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

[Bruce Griffiths]

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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