[time-nuts] Measuring phase shift between 1 Hz DMTD signals by I+Q processing

[Bruce Griffiths]

Bruce Griffiths wrote:
> Joe
>
> Joe Gwinn wrote:
>   
>> Bruce,
>>
>> At 1:00 AM +0000 7/26/09, time-nuts-request at febo.com wrote:
>>     
>>> Message: 3
>>> Date: Sun, 26 Jul 2009 12:29:24 +1200
>>> From: Bruce Griffiths <bruce.griffiths at xtra.co.nz>
>>> Subject: Re: [time-nuts] Measuring phase shift between 1 Hz DMTD
>>>     signals by I+Q processing
>>> To: Discussion of precise time and frequency measurement
>>>     <time-nuts at febo.com>
>>>
>>>
>>> If one uses a mixer output frequency of several kHz then one can avoid
>>> the flicker noise region if one uses a high pass filter between the ADCs
>>> and the mixer preamps.
>>>
>>> Does such a system have a performance advantage over direct RF sampling?
>>> Perhaps it does if and only if the phase noise floor of the lower
>>> bandwidth ADCs that are used is lower than the noise floor of the ADCs
>>> that would be required to sample the RF signals directly?
>>> The noise floor of state of the art ADCs suitable for direct RF sampling
>>> is around -150dBFS/Hz.
>>> The noise floor of  "typical" high resolution ADC(AD7762, AD7641)
>>> capable of sampling at around 1MSPS or so appear to be similar.
>>>       
>> It strikes me that one can do a double conversion here, with one
>> conversion done in analog hardware, the other in a DSP.
>>
>> The analog mixer would go from megahertz to say 100 KHz or 30 KHz, and
>> this kilohertz signal would be digitized, yielding a stream of I+Q
>> samples.
>>
>> The resulting stream of digital I+Q samples would then be numerically
>> mixed down to 1 Hz, and the relative phase between the two 1 Hz
>> signals would be measured.
>>
>> This two-step approach should neatly sidestep the flicker-noise issue.
>>
>> As for the ADC yielding I+Q samples, given the great oversampling
>> possible with current ADCs, one can use a single ADC and
>> mathematically generate the I+Q streams.  There are many methods,
>> invented because high=performance ADCs are very expensive, and because
>> it is difficult to find sufficiently well matched pairs of such ADCs.
>>
>>     
> That approach should also make it simpler to use a DDS to generate the
> offset frequency as one can use one of the DDS output frequencies that
> has no anharmonic spurs.
> It should even be possible to use triple balanced mixers (e.g.VAY-1,
> SRA-220, TFM-12MH, SAY-1) if the first IF frequency is high enough.
>   
>> Joe Gwinn
>>
>>     
>
> Bruce
>
>   
Correction:
After actually viewing the datasheets rather than relying on the IF spec
not extending to DC it turns out that only The VAY-1 and SRA-220 are
triple balanced mixers.
There are other Minicircuits triple balanced mixers but their IF
response doesnt extend low enough in frequency to allow use of an IF
frequency that is < 1MHz.

Bruce