Path: ...!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: Jeroen Belleman <jeroen@nospam.please>
Newsgroups: sci.electronics.design,comp.dsp
Subject: Re: DDS question: why sine lookup?
Date: Tue, 6 May 2025 21:00:33 +0200
Organization: A noiseless patient Spider
Lines: 29
Message-ID: <vvdm8h$3i2ju$2@dont-email.me>
References: <o3ak1k9ifikv6c1tmfnd89k6vfj4vigj37@4ax.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Injection-Date: Tue, 06 May 2025 21:00:34 +0200 (CEST)
Injection-Info: dont-email.me; posting-host="ea3ae17dc55693da09f73fc2a16d9888";
	logging-data="3738238"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX19MvvLvwcrObVfxX44x0O8k"
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.13.0
Cancel-Lock: sha1:8wHIdRjO69mfDah7XvrvU2v6+Ek=
In-Reply-To: <o3ak1k9ifikv6c1tmfnd89k6vfj4vigj37@4ax.com>
Content-Language: en-US, fr-FR, nl-NL
Bytes: 2284

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