[time-nuts] Question about precise frequency / phase measurement
magnus at rubidium.dyndns.org
Thu Apr 19 19:31:43 UTC 2012
On 19/04/12 21:10, skywatcher wrote:
> Hello @all,
> my name is Wolfgang and i'm new to the list. :)
> 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 ?
Have you looked at the PICTIC II project? It's not too bad.
There is several other possible projects to consider, but the PICTIC II
may be a good start. Also look at a divide down system such as the TADD
2 divider or the just released TADD 2 Mini.
There is a challenge in doing fairly high precission for low budget here
on the list. Besides measuring frequency, we pride ourselves in
measuring the frequency stability, Allan Deviation (ADEV), as good as
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