2-33
Note the frequency spectrum associated with each of these conditions. The carrier amplitude remains
constant, but the sidebands increase in amplitude in accordance with the amplitude of the modulating
square wave.
So far in pulse modulation, the same general rules apply as in AM. In view (C) the amplitude of the
square wave of voltage is equal to the peak voltage of the unmodulated carrier wave. This is 100-percent
modulation, just as in conventional AM. Note in the frequency spectrum that the sideband distribution is
also the same as in AM. Keep in mind that the total sideband power is 1/2 of the total power when the
modulator signal is a square wave. This is in contrast to 1/3 the total power with sine-wave modulation.
Now refer to view (D). The increase of the square-wave modulating voltage is greater in amplitude
than the unmodulated carrier. Notice that the sideband distribution does not change; but, as the sidebands
take on more of the transmitted power, so will the carrier.
Pulse Timing
Thus far, we have established a carrier and have caused its peaks to increase and decrease as a
modulating square wave is applied. Some pulse-modulation systems modulate a carrier in this manner.
Others produce no rf until pulsed; that is, rf occurs only during the actual pulse as shown in view (A) of
figure 2-30. For example, let’s start with an rf carrier frequency of 1 megahertz. Each cycle of the rf
requires a certain amount of time to complete. If we allow oscillations to occur for a given period of time
only during selected intervals, as in view (B), we are PULSING the system. Note that the pulse
transmitter does not produce an rf signal until one of the positive-going modulating pulses is applied. The
transmitter then produces the rf carrier until the positive input pulse ends and the input waveform again
becomes a negative potential.
Figure 2-30A.—Pulse transmission.
Figure 2-30B.—Pulse transmission.
Refer back to figure 1-41 and the over-modulation discussion in chapter 1. You will notice that the
overmodulation wave shape of view (D) in figure 2-29 and the pulse-modulation wave shape of figure
2-30, view (B), are very similar to figure 1-41.
Actually, both figure 1-41 and view (D) of figure 2-29 result from overmodulation. Even though the
output of the pulse transmitter in figure 2-30 looks like overmodulation, it is not; rather, it is pulsed.
