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

From: Mac (foo_at_bar.net)
Date: 01/31/05 

Date: Mon, 31 Jan 2005 05:27:04 GMT

On Sun, 30 Jan 2005 14:47:13 -0800, mike wrote:

> 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.

They are very good for high frequency resolution sine wave generation.
They are very good for generating extremely linear frequency ramps
(chirps). They are very good for exercising fine control over the phase of
a sinusoid.

When filtered appropriately, they produce good sine waves with reasonably
low phase noise, and relatively low (and predictable) spurs.

>
> 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.

Not just precise, but dead on (as long as your input clock is dead on).

>
> 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.
>

It's not really modulation. It is just a sampling issue. Again, if you
stay away from Nyquist, these chips do a good job. Within the Nyquist
region, there are spurs, but this is often tolerable. Especially if the
spurs are down by 45 dB.

> 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.
>

If you stay far away from Nyquist, the DDS's are great. If you get near
Nyquist, and don't mind a serious filter, DDS's are great.

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

--Mac

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