[sdiy] PLL's in synthesizers

[ASSI]

On Saturday 18 October 2003 02:36, James Patchell wrote:
> A few memebers have used PLLs in various projects...I personally, I
> have not found them to be all that useful for doing any synthesizer
> projects...which is why I suspect you don't see them mentioned much.

I was about to agree and then had a "wait a minute" moment...

A stock PLL consists of a phase-frequency discriminator that compares 
an input signal with the VCO signal in some way, a (linear) VCO and a 
loop filter. The output of the PFD goes to the loop filter and the 
filtered signal controls the VCO. This general topology can be used to 
lock phase, frequency or delay of input and output signal with some 
modifications to the detector and loop filter. Typical applications are 
clock recovery and deskewing, frequency synthesis from a reference and 
FM demodulation.

On the other hand, this seemingly simple structure can be bent in many 
ways. Replace the PFD with something else, like or-ed together zero 
detectors for input and output. Sync the VCO to some other input, or 
see what happens if you use the exponential input instead of the 
linear. Throw a monkey wrench in the loop filter path so that the loop 
becomes instable or cannot lock. Use a BPF/HPF instead of an LPF or 
perhaps a voltage controlled filter? Make wider loops over some more 
filters and/or VCO.

The best thing is, you probably already have everything you need if you 
have a modular synth and one can whack up some digital mayhem for the 
PFD pretty easily from a few CMOS gates.


Achim.

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