Hi All,

I saw a question about a  Phase Locked Loop as it applies to rolls
being copied.  Back before the days of stepping motors, Phase Locked
Loops were used to keep two motors at the same exact speed. (Old FAX
machines)

On the back end of each motor there was a sensor magnetic or optical.
The signals from the motors were feed into the PLL chip. If one motor
starts to go faster than the other or if it meets more resistance the
circuit will correct one or both motors.  To make the speed of the
motors even closer, the motors were usually geared down.  If only one
motor was used a reference frequency from a crystal clock circuit was
sent to one side of the PLL.

Now comes my opinion based on experiments I have done with PLL
circuits.  If the PLL controlled  the speed of a capstan or sprocket
that moves the paper than you have something, but if you are using the
PPL to turn the roll then the speed will vary  with the amount of paper
on the roll.  Take a good look at an expensive tape recorder, there
you will find a capstan that moves the tape. The take up reel does just
that, it takes up the tape but it does not drive the tape.

In the end a geared down 60 Hertz motor would be just as accurate but
the PLL motor's speed could be increased or decreased to the operator's
taste.

John Conrad Kleinbauer