Re: DDS, again

On Fri, 13 Dec 2024 09:09:57 -0800, john larkin <JL@gct.com> wrote:

On Fri, 13 Dec 2024 21:17:13 +1100, Chris Jones
<lugnut808@spam.yahoo.com> wrote:
>

On 11/12/2024 9:31 pm, john larkin wrote:

The "RF" mindset, that one has to do the sine and dac and hi-rent
lowpass filter and comparator, is not only a bunch of work and
expense, it gets really nasty at low frequencies.

>
Then don't do it at low frequencies. Do it at 2^N times the low
frequency that you want, and then after the comparator divide by 2^N.

>
I think that can be done, but very carefully. We need long-term phase
coherence to simulate a complex geared machine, and we need frequency
changes to happen at a selected phase angle, like top-dead-center of
some waveform of some part of a mechanism.
>
Using the MSB of the phase accumulator is sure appealing. Going
off-chip to a DAC and a filter and a comparator adds time lag and
jitter.
>
Most DDSs are RF oriented so don't care about time. Maybe some exotic
radars would.

Big radars in general care about time and phase a lot, not to mention
phase noise.  And long-term stability over time.

Joe Gwinn