Hi Wolfgang,
On 19/04/12 21:10, skywatcher wrote:
Hello @all,
my name is Wolfgang and i'm new to the list. :)
Welcome!
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
possible.
Cheers,
Magnus
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