PLL Frequency multiplier module
don at till.com
Sat Aug 26 17:10:20 CEST 1995
[Trying again; I have no idea what happened the first time. You know
how these wacky computers can be...]
Date: Fri, 25 Aug 1995 09:40:00 -0700 (PDT)
A question: Usually PPLs like symmetrical squares at their inputs.
How does it work and sound with narrow pulses? Any difference to
Check out the 4046 data sheet... this PLL has two phase comparators to
choose from. Phase comparator 1 is a regular old xor gate, while
phase comparator 2 is a little state machine and is able to lock over
a much larger range. It only cares about the low to high transitions
of the input signal, so any pulse width will work fine.
(A setup to switch between the two phase comparators would also be
cool. The xor gate has a limited range and will happily lock onto
nearby harmonics of the input signal. Tell me you can't find a use
But if you're gonna go to all this trouble, it only makes sense to
provide more output frequencies. Instead of just a x2 or a x3 output,
place a divider chain in there and generate a whole bunch of harmonics
simultaneously and put them to use. For example, use a multi-output
divide-by-12 arrangment. The following setup will give you six
seperate harmonic frequencies with a vco running at 12 times the input
frequency. Brackets  are flip-flop dividers.
vco --+--> [1/3] --+--> [1/2] --+--> [1/2] --+--> to phase detector
| | | |
+--> 12x +--> 4x +--> 2x +--> 1x
+---[1/2] --+--> [1/2] --> 3x
And of course if you go to that kind of trouble, subharmonics are a
must. You'll want 1/2 x, 1/4 x, 1/8 x, 3/2 x, etc.
(Note: To be really proper, you'll want a divide-by-three counter that
puts out a square wave.)
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