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

[Bob kb8tq]

Hi

Unfortunately the typical DDS generator has a “triangle wave” spur 
modulation on the phase of its output. Unless it’s a very unusual part
( = audio ) the DAC resolution limits things at a potentially nasty level.
When the phase truncation / modulation edges come along, they can 
create issues with the counter. 

Since this is time domain stuff and most designs are looking at frequency 
domain, it's an easy thing to miss. The typical app note goes straight 
to the frequency domain. They talk about the results, but not about 
a nice easy to understand phase error plot. ( = even finding a link with
a nice plot is not something Mr Google seems to want to do ….).

Bob

> On Aug 4, 2022, at 12:33 AM, Erik Kaashoek <erik at kaashoek.com> wrote:
> 
> Bob,
> 
> Thanks for the advice.
> I've updated the sweep from -1.5E-9 to +1.5E-9 with steps of 1E-11 and used a 10 second gate time.
> The used DDS signal generator was locked to the 10 MHz reference output of the counter
> The impact of being close to the reference is clearly visible in the updated plot [1]
> Measurement error well away from 10MHz is in the order of 1E-11
> Close to 10 MHz ( +/- 0.5E-9 ) this increases to 1E-10 which is still acceptable.
> Erik.
> 
> [1] http://athome.kaashoek.com/time-nuts/Freq_error.PNG
> 
> On 4-8-2022 1:04, Bob kb8tq wrote:
>> 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