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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