Re: DDS question: why sine lookup?

On 5/6/25 17:48, john larkin wrote:

A DDS clock generator uses an NCO (a phase accumulator) and takes some
number of MSBs, maps through a sine lookup table, drives a DAC and a
lowpass filter and finally a comparator. The DAC output gets pretty
ratty near Nyquist, and the filter smooths out and interpolates the
steps and reduces jitter.
 But why do the sine lookup? Why not use the phase accumulator MSBs
directly and get a sawtooth, and filter that?
 The lowpass filter looks backwards in time for a bunch of ugly samples
to average into a straight line. The older sine samples are the wrong
polarity! If the filter impulse response is basically zero over the
period of the sawtooth, and we compare near the peak, we'll average a
lot of steps and forget the big sawtooth reset. [...]

Two things are immediately obvious: First, the sawtooth will have
a variable frequency, and the filter won't have a zero response
for all possible frequencies.
Second, the usual reconstruction filters do *not* interpolate
into straight lines.
Beyond that, I would have to think this over a bit more.
Jeroen Belleman