[sdiy] 4069 sawtooth (René Schmitz) VCO

[Scott Gravenhorst]

I decided to play with this today and I have some
questions (about my results).

First, I just built the oscillator itself without
the PWM stuff.  Instead of the current sink
transistor, I put in a 10 K resistor.  This was
just to test the oscillator.  I got a sawtooth
waveform with a spike at the reset point.  I 
didn't care about the spike, but I noticed that
with the resistor as a current sink, the ramp was
*straight*, not humped as I would have expected
using a resistor and not a constant current sink.

So then I decided to add a linear V/I converter,
(opamp with NPN darlington B-E in the opamp
negative feedback loop).  What happened totally
baffles me.  Although the ciruit's frequency is
controllable, it isn't quite linear.  Something
near linear, but not *on* it.  This is probably
because now, my ramp is no longer straight!  I
understand why a ramp that is not straight will
not track linearly, what I do not understand
is why the ramp is not linear using this current
sink.

Any help?   René?

If I had an expo synth, I'd try René's current
sink, but that will do me no good since I need
to run this in linear mode.

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