy target of +/- 1e-9
> Is this a relevant measurement for detecting possible interaction between the two inputs of a frequency counter?
> Any suggestions for how to do a performance evaluation of a frequency counter?
> [1] http://athome.kaashoek.com/time-nuts/Freq_error.PNG
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