[sdiy] Need help with simple VCO design

[Julian Bunn]

Thanks Harry. I'm not sure I understand the distinction between a true VCO and
a current controlled VCO (CCO?). Certainly I am limited on the input to a voltage
swing between 6 and 12V. I should make some more measurements on the real circuit
to confirm my Spice model.

Julian

Harry Bissell wrote:
> 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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