[sdiy] Flangelicious noises - some queries about NCO jitter and resampling and similar

[Magnus Danielson]

Hi, René,

On 2020-01-28 14:07, René Schmitz wrote:
> Hi Richie,
>
> On 28.01.2020 13:23, rburnett at richieburnett.co.uk wrote:
> >>> What I don’t understand though is how this helps. Increasing the
> >>> output frequency is going to increase aliasing too, and dividing it
> >>> down again afterwards doesn’t seem to remove that to me. How does
> >>> this work please, René?
> >>
> >>  It's a standard trick. With higher DDS clock you get smaller
> >> time-errors, and then the divide down just removes transitions between
> >> the transitions. Higher synthesized frequency allows you to use more
> >> of the upper bits of the DDS, to achieve more effective bits useable
> >> in the DDS.
> >
> > Ok, so this allows you to make better use of the available DDS
> frequency
> > bit resolution, by synthesising a higher output frequency and then
> > dividing it down.  But it doesn't do anything to help with
> > jitter/aliasing at the top end.
>
> Here are your original points:
> 1) Resampling of the LFO output by the NCO reset
> 2) Frequency stepping caused by the NCO minimum frequency step
> 3) Jitter cycles caused by the NCO
>
> Well it helps updating your NCO more frequently, because each cycle is
> shorter now. I.e. higher sampling rate of your LFO.
>
> The NCOs minimum frequency step is reduced. Because the tuning word
> effectively shifts right, bringing in room for more LSBs.
>
> The cycle to cycle time variation (aka jitter) is still 1/f(clock) of
> your NCO. Not really an improvement there by the division, if you run
> the NCO at the same frequency as before. But if you can run the NCO
> clock higher then the variation is reduced.

But this is only because one uses only 1 bit output to represent the
phase. To get better you would use 10-12 bits or more out of the phase
accumulator to represent the phase, then sine-transform it. This is what
the modern DDS chips do and you get significantly better jitter then. If
you do not do it, your only way to improve things is to increase the DDS
clock, which will help even if better phase resolution is used. There is
a few other tweaks one can do, but usually that is not available or easy
for most folks.

Cheers,
Magnus