[time-nuts] DDS module

Bob Camp lists at rtty.us
Sun Jul 21 20:24:40 EDT 2013


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 <shalimr9 at 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 <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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> 
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