# [time-nuts] spur prediction DDS software

Magnus Danielson magnus at rubidium.dyndns.org
Sun Jan 30 12:58:18 UTC 2011

```On 27/01/11 14:33, jimlux wrote:
> On 1/27/11 5:17 AM, Luis Cupido wrote:
>> Hi,
>>
>> Is there a DDS spur prediction software around ?
>
> Not generically..
>
> There is a dissertation out there with some matlab code. I'll see if I
>
> Most of the time what I do is write a little program in matlab/octave,
> run a bunch of samples, and FFT it. That way you can model things like
> error feedback or truncation effects.

What I do is to write a little C program that keeps accumulating away.
Whenever the wrapped value becomes lower than the previous value I have
a cycle completed. Just print the values of each such phase-wrap and you
will readily see the phase movements. You will also notice that this is
a number of phase movements overlayed ontop of each other. Each such
overlay structure has an integer number of cycles over the DDS period.
It really takes very little time to break them out and get their
relative amplitudes using this method. This is a healthy exercise to do
to learn what DDS phase noise can result in.

Thinking a bit more about it, you realize that you can see the same
pattern as being the result of a sawtooth being wrapped up on itself
around the nyquist frequencies. Pondering some more over it you realize
that there is DDS frequencies in which these overtones will wrap on top
of each other in the wrapped spectrum, This condition will avoid
close-in spurs, so an analogue clean-up has a fair chance of doing
something with them. It also becomes clear that small shifts can produce
larger shifts of spurs...

Having a fixed M/N situation where N=2^n (common for many DDSes) will
limit the frequencies with the "good" condition of spurs being far away,
but shifting to a variable N improve things in this context, but makes
some control aspects messy.

A problem with using a 2^(n-1) < N < 2^n is that the phase-sequence will
run short compared to the usual binary bits taken from the top of the
DDS, so this will needs to be handled. It takes some extra tricks to
avoid the phase-jump and hence sawtooth phase modulation.

The good thing about DDSes is that you with very cheap hardware can get
a linear scale over a large frequency range. It will not necesserily
align up to the intended frequency (as dialed in). More elaborate
schemes will reduce these effects.

... and that was prior to do phase quantization prior to sine shaping.

Cheers,
Magnus

```