DDS issues...was sine generator ic solution

From: mike (spamme0_at_netscape.net)
Date: 01/30/05 

Date: Sun, 30 Jan 2005 14:47:13 -0800

DDS issues???

The concept of DDS frequency synthesizers comes up frequently.
I can't figure out what they're good for.
I took apart the prototype hardware, so you'll have to be
content with an Excel simulation.
Here's a plot of a DDS simulation.

http://nm7u.tripod.com/homepage/sine.jpg

If you looked at it on a real-time scope, you'd likely
be impressed. All the points lie exactly on the sine wave,
(within D/A resolution)
so statistically it's a real sinewave. Averaged over time
the AVERAGE frequency can be very precise.

But if you look at it on a storage scope, you can see that
each cycle is different. And the difference can change dramatically for
small changes in frequency. The graph shows how for some frequencies, the
output is amplitude modulated at a much lower frequency. You can't
take that out with a low-pass filter.

The graph is deceptive cause it
linearly interpolates the points. In actuality, there's a big
ole step at each point. This becomes painfully clear if you try to use
a comparator to generate a square wave. Or if you try to DDS anything
other than a sine wave.

Yes, if you filter it enough, you can make anything into
a sinewave. And if your hardware is a few orders of magnitude
faster than your ouput requirement, the filter is easier.

What am I missing that makes DDS useful in any time-domain application
or wideband frequency-domain application?
mike 

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Relevant Pages

  • Re: DDS issues...was sine generator ic solution
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