Bruce, At 10:16 PM +0000 12/23/08, time-nuts-requ...@febo.com wrote: > >Message: 1 >Date: Wed, 24 Dec 2008 08:47:21 +1300 >From: Bruce Griffiths <bruce.griffi...@xtra.co.nz> >Subject: Re: [time-nuts] Sub Pico Second Phase logger >To: Discussion of precise time and frequency measurement > <time-nuts@febo.com> > >[snip] > >>>>> > >>>> Using a DDS avoids the requirement for a pair of low phase noise VCOs. > >>>> > >>> If we can control the spurs, many DDS chips are very good. > >>> > >> [BG] A DDS, unlike a conventional digital frequency divider, >doesn't suffer > >> from aliasing of phase noise into the output passband. > > >> [JG] How true is this, in practice? A DDS is at the mercy of phase noise >> in its reference clock, by much the same mechanism as for a simple >> divider chain. And the variable-factor dividers (the M and N above) >> work in a manner similar to a DDS, but with far coarser increments >> and limits. Both DDS and M/N PLL chips use a PLL to clean up the >> resulting ref signal. Many DDS chips incorporate a M/N PLL to > > multiply the ref frequency. > > >For NIST's measurements of this effect see: >http://tf.nist.gov/timefreq/general/pdf/1380.pdf
Thanks for the reference. Very interesting paper, although speckled with typos and gaps. For instance, equation 5 is not grammatical, and appears to have been mangled. There is a later and longer IEEE paper by the same authors that I'll get in January. One would have expected this paper to have appeared 15 or 20 years earlier. I guess the effect wasn't elucidated until many people tried to lock microwave sources to 10 MHz. It also shows how to avoid the aliasing effect: Put an antialiasing filter and comparitor between stages. This is what Ascarrunz does in his US patent 6,278,330, specifically Figure 4. (This patent is what ref 3 of paper 1380 became.) >[snip] > >> The simplest way of achieving the required performance is preferable. >>> >> >> Yes, but aren't we Time Nuts? >> >> >Usually with finite budgets. Few dollars, many hours. Joe _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.