Hi If the DDS is acting pretty much as a divider you don't get a lot of spurs. The rest of the time there are spurs *somewhere* in the output spectrum. Put another way, there are thousands of "bad" tuning words for every good one. The good ones are evenly spaced over the range at a spacing determined by the DAC width and the phase truncation process.
Bob On Jul 21, 2013, at 7:07 AM, Tim Shoppa <[email protected]> wrote: > For the AD9850 (as well as the higher-tech more-bit-resolution modules) > there are programming words/freqs that are awful with a lot of close-in > spurs, and others that are comparatively clean (well, maybe just as many > spurs by some measure but they are far far away). > > If you go to a DDS with higher base frequency and more bits resolution > (e.g. comparing my AD9954 DDS with my AD9850 synth) these "bad freq words" > become less common/less severe. > > These frequency-dependent close-in spurs show up in a receiver, as sudden > appearance of raspies at certain receiving freqs while others just a > fraction of a Hz away sound clean. > > Several ham designs for the AD9850 follow it with a simple one-to-one > tracking PLL for some cleanup. > > There are some AD app notes that hint the patterns of the words with lots > of close in spurs but I've never found an easy programmatic way to skip > over them for the better words. > > http://www.analog.com/static/imported-files/application_notes/131351807AN_927.pdf > especially > section titled "Predicting and Exploiting Spur "Sweet Spots" in a DDS' > Tuning Range". > > Analog.com website has some tools to explore spurs vs tuning word: > > http://designtools.analog.com/dtDDSWeb/dtDDSMain.aspx > > Tim N3QE > > On Sun, Jul 21, 2013 at 4:21 AM, Nic McLean <[email protected]> wrote: > >> Hi Joe, >> It is good that they are that stable, but what is the phase noise like? If >> a >> ham across town is using them for WSPR and QRSS they are most probably OK >> in >> that regard, but not all DDS modules are. >> Best 73's >> Nic >> VK2KXN / VK5ZAT >> >> -----Original Message----- >> From: [email protected] [mailto:[email protected]] On >> Behalf Of Joseph Gray >> Sent: Sunday, 21 July 2013 2:04 PM >> To: Discussion of precise time and frequency measurement >> Subject: [time-nuts] DDS module >> >> I have a few of those $5 AD9850 DDS modules from China. I'm going to use >> one >> to replace a crystal in a transmitter. I attached it to a TI MSP430 >> Launchpad and programmed a fixed frequency. >> >> Just out of curiosity, I wanted to see how stable this thing is. Using >> cellophane tape, I put a few pieces of foam around the oscillator can. Then >> I hooked it up to my HP counter, which is locked to my GPSDO. >> >> I'm not logging, but I have kept an eye on the setup for well over 24 hours >> now. So far, the frequency has held to within 0.01-0.08 of the programmed >> value. >> >> The house thermostat is set at 75 F, but this room gets somewhat warmer due >> to the equipment in it. Also, this room gets the morning sun. >> >> Considering the cost, I am impressed with how well this little module works >> and how stable it is. There is an Amateur in town who has been using >> several >> of these modules to drive WSPR and QRSS transmitters. He has done up a poor >> man's oven to keep his transmitters on frequency. >> >> Joe Gray >> W5JG >> _______________________________________________ >> 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.
