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

skywatcher skywatcher at web.de
Thu Apr 19 19:10:03 UTC 2012


Hello @all,

my name is Wolfgang and i'm new to the list.  :)

I browsed through the list archive, but i didn't find the infos i need, 
so i decided to join the list
and to ask the experts directly.  :)

I want to measure the frequency difference between a 10 MHz OCXO and a 
10 MHz Rubidium.
I think that's what many people here have done many times... but i don't 
want to use expensive
equipment like time interval counters with picosecond resolution etc. I 
would prefer a cheap and
easy solution. I also would like to have an update rate of more than 1 
measurement per second,
or even more.

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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