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.