[time-nuts] DDS module

[Tim Shoppa]

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
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
>