You probably meant "as an integer divider", you don't get a lot of spurs.
Didier Bob Camp <[email protected]> wrote: >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. -- Sent from my Motorola Droid Razr 4G LTE wireless tracker while I do other things. _______________________________________________ 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.
