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

[Joseph M Gwinn]

Magnus,


time-nuts-bounces at febo.com wrote on 07/27/2009 12:53:19 AM:

> From:
> 
> Magnus Danielson <magnus at rubidium.dyndns.org>
> 
> To:
> 
> Discussion of precise time and frequency measurement 
<time-nuts at febo.com>
> 
> Date:
> 
> 07/27/2009 12:56 AM
> 
> Subject:
> 
> Re: [time-nuts] Measuring phase shift between 1 Hz DMTD signals
> by I+Q processing
> 
> Sent by:
> 
> time-nuts-bounces at febo.com
> 
> 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 onecan 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 30KHz, 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.

Ahh.  Now I see it.  Yes you did, in a reply to Bruce on 25 July 2009.


> > 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.

Exactly.

Joe Gwinn