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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design,comp.dsp
Subject: Re: DDS question: why sine lookup?
Date: Thu, 8 May 2025 02:11:43 +1000
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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