[time-nuts] Measuring phase shift between 1 Hz DMTD signals by I+Q processing
Joe Gwinn
joegwinn at comcast.net
Sun Jul 26 15:27:00 UTC 2009
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.
Joe Gwinn
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