LESSON 2
METHODS OF PRODUCING FM
SCOPE.....................................Study of the methods used to produce
direct and indirect FM; circuits used as
modulators in direct and indirect FM
systems.
CREDIT HOURS..............................2
TEXT ASSIGNMENT...........................TM 11-668, para 18-36;
Attached Memorandum, para 2-1 thru 2-4
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 frequency deviation produced by modulated oscillators.
2.
Determine the amplification factor, load impedance, and injected reactances for
reactance tube modulator stages.
3.
Identify and know the operation of the various modulator circuits.
4.
Use the block and schematic diagrams of an FM transmitter to determine whether
direct or indirect FM is being used.
ATTACHED MEMORANDUM
2-1.
ESTABLISHING FREQUENCY MODULATION
a. General.
The input and output waveforms of an FM oscillator are shown in
figure 2-1. In an FM transmitter, the modulation is accomplished at the oscillator
stage. A transistor oscillator can be modulated in the same manner as an electron-
tube oscillator or by varying the oscillator gain at the modulating rate. The same
amplitude-modulated oscillator used in an AM transmitter can be used in an FM
transmitter.
The unwanted amplitude changes can be removed by a limiter stage
before the carrier signal is increased in frequency and magnitude.
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