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

[Scott Gravenhorst]

Frequency multiplication is a typical application.
Instead of the VCO square wave output driving
one input of the phase comparator directly, a 
divide by N digital counter can be inserted
between the VCO output and the phase comparator
input.  The VCO will track the input signal's
frequency times N.  Eg. Use a divide by 2 and get
an octave above, use a divide by 3 and get a
perfect 5th in the next octave up.  The counter
should ideally be a symetrical one.

"Jon Darby" <jdarby at lplizard.com> wrote:
>Thanks, Magnus, I appreciate the primer. What is a typical application of
>the phase-locked loop? I can kind of pick up smatterings of filter
>applications, but don't understand what role it plays. I'll look for the
>book tonight. The 4046 seems really interesting from the datasheet and I
>feel like I'm beginning to take that first dive into Lake Understanding The
>Application of Electronic Components.
>
>Thanks again,
>Jon Darby
>
>----- Original Message -----
>From: "Magnus Danielson" <cfmd at swipnet.se>
>To: <jdarby at lplizard.com>
>Cc: <synth-diy at dropmix.xs4all.nl>
>Sent: Friday, August 31, 2001 6:52 PM
>Subject: Re: [sdiy] Phase-locked loops and the root of my headache
>
>
>> 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
>
>

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