[time-nuts] DDS module

[Bob Camp]

Hi

It's a bit more complex than an integer divide, but yes that's the right idea. You want something that will fit both in the truncation word and inside the range of the DAC.

If you have a 12 bit DAC and the truncation word  ( sine table input , what ever you call it) is more than that, then it's going to be your high frequency clock divided by 2^12

Bob

On Jul 21, 2013, at 6:26 PM, Didier Juges <shalimr9 at gmail.com> wrote:

> 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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