[sdiy] Need help with simple VCO design

[Harry Bissell]

It looks like the core is a true VOLTAGE controlled oscillator.. the
resistors in the input stage are how the capacitor is charged. Most really
wide range designs are a CURRENT controlled oscillator.

Those fixed resistors mean that eventually you will need a really HUGE
input voltage to get more current in the core. The up side is that you might be able to use
really large voltages as an input, some of these true VCO designs can have as much as
(ohh...) 50V on the input. Of course your power supply limits that completely, you never see
more than maybe 12-15V.

I think the circuit might run to one rail when stalled, and it takes a long time to
get to the trip point of the schmitt trigger stage. This might be similar to how a
555 based astable oscillator works (the first cycle starts from 0V and later cycles go between
1/3 and 2/3Vcc).

Thats my guess. This is a classic design but sooner or later you run out of supply voltage.

Replacing the resistors in the input stage with a controlled current source makes it have
a much wider range but makes the rest of the core much more difficult.

My quick guesses....

H^) harry




----- Original Message -----
From: Julian Bunn <Julian.Bunn at caltech.edu>
To: Synth-diy at dropmix.xs4all.nl
Sent: Sat, 26 Dec 2009 21:53:47 -0500 (EST)
Subject: [sdiy] Need help with simple VCO design

This is the schematic of a VCO I'm playing with:

http://farm3.static.flickr.com/2488/4217027179_c41d9bedd3_o.png

It is (very) loosely based on a circuit that appeared in "Applied Ideas"
in Electronic Engineering magazine in Jan 1975.

Using a Lin voltage input of between 0 and 6V, the circuit produces
a ramp/triangle/saw (depending on the position of the "Shape"
pot) of between ten or so Hz up to around 10kHz, which is fine
for my purposes.

However, I don't get the range from the Log input: it flattens off
too quickly, and I'm not sure why. I basically just bolted on the
log current generator without really thinking too hard about it.

Another problem is that, at low input voltages, the VCO is slow
to start - it takes a perceptible time after the input voltage is
applied before the oscillations start - and this behaviour is
confirmed in Spice. The time to start is much greater than the period
of the oscillations.

I'd welcome some advice on changes/adjustments I could try to
eliminate these problems.

Thanks,
Julian
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-- 
Harry Bissell & Nora Abdullah 4eva