1-54
modulation is applied. The FCC defines HIGH-LEVEL MODULATION in the Code of Federal
Regulations as "modulation produced in the plate circuit of the last radio stage of the system." This same
document defines LOW-LEVEL MODULATION as "modulation produced in an earlier stage than the
final."
Q-36. What is percent of modulation?
Q-37. With a single modulating tone, what is the amplitude of the sideband frequencies at 100-percent
modulation?
Q-38. What is the formula for percent of modulation?
Q-39. What is high-level modulation?
MODULATION SYSTEMS
To complete your understanding of AM modulation, we are now going to analyze the operation of a
typical plate modulator. Detailed circuit descriptions will be used to give you an understanding of a basic
AM plate modulator. In addition, we will cover basic circuit descriptions for cathode and grid electron-
tube modulators and for base, emitter, and collector transistor modulators in this chapter.
Plate Modulator
Figure 1-45 is a basic plate-modulator circuit. Plate modulation permits the transmitter to operate
with high efficiency. It is the simplest of the modulators available and is also the easiest to adjust for
proper operation. The modulator is coupled to the plate circuit of the final rf amplifier through the
modulation transformer. For 100-percent modulation, the modulator must supply enough power to cause
the plate voltage of the final rf amplifier to vary between 0 and twice the dc operating plate voltage. The
modulator tube (V2) is a power amplifier biased so that it operates class A. The final rf power amplifier
(V1) is biased in the nonlinear portion of its operating range (class C). This provides for efficient
operation of V1 and produces the necessary heterodyning action between the rf carrier and the af
modulating frequencies.
