Hello Alberto, hello all,
I thought again and, yes, the high order bit of the phase accumulator is not
the squared up version of the sine output, but, after the phase accumulator,
a DDS has a sine lookup ROM. What I really meant was the high order
bit of the output of this ROM. Using this bit, I feel confident to state again
that you don't need a DAC to generate a square wave of the wanted frequency.
With the AD9850 you have to use the comparator, as you do not have external
access to the output of the sine ROM.
You are right saying that a PIC probably doesn't have enough horse power
for this task. Maybe some chips of the Scenic family ???
The problem is the discrete time the DDS uses. The high order bit
only changes at the clock edges. The result is a square wave with
heavy phase jitter. For example, draw a sine wave (3 waves),
sample it at about 8/3 of its frequency in your sketch, and generate
the square wave from these sample values (all on the paper).
The one-periods and the zero-periods then are one or two clock cycles
long. That's what Andy in other words already said.
If a DAC is used then time is continuous and a clean square wave can
be obtained.
73 de Klaus, DJ5HG
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