[time-nuts] Question about frequency counter testing

Oleg Skydan olegskydan at gmail.com
Mon May 14 13:50:21 EDT 2018


From: "Bob kb8tq" <kb8tq at n1k.org>
>> If such conditions detected, I avoid problem by changing the counter 
>> clock. But it does not solve the effects at "about OCXO" * N or "about 
>> OCXO" / M. It is related to HW and I can probably control it only 
>> partially. I will try to improve clock and reference isolation in the 
>> "normal" HW and of cause I will thoroughly test such frequencies when 
>> that HW will be ready.
> It’s a very common problem in this sort of counter. The “experts” have a 
> lot of trouble with it
> on their designs. One answer with simple enough hardware could be to run 
> *two* clocks
> all the time. Digitize them both and process the results from both.

I thought about such solution, unfortunately it can not be implemented 
because of HW limitations. Switching 400MHz clock is also not ideal 
solution, cause it will make trouble to GPS correction calculations, the 
latter can be fixed in software, but it is not an elegant solution. It all 
still needs some polishing...

> still have the issue of a frequency that is a multiple (or sub multiple) 
> of both clocks.

The clocks (if we are talking about 400MHz) has very interesting values like 
397501220.703Hz or 395001831.055Hz , so it will really occur very rarely. 
Also I am not limited by two or three values, so clock switching should 
solve the problem, but not in elegant way, cause it breaks normal work of 
GPS frequency correction algorithm, so additional steps to fix that will be 
required :-\.

BTW, after quick check of the GPS module specs and OCXO's one it looks like 
a very simple algorithm can be used for frequency correction. OCXO frequency 
can be measured against GPS for a long enough period (some thousands of 
seconds, LR algorithm can be used here also) and we have got a correction 
coefficient. It can be updated at a rate of one second (probably we do not 
need to do it as fast). I do not believe it can be as simple. I feel I 
missed something :)...

All the best!

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