Basic Phase Locked Loops
Phase locked loops are primarily used in communications circuits to demodulate
FM and FSK signals. A complete understanding of their operation
requires a knowledge of second order differential equations and
transforms. However, it is possible
to examine the operation of a simple PLL with little more than a
basic knowledge of trigonometry.
AN104A – PLL Hardware
Design and Simulation by Elanix
AN46 – The PLL IC
as a Communication System Building Block by National Semiconductor
PLLs for HF Transmitters and Receivers Part
1, Part 2, Part
TLC2932 PLL by Texas
Phase Locked Loop
These loops can be either digital or analog in nature however,
their operation is easiest to analyze when they are composed of
linear analog circuits.
let the input =
let the VCO output =
Then the multiplier output is:
After the LPF we obtain:
is irrelevant and the circuit is not in lock.
output is positive
output is negative
Therefore this signal can be used to drive the VCO in such
a way that it will force . This is the frequency locked
When , then the output of the LPF
is , where q
is the phase difference between the input and VCO output. Since
this term is an odd function, it can be used to drive the VCO such
that phase lock is maintained. Note that the zero error signal occurs
when the input and VCO output are in phase quadrature.