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From: Martin Brown <'''newspam'''@nonad.co.uk>
Newsgroups: sci.electronics.design,comp.dsp
Subject: Re: DDS question: why sine lookup?
Date: Wed, 7 May 2025 20:32:41 +0100
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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