I'm looking at using a Direct Digital Synthesizer to generate the local
oscillator for an FM receiver. I'm considering using a D FF as the mixer.
The output frequency would be the difference between the frequencies on
the clock and D inputs. I have an Analog Devices DDS evaluation kit. It
allows you to get a square, sawtooth, or sine wave output. When using the
square wave output, which is the msb of the phase accumulator, I can see a
lot of jitter in the output if the output frequency is not an integer
fraction of the clock frequency. This makes sense, since the addition of
the next step in the phase accumulation will have a somewhat random
relationship to the msb rollover if there is not an integer relationship.
The sine output does not show this jitter, since when the msb of the phase
accumulator changes, the sine output doesn't just change state but,
instead, goes to some appropriate value on the sine wave.

So, the question is what to drive the mixer with. If I drive it with the
square wave output, the jitter will frequency modulate the output. But, if
the spectrum of the jitter is far above the modulating frequencies, it can
be removed by the low pass filter in the demodulator.

Another, more complex, approach would be to take the sine wave and change
it to a square wave using a comparator. That would also introduce some
jitter, but not as bad as I'm seeing on the square wave.

Another approach would be to use an analog mixer instead of the D FF. I
could just use an analog multiplier to do the mix, then filter out the sum
frequency, leaving just the difference.

So... Does anyone have a feal for the spectrum of the jitter on the square
wave output of a DDS synthesizer? This is running on a 25MHz clock, and
I'm generating frequencies of 2.95MHz and 3.45MHz.

THANKS!

Harold



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