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.
