[sdiy] Hopf bifurcation VCOs

[JH.]

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

As long as both outputs increase in the same way, it's ok for a
frequency shifter.

>
> 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.

This is good news!

> It should be fairly simple to make a kickstart
> circuit to ensure things get running at power up.

Yes, either force a high frequency on power-on, or a stronger nonlinearity,
or maybe even both.

JH.