[sdiy] Clock generation

[Tom Arnold]

On Mon, Mar 13, 2006 at 07:13:00PM -0700, Eric Brombaugh wrote:
> One thing you shouldn't forget is Nyquist! BBDs (and switched cap 
> filters for that matter) are sampled devices. The filter has a fixed 
> ratio between the corner frequency and the clock frequency. The corner 
> frequency should be set to 1/2 (or less) of the BBD clock rate. This 
> means that if you choose a filter with a 50:1 Fclk/Fc ratio, you should 
> only run it 25x the BBD frequency to avoid passing frequencies greater 
> than the BBD Nyquist rate.

Max corner frequency of the units I have is 20khz.  I hadnt fully considered
where the filter clock would be to hit that corner.  One thing to consider
is that the BBD clock is two phase, so for every clock "cycle" there are two
shifts, so you can safely consider a standard clock as the 1/2 sampling rate
frequency.

> That's attractive from a 'gee-whiz' standpoint, but I'd be really 
> careful about driving your audio sampling clocks from a microprocessor. 
> The processor can't generate accurate jitter-free clocks since it can 
> only create edges at integer multiples of its own clock period. That 
> means your BBD and switched cap clocks will be jittering like crazy, 
> introducing all kinds of phase distortion into the audio signal. That 
> might be interesting/useful in some situations, but less than ideal for 
> most applications.

Yet something else I hadnt fully considered. *sigh*
Think I'll play with a PLL when I get home from work tonight.
I havent gotten an answer from Mixed Sig yet on behavior when the clock rate
is much higher then max.  I'm hoping its tolerable.

-- 
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 - Tom Arnold       -  Free Synth DIY webspace http://www.sdiy.org 
 - SynthGeek        -
 - BBD Fanatic      -  "...is it a virus, a drug, or a religion?" 
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