[sdiy] Hopf bifurcation VCOs

[Ian Fritz]

Here are some more results on the new QVCO.

Distortion is low throughout the 20Hz - 10kHz range (second harmonic 50dB 
down).  This is all the frequency range I can measure going through my 
sound card.

The oscillator runs easily to beyond 30kHz.  At that frequency there is 
some increase in amplitude, probably due to opamp phase shifts.

At the low end, I checked operation at 0.5Hz.  Here there is an interesting 
issue.  On power-up it takes the system about 2 minutes to build up to full 
amplitude with both nonlinear circuits in place, and about twice that with 
just one, as in the posted schematic. However, once the system is running 
it changes frequency rapidly when the control current is changed, probably 
because the quadrature phase relation is already established.  So I think 
it can be swept OK.

The low frequency buildup can be reduced to about 15 sec by reducing the 
feedback resistor from 1 MOhm to 100kOhm.  This increases the distortion a 
bit, with the second harmonic now down about 30dB.

Again, this issue seems to only be problematic if the circuit is powered up 
at very low freequencies.  It should be fairly simple to make a kickstart 
circuit to ensure things get running at power up.

   Ian