LESSON 3
FM TRANSMITTER CIRCUITS
SCOPE.....................................Principles of operation of frequency
multipliers
and
power
amplifiers;
problems
in
FM
transmitters.
CREDIT HOURS..............................2
TEXT ASSIGNMENT...........................TM 11-668, para 39-41;
Attached Memorandum, para 3-1 thru 3-7
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. Determine the amount of multiplication and the number of multiplier stages
needed to produce a given frequency.
2. Explain that frequency multipliers are basically RF amplifiers and how they are
tuned to the frequency of the desired harmonic.
3. Describe the input, output, and operating characteristics of the different
power amplifier circuits.
4. Calculate the impedances needed for maximum power transfer between stages in
the FM transmitter.
ATTACHED MEMORANDUM
3-1.
MULTIPLIER THEORY
a. Design Considerations. The frequency multiplier normally consists of a class
C amplifier with its output tuned to a multiple of the input frequency.
The
considerations involved in the design of transistor frequency multipliers are much
the same as those in electron-tube class C amplifiers, with one exception--the
harmonic content of the collector-current pulse is very sensitive to the phase
angle of the collector current flow.
The correct collector phase angle must be
chosen with respect to the input for the desired frequency ratio.
b. Efficiency. The optimum phase angle, expressed in degrees, is approximately
180 divided by the order of the harmonic; thus, when doubling, 90 should be used.
The collector circuit efficiency decreases as the ratios increase and are given
approximately 100 divided by the order of the harmonic.
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