[time-nuts] Question about precise frequency / phase measurement

Azelio Boriani azelio.boriani at screen.it
Thu Apr 19 19:55:55 UTC 2012


Of course, there are PICTIC II, DMTD, DDMTD, SR620, HP5370B, Wavecrest,
PM6681, HP53132. The simplest is using a scope and... wait.

On Thu, Apr 19, 2012 at 9:53 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz
> wrote:

> Use a dual mixer system with an offset LO.
>
> Bruce
>
> Azelio Boriani wrote:
>
>> 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<
>> magnus at 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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>>>
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>
>
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