Hi The 9852 can be dithered from an external source. Of course, to be effective, you need a source that can calculate and transmit a multi megabit / second bit stream to do it. That gets you right back to the FPGA. The net result is not as good as a full blown FPGA approach, but it is indeed better than the stock part.
Bob On May 28, 2012, at 8:12 AM, Ulrich Bangert wrote: > Gentlemen, > > in general i can second Bruce's comments on the topic. But since I > constructed a DDS based GPSDO of my own and constructed two DDS based offset > generators of my own I would like to add some comments that arise mainly > from experience: > > 1) The absolute size of the phase and amplitude truncation errors are a > direct function of the accumulator width and the dac bit width. Two designs > that are different in these parameters my behave VERY different. > > 2) A AD9850 32 bit accumulator / 10 bit dac design is definitely NOT suited > for serious time applications. > > 3) A AD9852 48 bit accumulator / 12 bit dac design is - whith some > compromises - suited for serious time applications. > > 4) If one wants to do better a fpga solution is needed. Note that the > current XILINX and ALTERA DDS ip cores offer no higher accumulator > resolution than 48 bit, so you won't be able to increase the frequency > resolution with them against the AD9850. Nevertheless they feature a number > of interpolation and dithering tricks to decrease the phase and amplitude > truncation errors. Note that you can take advantage out of these tricks then > and only then when the DDS is allowed to produce an output word with > sufficient bit width. As long as the NCO is completely INSIDE the fpga (as > in John Miles's Timepod project) wide output words are not a problem. > However if you are thinking of producing a real world signal keep in mind > what bit width your external dac has to avoid an overshot in complexity in > the fpga compared to what the dac can deliver. I would judge that a 16 bit > output width in conjunction with a 16 bit dac as the AD5762 or the LTC1668 > is currently the best that can be done. The AD offers the higher update rate > but the LTC is easier to connect to the DDS ip cores due to its parallel > data input design. Design a low tempco reconstruction filter for the dac and > please don't ask me how to do because I don't know. > > 5) Although Rick is a member of the group I am always very astonished that > his synthesizer publications specially > > http://www.karlquist.com/FCS95.pdf > > are not well known among the group members. I assume the design described > here makes an excellent tuning circuit for use in an GPSDO albeit the > repeated dividing and mixing increases the complexity. I am not aware which > impact the repeated filtering has on the overall tempco. My guess is that > the Symmetricom AOG 110 must be similar in design. > > Best regards > Ulrich Bangert > >> -----Ursprungliche Nachricht----- >> Von: time-nuts-boun...@febo.com >> [mailto:time-nuts-boun...@febo.com] Im Auftrag von Bruce Griffiths >> Gesendet: Montag, 28. Mai 2012 10:41 >> An: Discussion of precise time and frequency measurement >> Betreff: Re: [time-nuts] DDS in GPSDO design? >> >> >> Poul-Henning Kamp wrote: >>> In message<4fc2a5e9.7050...@partiallystapled.com>, Michael Tharp >>> writes: >>> >>> >>>> Does anyone have any comments or experience with DDS-based >> frequency >>>> references? >>>> >>> I belive that is basically how the optional "output >> generator" works >>> for hydrogen masers. >>> >>> >> The Symmetricom AOG 110 has a very small fractional frequency >> adjustment >> range of 5E-8 albeit with a resolution of 1E-19. >> Thus it is relatively simple to ensure that the effect of any >> resisual >> DDS phase truncation and other spurs is very small. >> >> Bruce >> >> _______________________________________________ >> 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. > > > _______________________________________________ > 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. _______________________________________________ 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.
