[sdiy] Phase-locked loops and the root of my headache

[Magnus Danielson]

From: "Jon Darby" <jdarby at lplizard.com>
Subject: [sdiy] Phase-locked loops and the root of my headache
Date: Fri, 31 Aug 2001 17:41:50 -0400

> Howdy, everyone!

Howdy Jon,

>     I would really appreciate it if someone could give me (us?) an overview
> of PLLs and their audio applications, particularly the 4046A PLL with VCO
> chip. At $1.02 a pop I had to pick one up to play with it, but do not
> understand exactly what a phase locked loop is.

The 4046 (and its derivates) are absolute classics in the PLL world.

Let me tell you, I have several books on the topic itself, so it is
not something one summarizes with deep insight, but an initial tour of
what can be acheived is not a problem.

> Reading the data sheet on the 74HCT4046, it appears the the three
> different comparators are not inteded to be used simultaneously?

This is correct. They share common inputs, but you use the output of
your choice to best match your application. At rare cases one uses one
of the outputs for lock detection, so then you effectively use two outputs.

> Does the frequency or whatever is fed into teh comparator input
> control the frequency of oscillation of the VCO?

Yes, but not only the frequency, you will also lock the VCO output
phase in relation to the input signals phase. However, this phase
relationship may be many different things than 0 degrees.

Also note, you must set up the VCO for the propper frequency
range. While it may be very wide, the center frequency of the
oscillator is of interest since it is actually from this you modulate
the VCO up or down.

> If so, would an external master oscillator drive the PLL's range that it
> allows the VCO to oscillate within?

Yes, you can make the VCO track the frequency of some reference (or
master) oscillator/signal.

> Uggh, I'm talking out of my rear at this
> point. If anyone has a pointer or two where to start to understand PLLs or
> little projects I could do with this beast for fun fun audio fun it would
> really be swell.

There is one book called "Phase Locked Loop" by Best which could be a
good starter book. It has a very practical side to it and talk about
actual components. Examples with 4046 is there for instance. It comes
with a program which allows you to simulate the behaviour of a PLL, so
you can get a feel if you are in the right neighborhood before
actually building.

I do not know of a good web-site thought. The datasheets is about it
as I know it from the top of my head. Philips 74HC4046, 74HC7046 and
74HC9046 datasheets contain quite alot of design help for a
datasheet. They basically tossed in appnotes into it.

One of the things which one should know is that you are not required
to use the VCO sitting in the 4046, you can use another VCO. The expo
input is however an issue which most probably causes some upset to the
system.

Cheers,
Magnus