AW: AW: more frequency tracking..

[Magnus Danielson]

>>>>> "HJ" == Haible Juergen <Juergen.Haible at nbgm.siemens.de> writes:

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

 HJ> Ahh, I remember this one. Interesting !

Yeah.

 >> 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.

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

Hmm... interesting. However, I think that this kind of PLL does
however make very abrupt jumps in their phase and thus frequency each
time that they compensate. Such jumps may not allways be advantagous.
There is even cases where such jumps is nothing but catastrophic.
I can't help it, but for some reason I like smooth curves :)

But as allways, look at each and every case and find what are the
limits for this applications and then find the solution for that
application.

 HJ> (My prof liked the mathematical proof better, but the lazy guy
 HJ> that I am, I prefer graphical deduction.)

But then, I have a friends doctorate paper at home, and it is such a
mathematical proof of the whole thing that most people in the
buissness does not understand it. Let's just say that his family stood
no chance of understanding basically anything between Wellcome and
Thanks for comming on his defence. I know that I didn't get much anyway.

Cheers,
Magnus