Bruce, At 3:54 AM +0000 12/19/08, [email protected] wrote: > >Message: 5 >Date: Fri, 19 Dec 2008 16:51:55 +1300 >From: Bruce Griffiths <[email protected]> >Subject: Re: [time-nuts] Sub Pico Second Phase logger >To: Discussion of precise time and frequency measurement > <[email protected]> > >Joe > >Joe Gwinn wrote: >> At 11:48 PM +0000 12/18/08, [email protected] wrote: >> >>> Message: 5 >>> Date: Fri, 19 Dec 2008 12:48:27 +1300 >>> From: Bruce Griffiths <[email protected]> >>> Subject: Re: [time-nuts] Sub Pico Second Phase logger >>> To: Discussion of precise time and frequency measurement > >> <[email protected]> > >> > >> [snip] > > >>> Using a second sound card to generate the test signal may overcome this >>> problem at increased cost, and for some it may not even be an option. > >> >> >> It may not work with PCI soundcards, as the card clock may be >> synchronized to the PCI bus clock. Firewire/USB cards will have >> their own independent clocks. >> >> >Most recent design PCI sound cards have their own independent crystal >oscillator.
It won't be a great crystal, but a crystal nonetheless. >Some claim to be able to sync to an SPDIF input but the resultant jitter >may be large. Why large jitter? Bad implementation? > > >>> >> 10Hz resolution whilst avoiding phase truncation spurs is impractical >>> >>>>> with a DDS chip by itself. >>>>> Depending on the DDS and its clock frequency, the frequency spacing of >>>>> phase truncation spur free outputs may be as large as several kHz. >>>>> >>>>> > >>> Is this true of concatenated DDS chips? I recall a patent to the > >>> contrary. > >> > > >> Which patent? > > >> Hmm. It's at work. I'll look it up in January. As I recall, the >> second DDS made a small integer conversion, and so had low spurs, >> while the first DDS was set to whatever was needed. >> >> >Do you mean US5598440? Yes, that's it. > >> If the zero crossings are time stamped and do not occur simultaneously >>> in each channel then the phase noise of the offset oscillator will > >> affect the measurement. > > >> I'm not following. Please expand. The zero crossings are never >> aligned unless there is no phase delay. >> >> >> >Yes, thats the point, the offset generator phase noise contribution isnt >the same for both zero crossings. >Greenhall et al correct for this to some extent, but at least for short >tau, one is then no longer measuring ADEV, MDEV etc. >There is some advantage in having a higher beat frequency as the offset >generator phase noise has less time to accumulate. I recall reading an article on this by Greenhall, probably _the_ article. My reaction was that I would be fortunate to have a setup where I could even detect such a problem. > >> >> A few divide and mix stages will be required to achieve a spur free >>> >>>>> resolution of 10Hz. >>>>> >>>>> >>>> That is a traditional approach. But are there alternate approaches that >>>> have now become practical? >>>> >>>> >>>> >>> Diophantine frequency synthesis? >>> >> >> From the sound of the name I think so, at least in the last DDS >> stage, as done by that patent. >> >> But I was fishing. >> >> >Conventional Diophantine synthesis uses number theory together with 2 or >3 conventional synthesiser loops to achieve very fine resolution whilst >maintaining a high PLL phase detector input frequency. In a sense, the concatenated DDS approach is a divide-and-mix chain. Perhaps there is a parallel here. _______________________________________________ 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.
