The traditional approach was to use a double-balanced mixer configured as a phase detector, pass the phase detector output through a low-pass filter (with 1 Hz bandwidth), and plot the result using a chart recorder. The chart recorder would also have marks from some kind of accurate clock. After a few days of recording, the beat frequency is easily determined, as is variations over the day. The present-day equivalent replaces the chart recorder with a recording digital voltmeter of some kind.
Joe Gwinn From: Azelio Boriani <azelio.bori...@screen.it> To: Discussion of precise time and frequency measurement <time-nuts@febo.com> Date: 04/19/2012 03:38 PM Subject: Re: [time-nuts] Question about precise frequency / phase measurement Sent by: time-nuts-boun...@febo.com And, if you are measuring, by analog mixing, two very slightly different frequencies, what do you expect to obtain if not a signal that is slow, very slow. How can you measure milliHertz or microHertz without waiting? On Thu, Apr 19, 2012 at 9:31 PM, Magnus Danielson < mag...@rubidium.dyndns.org> wrote: > 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 > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
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