more frequency tracking..
magnus at analogue.org
Thu Jul 2 01:20:58 CEST 1998
>>>>> "ED" == Ethan Duni <eduni at ucsd.edu> writes:
This is a little late reply (posted in 16/6) but I have been out of
luck with my Internet line so...
ED> anyone have any thoughts on using a PLL to extract a clean sine from an
ED> arbitrary signal and then running that across a series RC, tapping off of
ED> one of the components then running that to a buffer and then rectifying and
ED> filtering to get a frequency tracking CV? how large a delay is usually
ED> introduced in a PLL before the VCO matches the input frequency? which PLL's
ED> have the lowest delay?
The idea of using PLL to have a VCO track along isn't new, the Moog 921
oscillators have a PLL mode.
I am not really sure that I get your idea of what you want to do, but
I can say something about PLLs. First of all is there a wide class of
PLLs each having their usefull properties, they can have diffrent lock
range, track range etc. The actual lock time wil vary greatly with
factors such as which phase detector and which loop filter is being
used just as well as which input signal is you trying to lock. There
are signal situations that a PLL may chase forever... if your signal
will have a too step frequency change per time unit may a PLL not be
able to lock it or even track it. You can unlock a PLL with a quick
chirp or frequency step.
The type of filter will also greatly change these properties.
A PLL may be of diffrent degrees, depending on how many poles the
system contains. The simple rule for counting degree is that there is
one pole in the VCO and all the others is in the loop filter.
A first degree is pretty useless for most applications, it has no loop
filter and will make abrupt jumps and has no memory what so
ever. Whenever there is a phase difference it will cause a jump on the
output, not very neat.
A second degree PLL is very common, the loop filter is usually either
a RC-lowpass or a RCR-lowpass lag filter. There is many good books
describing these. The second degree PLLs can do stuff like frequency
multiplication, tracking, FM-demoduling, phase-jitter swallowing etc.
The properties is also governed by the phase comparator used, you may
use 4-quadrant multipliers, XOR gate, state-machine (the 4046) or
JK-flipflop phasedetectors, each with their own property. The
selection of the phase detector may be as crusuial as correctly
dimensioning the loop filter.
Higher degrees PLLs are uncommon but have their own right in various
applications (like moving and accelerating GPS receivers).
Dimensioning a PLL can be a hairpulling experience until one get the
basics straight... but it does not need to be too hard to get up and
playing. Take a 4046 (or a 74HC4046) build a simple RC filter with a
tau of say 20 ms to start with, hook the 4046 up according to the
datasheet, use either of the phase detectors (the 74HC4046 has a
third, a JK-flipflop) and start of with the internal VCO set for audio
range and start playing with it. The simple RC filter will have a
large locking range.
The 4046 is a very simple and easy to use standard PLL for a low cost,
it is a great tool to start fooling around on and you may dump the VCO
on it if you feel like it.
There is a neat little thing in it worth using, the loopfilter output
is being feed to the input of the VCO which starts of with a MOSFET
follower that has an input impedance at about 1 Tohm and a output
impedance way down, so you will not need to worry much about how you
load your passive filter or even much on how you select components.
You may also have an output being taped wiether or not you are using
the internal VCO. When you use the 4046 as a FM demodulator you tap
the output signal at the output of this buffer, most usefull.
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