[sdiy] Clock generation

[Tom Arnold]

On Mon, Mar 13, 2006 at 05:10:36PM -0700, Eric Brombaugh wrote:
> * What are you using for a VCO? I've always used the CD4046 for that and 
> it seems to top out around 1-2MHz. The 74HC4046 is good to 10MHz though 
> and is a pin-for-pin replacement for Vdd<6V.

I've only played with a 555 based VCO.  Seems to top out around 4mhz which
is more then high enough.  I've had a little trouble getting two to track
cleanly over that sort of range which could be a problem.

> * What switched cap filter are you using? By choosing the right resistor 
> ratios you can alter the Fclk/Fc ratio on some of them. That way you 
> don't need to operate the filter at 50x the BBD clock.

For various reasons I'm using ones from Mixed Signal.  They offer 50:1 and
100:1 ratios. 7pole.  Good price.  Good people to deal with.

> * You could use 1 VCO to generate the BBD clock over the full 
> 10kHz-100kHz range and a 4046 PLL to generate 50x clock for the 
> switched-cap filter from that rate.

I didnt consider using a PLL because of the tracking slop, but I suppose I
should since we're talking about fairly slow rates of change it wouldnt be
bad at all.

One thing I have pondered would be using a microcontroller to generate the
clock.  The code would be trivial, the two clocks would track each other
dead-on.  The parts count would be low.  Use an A/D to track the inbound CV
and set the timing loop.   Might not be a bad way to do it.

-- 
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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?" 
 - echo evho wjxo   -    Juanita Shrugs. "What's the difference?" 
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