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
>
>
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