Re: DDS question: why sine lookup?

On 8/05/2025 1:31 am, Phil Hobbs wrote:

On 2025-05-07 09:55, john larkin wrote:

On Tue, 6 May 2025 16:46:16 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
>

On 2025-05-06 15:00, Jeroen Belleman wrote:

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

<snip>

Once you have a lookup table, a sine is as easy as anything else, and
minimizes the demands on the DAC, filters and amplifiers.

>
I'm Spicing things and what I'm seeing in the FFT of my DAC output,
with the sawtooth, is giant subharmonics at some magic frequencies.
Those contribute the most period jitter.

 Right, those are the nasty ones I'm talking about.  They're smaller with a sine output, because the nasty tall spike (which contributes power quadratically) isn't there.  They're closer in and much smaller with higher resolution DACs, and go away entirely when the DAC has the full resolution of the accumulator, because what you have then is a correctly sampled sine wave.

Seems unlikely to be true. A DAC synthesises a staircase approximation to a sine wave, where the individual steps can be seen as sawtooth elements added onto the sine wave. The imperfections of the DAC make each little sawtooth a bit different from the next in amplitude, and the fact that the sine wave has a variable slope means that each sawtooth element has a different period.
They can't go away entirely. You can filter them out pretty effectively
<snip>
--
Bill Sloman, Sydney