[sdiy] Hopf bifurcation VCOs

[jhaible at debitel.net]

> I think it is OK to just regulate just x^2, because then the 
> correct y must follow because of the integration!  In fact, I didn't 
> mention it yesterday, but I have found that only one of the NL circuits is 
> needed. Same reason.

But isn't THD at low frequencies much better if you use both terms?

(I'm just speaking of the Tietze/Schenk version here, as I haven't seen your
implementation yet.)

Normally, when you control the loop gain of a sine VCO, you have some device
for mearuring the amplitude, which has some ripple, which will either cause
some THD in your sine wave, or - if you smooth it with long time constants -
will react slowly. So for low oscillator frequencies, it's always a tradeoff
between THD and regulation time.

Now when you have a QVCO, and use (x**2 + y**2) instead of an ordinary level
detector, you get (ideally) *no* ripple, because 
(sin(wt))**2 + (cos(wt))**2 = 1
for all t. So using two multipliers you get the amplitude information all the 
time, and not just around the peaks of either sin or cos.
That should be an advantage.

JH.

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