[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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