[sdiy] Need help with simple VCO design

Julian Bunn Julian.Bunn at caltech.edu
Sun Dec 27 03:53:47 CET 2009


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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