Bruce Griffiths wrote:
Magnus Danielson wrote:
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
can find a link
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 AD9913 (when operating in the variable modulus mode) merely adds
an extra 1 (lsb) to the main accumulator whenever the auxiliary
(variable modulus) accumulator overflows.
Thus to avoid phase modulation of the output the combined result of
the 2 accumulators has to result in the main accumulator always having
zeros in the lower b-p bits where p is the width of the truncated
phase word and b is the width of the main accumulator.
More precisely the phase truncation error must be constant (ie the phase
truncation error, the least significant b-p bits of the main accumulator
should always have the same value).
This requires that the auxiliary accumulator overflow every cycle and
the ls p-b bits of the main frequency tuning word have the value 2^(p-b) -1.
ie using the auxiliary accumulator doesnt add any extra phase truncation
spur free frequencies compared to those available when only the main
accumulator is used.
Thus although using the auxiliary accumulator may allow frequencies like
10MHz - 10Hz to be generated from a 100MHz DDS clock they will not be
phase truncation spur free.
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
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Bruce
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