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

From: John Larkin (jjSNIPlarkin_at_highTHISlandPLEASEtechnology.XXX)
Date: 01/30/05 

Date: Sun, 30 Jan 2005 15:23:04 -0800

On Sun, 30 Jan 2005 14:47:13 -0800, mike <spamme0@netscape.net> 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.
>
>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

I can't see the pic, but a raw DDS output does indeed look nasty, and
gets worse as you approach Fclk/2, the Nyquist frequency. But if you
run it through a lowpass filter, you get a nice sine wave with
respectably low jitter. The Sampling Theorem says so, and it works.

We build an arbitrary waveform generator that includes four DDS clock
sources. The crystal oscillator freq is 40 MHz, and we synthesize
wavegen clocks up to 15 MHz. The DDS chip output feeds a 4-pole LC
filter and a schmitt trigger gate and makes a pretty nice clock. In
retrospect, we might have used a better filter, elliptical maybe, and
got a bit less jitter, but it's not bad. The filters were tweaked to
peak at 15 MHz to compensate for losses and sinc distortion. I posted
a graph of DDS jitter vs frequency to a.b.s.e. a while back.

If the output frequency is, say 5:1 or 10:1 below the clock frequency,
the filter is easy and jitter will be very low. The rub is that at
very low frequencies the filter essentially disappears and you're left
with the dds dac steps re-emerging, so jitter trends toward some fixed
fraction of the output period, 1/10,000 maybe, depending on the number
of dac bits. In that case, one can switch filters, or keep the dds
output up where the filter's still effective, and divide the schmitt
output digitally to get a low-jitter lf clock.

I haven't seen the lf modulation you refer to. Very close to Nyquist,
you'll get your desired signal and its image flipped about Fclk/2,
which would take a brickwall filter to separate.

Really, these things are great! But without a filter, they're junk.

It is interesting that the Analog Devices datasheets used to show
typical filters, and now they don't. And their eval boards used to
include filters, and now don't. It's almost as if they're pretending
that you don't need a filter.

John