[time-nuts] DDS module

Didier Juges shalimr9 at gmail.com
Sun Jul 21 17:17:19 EDT 2013


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 <planophore at 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
>
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