Bill, yes, I have been thinking exactly this way too. The easist way would be to divide the reference by 2 giving 5 Mhz and then use an odd integer multiplier in the DDS. However, I believe that the AD9851 is limited concerning its multiplier range, so it might be necessary to use an AD9852/54 which again is a complication (that I don't like) of an otherwise "easy" project.
73 Ulrich > -----Ursprungliche Nachricht----- > Von: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] Im Auftrag von WB6BNQ > Gesendet: Donnerstag, 5. Juni 2008 00:41 > An: Discussion of precise time and frequency measurement > Betreff: Re: [time-nuts] quick and very dirty phase comparator > > > Ulrich, > > One of things I noticed when playing with that DDS design > tool is you can achieve the same output frequency using a > different clock frequency, thus removing the spurs. THe > trick is to look at clock frequencies that you can lock to > your house reference for stability. Also, it might be > possible to use cascading DDS chips, one providing the clock > for the other. > > What are the thoughts on such an approach ? > > Bill....WB6BNQ > > > Ulrich Bangert wrote: > > > Bruce, > > > > > Even a DDS followed by a PLL cleanup loop (10811 plus analog PD > > > etc.) should work well although with a binary tuning word > obtaining > > > an exact 10.00001MHz (or alternatively 9.99999 MHz) output isnt > > > possible. A DDS has some advantages over a synthesizer using > > > dividers in that > > > additional noise isnt aliased into the output. > > > > Since I am well familiar with the Analog Devices DDS circuits, this > > has been my very first idea. The most simple one for that purpose > > would be a AD9851 (180 MHz, 32 Bit, built in clock multiplier). But > > when I used the DDS design tool available on the AD web pages I > > received a big warning saying that using a "clock X multiplier" > > frequency that is a near integer of the output frequency generates > > lots of unwanted spurs. Which was new to me since I do so > in my GPSDO > > but should they not know better? This is why I dropped the > thoughts on > > DDS. > > > > Best regards > > Ulrich Bangert > > > > > -----Ursprungliche Nachricht----- > > > Von: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] > > > Im Auftrag von Bruce Griffiths > > > Gesendet: Mittwoch, 4. Juni 2008 10:44 > > > An: Discussion of precise time and frequency measurement > > > Betreff: Re: [time-nuts] quick and very dirty phase comparator > > > > > > > > > Ulrich Bangert wrote: > > > > Bruce, > > > > > > > > thank you for correcting me. Here I have clearly fooled myself. > > > > However your posting originated some new ideas: With the > > > GCD becoming > > > > THAT low an analogue phase lock to a 10 MHz reference > will not be > > > > easy. But if we stop to think about phase locked VCXOs > we need not > > > > bother anymore about odd exotic xtal frequencies > > > Yes you would need a VCXO with low close in phase noise for the > > > 17.73447MHz source. That crystal frequency isnt too exotic as RS > > > components have suitable crystals, if you build your own VCXO. > > > > at all that may generate us a GCD of 10. > > > > Instead we are free to choose for example 10000010 Hz for the > > > > controller's frequency. Which brings us back to a > construction of > > > > a good offset generator. > > > > > > > > Until now I have believed that a good (low phase noise, high > > > > stability) offset generator would involve > > > > > > > > a) a number of single sideband mixers (as described in > > > > www.horology.jpl.nasa.gov/papers/fssa.pdf) > > > > > > > > or > > > > > > > > b) the well known offset synthesizer circuitry as described by > > > > Rick Karlquist. > > > > > > > > I am sure that both ideas work excellent, although I am > > > unsure whether > > > > a) can generate an 10 Hz offset. However, both methods involve > > > > circuitry that I would not call exactly "quick and dirty" and > > > > their use would overstress the try to make something really > > > simple. On a new > > > > internet search for "offset generator" I came over this one: > > > > > > > > > > > 10Hz offset by method a is trivial (9.99999MHz is just as > useful as > > > 10.00001MHz): > > > > > > 1) Use a LSB mixer to mix 10MHz with 10MHz/1000 to > generate 9.99MHz > > > > > > 2) Bandpass filter this and then use a USB mixer to mix > 9.99MHz with > > > 9.99MHz/1000 to generate 9.99999MHz. > > > > > > 3) Use a PLL to phase lock a low noise VCXO (spare 10811A > or similar > > > detuned mechanically by 10Hz??) to the 9.99999MHz output > to remove > > > spurs etc. > > > > > > Even a DDS followed by a PLL cleanup loop (10811 plus analog PD > > > etc.) should work well although with a binary tuning word > obtaining > > > an exact 10.00001MHz (or alternatively 9.99999 MHz) output isnt > > > possible. A DDS has some advantages over a synthesizer using > > > dividers in that > > > additional noise isnt aliased into the output. > > > > > > > www.diva-portal.org/diva/getDocument?> > > urn_nbn_se_liu_diva-1838-1__fullt > > > > ex > > > > t.pdf > > > > > > > > What do you think about that topology? Let the "IF in" be > > > the needed > > > > offset and the "offset OSC" be our 10 MHz reference. Would that > > > > not make an really easy way to generate an precise offset with > > > the wanted > > > > features? > > > > > > > > > > > Not much (can work well with 20kHz offset but not with > 10Hz offset), > > > direct generation mixing 10Hz with 10MHz like this > requires a rather > > > low PLL bandwidth. > > > Also filtering out the unwanted sideband may be problematic. > > > > > > > Best regards > > > > Ulrich Bangert > > > > > > > > > > > Bruce > > > > > > > > > _______________________________________________ > > > 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. > > > > > > > _______________________________________________ > > 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. > > > _______________________________________________ > 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. > _______________________________________________ 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.
