----- Original Message -----
From: Harold M Hallikainen <haroldhallikainen@JUNO.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Thursday, October 21, 1999 5:17 AM
Subject: Re: PROGRAMMABLE OSCILLATORS


>         Fox Electronics has programmable oscillator modules.  Some are
> programmed at the factory and others can be loaded serially on the fly.
> In addition, of course, there are the Analog Devices DDS synthesizer
> chips.

What a hideously expensive solution ! A DDS to clock a 2 bit micro - this is
a joke right?

When I was a lad there was such a thing as a Phase Locked Loop.  This came
from the 4000 series CMOS range and was called 4046.  Add a crystal,
programmable divider, loop filter and you're away.  You might even want to
control it with a PIC.

DDS is a relative new comer on the frequency synthesis scene - its been done
with PLL's for donkey's years.  They (DDS) have exceptional resolution and
tuning times and are widely used in comms and frequency agile systems.

However, as with all things digital (read sampled), Nyquist comes into play.
EG at multiples of the clock frequency, the fundamental output is *zero* (it
follows sinx/x envelope).   Aliasing means that the max o/p freq will be
clk/2.   Granted that for clock generation, you may be able to tolerate high
levels of spurious signals, but this is also a drawback of DDS synthesis.

It makes me shudder every time I hear some-one *casually* suggest the use of
DDS.  Consider these points:

1:    DDS is expensive (relatively)
2:    DDS requires high clocking to avoid aliasing
3:    DDS generates high levels of close in spurious
4:    DDS output follows sinx/x envelope and is *not* flat

I would refer those interested to "Digital Techniques in Frequency
Synthesis" -Goldberg  ISBN 007024166