文档介绍：h hs okdLo Ca omncto ytmBidn lc AN-46 Block Building munication a as IC Loop Locked Phase The
National Semiconductor
The Phase Locked Loop IC Application Note 46
as munication Thomas B. Mills
System Building Block June 1971
INTRODUCTION
The phase locked loop has been found to be a useful ele- It can be seen that the action of the VCO is that of an
ment in many types munication systems. It is used in integrator in the feedback loop when the phase locked loop
two fundamentally different ways: (1) as a demodulator, is considered in servo theory.
where it is used to follow phase or frequency modulation A better understanding of the operation of the loop may be
and (2) to track a carrier or synchronizing signal which may obtained by considering that initially, the loop is not in lock,
vary in frequency with time.
but that the frequency of the input signal ei and VCO eo are
When operating as a demodulator, the phase locked loop very close in frequency. Under these conditions ed will be a
may be thought of as a matched filter operating as a coher- beat note, the frequency of which is equal to the frequency
ent detector. When used to track a carrier, it may be thought difference of eo and ei. This signal is also applied to the
of as a narrow-band filter for removing noise from a signal. VCO input, since it is low enough to pass through the filter.
Recently, a phase locked loop has been built on a monolith- The instantaneous frequency of the VCO is therefore
ic integrated circuit, incorporating the basic elements neces- changing and at some point in time, if the VCO frequency
sary for operation: a double balanced phase detector and a equals the input frequency, lock will result. At this instant, ef
highly linear voltage controlled oscillator, the frequency of will assume a level sufficient to hold the VCO frequency in
which can be varied with either a resistor or capacitor. lock with the input frequency. If the tuning of the VCO is