Magnus,
[email protected] wrote on 07/27/2009 12:53:19 AM: > From: > > Magnus Danielson <[email protected]> > > To: > > Discussion of precise time and frequency measurement <[email protected]> > > 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: > > [email protected] > > Joe Gwinn wrote: > > Bruce, > > > > At 1:00 AM +0000 7/26/09, [email protected] wrote: > >> Message: 3 > >> Date: Sun, 26 Jul 2009 12:29:24 +1200 > >> From: Bruce Griffiths <[email protected]> > >> Subject: Re: [time-nuts] Measuring phase shift between 1 Hz DMTD > >> signals by I+Q processing > >> To: Discussion of precise time and frequency measurement > >> <[email protected]> > >> > >> > >> 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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
