AW: AW: more frequency tracking..

[Haible Juergen]

	> it is a 4-quadrant multiplier or something, thus a classic phase
detector.

Ahh, I remember this one. Interesting !

>  >HJ> BTW, it can be shown that the schmitt trigger / integrator
	> HJ> triangle / square oscillator that is widely used in LFOs will
	> HJ> act as 1st order zero crossing PLL when you add one more
resistor
>  >HJ> to the integrator input summing node and use this to feed in your
>  >HJ> signal. The ratio between the two resistors will then set the
>  >HJ> hold range of the PLL. I mention this, because I remember seeing
	> HJ> a similar circuit in Buchla's famous noise and random module,
>  >HJ> but I may be wrong here. (Have to look again.)
	>
	>This is interesting. I will try to meditate a bit over it, but I
think
	>I can see how the schmitt-trigger can become a phase comparator.

Think of an XOR gate as phase dedector for a start. The phase
difference output is a PWM-ed pulse CV (as there is no loop 
filter) instead of an analogue voltage.
To make a PWM-controlled VCO from the basic oscillator, you need
to change direction of the charge / discharge operation of the
capacitor, depending on the current state of the oscillator.
So the whole PLL uses (theoretically) one XOR for the PD,
and another XOR (in series) for the VCO function, with the second
inputs of both XOR gates connected to the VCO output.
These two XOR fuctions cancel, of course, and the result is that
it reduces to a simple resistor. If you draw it to a piece of
paper, it's quite obvious.
(My prof liked the mathematical proof better, but the lazy guy
that I am, I prefer graphical deduction.)

	JH.