[sdiy] NCO Jitter (was Large Numbers)

[Vinicius Brazil]

Hi Tim,

My NCO´s operates at a minimum of 2MHz

- Vinicius

On Sat, Jul 14, 2018 at 12:59 PM, Eric Brombaugh <ebrombaugh1 at cox.net>
wrote:

> If you oversample your NCOs by a factor of 20 from a final output rate
> of 48kHz that means they're running at approx 1MSPS. If the NCOs are
> simply generating naive sawtooth waveforms that implies that the
> audible aliased harmonics of a 1kHz note will be roughly the 90th
> harmonic, at which point the amplitude is 1/90 which is about 40dB down
> - not inaudible but certainly not in your face either.
>
> Don't forget that when you oversample your NCOs by a factor of 20
> you'll need to use bandlimited resampling to decimate them down to the
> final 48kHz output rate. This will require a decent filter to prevent
> the unwanted high frequency content from aliasing back into the audible
> range. Ideally a FIR filter with flat response from DC to 20kHz or so,
> but you can get away with simpler polynomial or CIC structures and
> still have decent sounding results.
>
> Eric
>
> On Sat, 2018-07-14 at 08:41 -0700, Tim Ressel wrote:
> > So how much overclocking on the an NCO is needed to reduce the jitter
> > to
> > an acceptable level? I will be sampling at 48K. If I run the NCOs at
> > 20x
> > that rate, is that good enough?  Keeping in mind I have 10 NCOs to
> > operate and a proc running at 180 MHz.
> >
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