Hi A number of them are sigma delta PLL's and have some really nasty jitter and spur issues. Anything that shows up listed as "user programmable" - watch out.
Bob On Jul 21, 2013, at 5:17 PM, Didier Juges <shali...@gmail.com> wrote: > Some of the small surface mount canned oscillators are actually pll and not > very good at that. I have a page about it somewhere on my web site. > > Didier > KO4BB.com > > Graham <planoph...@aei.ca> wrote: >> Good morning Joe (and all), >> >> I have been playing around with a few of these DDS modules as well as a >> >> couple of the lower end DDS chips (i.e. AD9834, AD9850, AD9851) and >> they >> do work well but they are not perfect. >> >> I have a Fox Delta Antenna SWR analyzer which has an AD9850 and a >> 125Mhz >> surface mount oscillator and although I have not done any extensive >> testing of the devices stability it is relatively stable to a N3ZI >> design DDS VFO which uses a AD9834 and a 80Mhz dip can (ie. not surface >> >> mount) oscillator. >> >> The AD9834/80Mhz dip can oscillator DDS wanders quite a bit and when I >> listen to it on a receiver it sounds "raspy". I have simple heater >> attached to the dip can - it is just as bad with as without. There has >> been some discussion on the Knights QRSS reflector on the subject. If >> you are not yet a member, you can join here: >> http://mail.cnts.be/mailman/listinfo/knightsqrss_cnts.be >> >> I am guessing "the amateur in town" is Dave WA5DJJ. He and I have >> exchanged a few emails on this subject (and others). There is a fellow >> in Australia as well which I have exchanged a few ideas. >> >> One common method of taming some of wandering habits of the inexpensive >> >> DDS modules is to put a small coin on the dip can oscillator. I noted >> this behavior as well some time ago. I can't quite put my finger on why >> >> it helps. Some postulate that it is adding just enough thermal mass to >> keep drift down when there is moving air around the can oscillator. >> >> What I have noticed is that the DDS with the surface mount oscillator >> is >> more stable (remember, no extensive and exhaustive testing at this >> point, just simple observation). I have also noticed that if I have one >> >> of the DDS modules on my bench that when I monitor the output of said >> device that tapping on the desk also seems to cause some wandering of >> the synthesized frequency indicating that the the dip can oscillator is >> >> possibly microphonic in some way. >> >> It seems at this point as though the SMD oscillators are a better >> choice >> between these two low cost options. Of course, TCXO or OCXO would be a >> better choice. >> >> I stumbled across an Analog Devices application note AN-419 title "A >> Discrete Low Phase Noise 125Mhz Crystal Oscillator for the AD9850 >> Complete Direct Digital Synthesizer" which has some interesting tid >> bits >> of information. I am using this as a starting point for some >> experimenting with reference clock sources for DDS's. One bit of >> information that no doubt is in the DDS chip's data sheet is that the >> reference clock needs to be at least 3 Vp-p and centered on 1/2 VDD. >> Seems so obvious but makes me wonder if adding a small coin to top of >> the DIP can oscillator is not only helping thermal wise but is somehow >> helping improve phase noise. >> >> I think we have tended to think of a DDS as being "it". That is, we can >> >> generate something stable and precise digitally and it will be good no >> matter what. What we (I) forgot when first exploring the devices was >> that what we get out the synthesized end is (relatively speaking) no >> better than what we put in (i.e. the reference clock), another example >> of GIGO. >> >> cheers, Graham ve3gtc >> >> >> >> On 13-07-21 04:03 AM, Joseph Gray wrote: >>> 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 -- time-nuts@febo.com >> 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 -- 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.