[time-nuts] DDS module

[Didier Juges]

You probably meant "as an integer divider", you don't get a lot of spurs.

Didier

Bob Camp <lists at rtty.us> wrote:
>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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