[time-nuts] Question about precise frequency / phase measurement
jfor at quikus.com
Thu Apr 19 19:24:02 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.
You would only see a sine wave if the difference frequency was stable an
such that the LPF cut off all but the fundamental.
If you want sine wave output, use an analog mixer on sine waves, and don't
> 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,
> time-nuts mailing list -- time-nuts at febo.com
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