# phase lock loop (PLL) HELP

B

#### biras

Jan 1, 1970
0
HI
i need help for design the phase lock loop . i feel trouble in loop
filter design. so please guid me
Thanks
R.Birasanna

A

#### Andrew Holme

Jan 1, 1970
0
biras said:
HI
i need help for design the phase lock loop . i feel trouble in loop
filter design. so please guid me
Thanks
R.Birasanna

You'll have to be a bit more specific than that. How far have you got?
What don't you understand?

C

#### Chris

Jan 1, 1970
0
biras said:
HI
i need help for design the phase lock loop . i feel trouble in loop
filter design. so please guid me
Thanks
R.Birasanna

Hi, Biras. You're not the only one who "feels trouble" in loop filter
design. ;-)
From your posts in other groups the last couple of days, I'm hearing
you say you've got a 40-70 Hz input signal and a 4046. You'd like to
use the 4046 PLL to generate and lock a 256KHz to 448KHz signal.
You've got a 5VDC power supply. You're stuck on the loop filter, and
this apparently isn't a homework problem.

If you're looking for a "cookbook" answer, you should look at the end
of Chapter 7 of Don Lancaster's CMOS Cookbook, available from many
libraries as well as from amazon.com or through Mr. Lancaster's
website:

http://www.tinaja.com/

He discusses the 4046 in enough depth to allow you to get a good start,
and hack a loop filter together that's good enough. Assuming you have
a clean digital CMOS-level input signal (needn't be 50% duty cycle),
Mr. Lancaster might have you start out with the wideband phase detector
(wider range, more stable, no requirement for 50% duty cycle on either
input) and something like this (view in fixed font or M\$ Notepad):

 470pF
___ || ___
 .--|___|-. .--||--. .-|___|-.
| 10K | | || | | 15K | .---------.
 | | | | | | | |
=== 11| |6 7| |12 === | +12V |
 GND .-----o--o------o--o-----.GND | |\| |
| | '---|-\ | V(t)
 5| 200KHz-500KHz |9 | >--o---o
.-----o VCO o-------o----|+/
 | | (1/2 CD4046) | | |/|
| | | | -12V
 === '-----------o------------' |
GND 4| |
 o--------------------(-------o
| | f(out)
 .-----------o------------. | 256-448KHz
| | |
 | Divide-by-6400 | |
| Counter | |
 | (U-Do-It) | |
| | |
 '-----------o------------' |
| |
 3| |
.-----------o------------. |
 | | |
14| Wideband Phase | ___ | ___
 o----------o Detector 13 o-|___|-o-|___|--.
f(in) | (1/2 CD4046) | 1M 100K |+
 40-70Hz | | ---
'------------------------' 1uF ---
 Tantalum|
===
` GND
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

The values for the two resistors and the cap for the VCO come from the
equations in the datasheet (but please check for yourself -- no
guarantees on the maths at these prices). The design of the loop
filter at the output of the wideband phase discriminator is of course
the most important part of the circuit. Mr. Lancaster suggests the
"fire, ready, aim" method here. By his cookbook method, you would
apply a step change to the input signal frequency, then observe the
effect on the loop filter output/VCO input node (pin 9), and change the
filter components iteratively to get the response you need. You would
use a high input impedance, fairly fast op amp to monitor the voltage
V(t) without bogging down the loop filter cap with the input impedance
of your meter. You should also monitor the output on pin 1 to see if a
good lock is occurring.

It wouldn't be fair to Mr. Lancaster to go through the whole technique
here -- and I believe this is the best part of the book. Not cricket
to give it away and all. I highly recommend you buy, beg or borrow his
CMOS Cookbook, and look at his treatment of the 4046. Using it with an
oscilloscope, a newbie or digital guy can actually make a lower
frequency PLL work pretty well (which is good enough, most of the time)
without the maths. But I can almost guarantee it won't be good enough
unless you go through the technique to trade off between loop settling
time and damping for your application. If you've got a high impedance
input fast op amp (like the LF411) to buffer pin 9, all you need is the
scope. I can guarantee that, with the book in one hand and scope probe
in the other, you should get a satisfactory answer to your puzzle (at
least as long as you're not asking for anything unreasonable from the
loop filter).

Not only that, but after all, the loop filter is supposed to be
tailored to your needs. You neglected to mention any of your loop
filter requirements. The above circuit is likely too slow for what you
need.

You don't need the op amp in your final circuit, of course.

If you are interested in the maths, look at "Phaselock Techniques" by
Floyd Gardner.

Good luck
Chris

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