[time-nuts] Question about precise frequency / phase measurement

SAIDJACK at aol.com SAIDJACK at aol.com
Thu Apr 19 19:49:03 UTC 2012


Hi Wolfgang,
 
one of the easiest and very accurate ways to do this is simply to measure  
the drift of the two 10MHz signals on an oscilloscope. Adjust the OCXO so  
that this drift between the two traces is as slow as you can get it. Then 
simply  measure it over time. Use one signal for trigger, the other to display 
if  you only have a one channel scope.
 
If you get say 10ns drift over 1 hour (which you can easily measure even  
with the cheapest scopes), that is a resolution of 10ns/3600s = 2.78E012.
 
Or in other words 27.7uHz!
 
This has been discussed before and documented in the time nuts archives  
some time ago.
 
bye,
Said
 
 
 
 
In a message dated 4/19/2012 12:10:53 Pacific Daylight Time,  
skywatcher at web.de writes:

My first  approach was to use a simple XOR phase comparator. I tried a 
74HCT86 and a  74HCT4046.
It works, but it's very noisy, so i don't get better than about  10 mHz 
frequency resolution.
If i look at the lowpass-filtered output i  don't see a nice sine or 
triangular wave, but it looks more
than a  triangular wave with round tops and some bumps between them. 
Another  problem is that the
difference frequency gets very low when the frequencies  are very close, 
so it's not enough to look
only for zero crossings of  the difference signal.

Does anybody know a possibility to get a  resolution < 1 mHz ?

Best regards,
Wolfgang





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