[time-nuts] Measuring phase shift between 1 Hz DMTD signals by I+Q processing
magnus at rubidium.dyndns.org
Mon Jul 27 04:53:19 UTC 2009
Joe Gwinn wrote:
> 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.
I just proposed that method to avoid flicker.
> 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.
Well, it's just like normal radio when first LO just brings the
frequency into range and second LO does I/Q separation.
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