[sdiy] Current driving a CD4046

[Scott Gravenhorst]

Grant Richter <grichter at asapnet.net> wrote:
>Just as a note of interest.
>
>I needed a very wide range VCO for playing the 256 step pages of the  
>Envelooper MARF.
>
>I knew a CD4046 would run 256 kHz at 5 volts which I needed to play  
>the 256 steps in one millisecond.
>
>I hooked pin 9 to +2.5 volts through a voltage divider and ran a  
>regular NPN exponential current source through a 10K to pin 11 (max  
>frequency). It produces a square wave from 15 Hz to 280 kHz with a  
>100 pF timing cap.
>
>Since I don't need perfect 1 volt per octave tracking (just close  
>enough to half / double rhythms), I didn't test for precise 1 volt  
>per octave pitch performance.
>
>With more work it might work well enough to drive a counter for a  
>wavetable VCO, a Walsh generator or something else that needs an RF  
>clock at 5 volts.

I've played in this sandbox...  I got some help from a colorful character who went
by several names, one was Bjorn Julin.  He had a circuit that is basically what you
describe but without the expo so it was linear (or it was supposed to be).  He
claimed that it was linear, but I tried several times and couldn't get it linear
enough for music even over a small range (3 octaves was too much to hope for).  I
hope you have better luck than I did.  However, if that's not important, it
certainly is voltage controlled and should work fine as a sequencer clock.  The
CD4046 is spec'd at 1.3 MHz while the 74HC4046 is spec'd at 17 MHz.

Unless you can get more linearity out of it than I did, I'm afraid it won't be much
use as a wavetable VCO or Walsh clock.  Also, IMHO, if you want Walsh, go for an FPGA.




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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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