[sdiy] Re: [sdiy] 4069 sawtooth (René Schmitz) VCO

[René Schmitz]

At 18:52 28.10.01, you wrote:
>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.

Thats because the input of the integrator gate is held
at a pretty much fixed voltage. So when you put that 
10k at another fixed voltage (like GND), a constant 
current flows. Its an integrator. 

In theory that could already be used for linear 
control. But in practice the control voltage would 
need to be shifted to the voltage of the gate.

Btw, my oscillator has that spike too, the unconventional 
reset scheme does that.

Oh, you should really add the PWM stuff, because 
this circuit has a nice floppy square that somehow 
reminds me of the 303 square.

>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.

I have a suspicion, have you been monitoring at the 
circuits output?! If so, then the cap at the output might 
be responsible for the no longer straight ramp (at lower 
frequencies anyway. I figure that using the 10k the 
frequency was rather high, so you might not have 
encountered the highpass action of the output cap.) 
The mistracking could be due to the current that the 
base robs, the control currents are rather tiny, and a 
ever so small base current for the darlington could 
disturb the linearity. You could try a n-JFET instead of 
the darlington.

Just wildly guessing of course, otherwise I'll be as baffled as 
you, and need to try this my self.

Cheers,
 René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159