[time-nuts] DDS module

Bob Camp lists at rtty.us
Sun Jul 21 10:51:01 EDT 2013


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 <tshoppa at gmail.com> 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 <mcleannb at bigpond.com> 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: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] 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
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