5-2
MODULATION MEASUREMENTS
Modulation measurements are sometimes required during tuning procedures to adjust transmitting
equipment for the proper amount of modulation. During maintenance tests of modulated transmitter
equipment, you should determine the amount of distortion in the output signal and the modulation level or
index. The modulation level in multiplexing equipment is usually set at the factory or during corrective
maintenance procedures. Proper adjustment of the input signal level and automatic signal-level regulation
circuits provides the correct amount of modulation. Defects in modulation circuits of a transmitter can be
detected by measurements of the quality of the received signals at the receiver. Corrective maintenance
analysis of multiplex equipment modulation circuits can usually be made by signal-level measurements.
Some radio transmitters, when operating in the AM mode, must be adjusted for correct modulation
during normal tuning procedures. If the modulation level is low, the transmitter is not operating at its
maximum efficiency. On the other hand, modulation in excess of 100% produces serious distortion. Since
neither of these conditions is desirable, amplitude modulation should be maintained between 60% and
95% when possible. The modulation level or index of AM and fm radio transmitters that operate in the
vhf range is initially adjusted by the manufacturer or during corrective maintenance. The amplifier gain of
the modulator can be initially adjusted by reference to the modulation meter provided on the front panel
of the equipment.
Pulse modulation of radar and radio beacon signals can be measured by waveform displays presented
on a standard oscilloscope. The amount of usable energy in a pulsed waveform, as measured by a
spectrum analyzer, is also an indication of the pulse modulation quality.
Attaining 100% amplitude modulation of an rf carrier with a sine wave requires a modulating power
equal to one-half of the rf carrier power. Under this condition, the average power of the modulated carrier
is equal to 1.5 times the average unmodulated carrier power. The added power is divided equally between
the upper and lower sidebands. During the peaks of 100% modulation, the amplitude of the carrier is
doubled. This will cause the instantaneous peak power to be four times the instantaneous unmodulated
peak power P = E2/R. When voice modulation is employed, only the highest amplitude peaks can be
allowed to modulate the carrier 100%. Since many speech components do not modulate the carrier 100%,
the average power required for voice modulation is less than that required for modulation with a sine
wave. Voice peaks usually modulate a carrier 100% when the modulation increases the average carrier
output power 25% over its normal value.
Q-1.
What is the result of overmodulating an AM signal?
Q-2.
For AM transmissions, the carrier is normally modulated within what range?
AMPLITUDE-MODULATION MEASUREMENTS
An increase in the power output of an AM transmitter is indicated by an increase in antenna current.
The increase can be taken as a measure of the degree of modulation and can be expressed as a percentage,
as shown in figure 5-1. The graph for this figure was developed from the relationship existing between the
carrier power and the increased power resulting from the added modulation power. The formula for
calculating the PERCENTAGE of MODULATION is as follows:
