[sdiy] very simple vco

[Scott Gravenhorst]

Paul Perry <pfperry at melbpc.org.au> wrote:
>Tony Algggod says:
>>Check out the 74HC4046 chip too. Nice little square wave VCO for
>>pennies. 'Linear' repsonse though.
>
>True.. can anyone see how t\to get an approximate saw from it?
>There is a pretty hopeful sort of irregular waveform from some part of it
>from memory, but I couldn't wrestle it to the ground.. any ideas??
>
>paul perry (Melbourne Australia, scattered showers)

Paul,

A guy who used to hang out here, Bjorn Julin (AKA Bjorn Nillson, AKA
several nasty names, some of you may remember this colorful individual)
did a schematic for this.  

At one point in time, I had played with this (actually a 4046) and found a
really good data sheet that explained in detail how the internal VCO
worked.  There's a flip flop arrangement in there that alternately causes
one side of the integrator cap to be grounded and charge current it sent
to the other side.  So if you look at one side of the cap to ground with a
scope, you do get a strange waveform.  Bjorn used a differential
arrangement of 3 JFET input opamps to produce sawtooth by watching both
pins of the capacitor.  I think that it might be easy to make it produce
triangle as well.

I was originally trying to make a musically useful VCO with this and never
quite got there.  The spec sheet says the VCO is linear...  My attempt to
add a linear current source didn't work very well, but I'm not sure I did
it correctly or if this is even the right approach to improve it's linearity.

I still want to investigate this thing as a linear VCO.

I have posted the PDF to my website temporarily at:

http://home1.gte.net/res0658s/4046saw.pdf

I'll leave it there for a week.



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