LESSON 4
SPECIAL CIRCUITS AND ALIGNMENT
SCOPE.....................................Purpose
and
operation
of
squelch
circuits; general alignment procedures
for FM receivers; typical equipment in
military use.
CREDIT HOURS..............................2
TEXT ASSIGNMENT...........................TM 11-668, para 82-99;
Attached Memorandum, para 4-1 thru 4-13
MATERIALS REQUIRED........................None
SUGGESTIONS...............................Read the assignment in TM 11-668 before
you read the attached memorandum.
LESSON OBJECTIVES
When you have completed this lesson, you will be able to:
1.
Identify and analyze the squelch circuits used in FM receivers.
2.
Align an FM receiver by the visual and meter methods.
3. Analyze circuit operation
and
to
troubleshoot
both
electron
tube
and
transistorized FM receivers.
ATTACHED MEMORANDUM
Section I.
SPECIAL CIRCUITS AND ALIGNMENT
4-1.
AUTOMATIC GAIN CONTROL (AGC)
Tuned amplifiers are used extensively in radio and television receivers as RF
and IF amplifiers.
In these applications, it is often desired to automatically
vary the gain of the amplifier in accordance with the strength of the RF signal
carrier received at the antenna; that is, lower gain is required for strong signals
and higher gain is required for weak signals. One component of the output of the
second detector of a receiver is a dc voltage that is directly proportional to the
strength of the RF signal carrier received.
It is this dc component that can be
used to vary the gain of the tuned amplifiers.
a. Dc Emitter Current Control.
The power gain of the common-emitter amplifier
shown in A of figure 4-1 is controlled by feeding the AGC voltage to the base of
the amplifier to vary the dc emitter current.
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