Re: DDS question: why sine lookup?

On 06/05/2025 16:48, 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?

A saw tooth wave has a huge step like discontinuity in it which looks very ugly in the frequency domain with strong harmonics. Strong sharp features in time domain are broad in frequency space and vice versa.
If you wanted something a bit different then detecting the phase accumulator overflow and reversing the count sense to get a triangle wave might be an option (at half the frequency). Needs some very careful maths at the boundary flips to avoid introducing jitter.
 From that triangle wave you can use HP's wizard diode shaping network trick to get a pretty good clean sine wave.
--
Martin Brown