each time the grid is driven positive. These rf pulses are referred to as current pulses and are shown in
view (C). The plate tank output circuit (T3) is shocked into oscillation by these current pulses and the rf
output waveform shown in view (E) is developed. The rf plate voltage waveform is shown in view (D).
An audio-modulating voltage applied to the grid of V2 is amplified by the modulator and coupled to
the plate of V1 by modulation transformer T2. The secondary of T2 is in series with the plate-supply
voltage (E) of V1. The modulating voltage will either add to or subtract from the plate voltage of V1. This
is shown in view (A) at time 2 and time 3. At time 2 in view (A), the plate supply voltage for V1
increases to twice its normal value and the rf plate current pulses double, as shown in view (C). At time 3
in view (A), the supply voltage is reduced to 0 and the rf plate current decreases to 0, as shown in view
(C). These changes in rf plate current cause rf tank T3 voltage to double at time 2 and to decrease to 0 at
time 3, as shown in view (E). This action results in the modulation envelope shown in view (E) that
represents 100-percent modulation. This is transformer-coupled out of tank circuit T3 to an antenna.
Because of the oscillating action of tank circuit T3, V1 has to be rated to handle at least four times its
normal plate supply voltage (Eb), as shown by the plate voltage waveform in view (D).
Heterodyning the audio frequency intelligence from the modulator (V2) with the carrier in the plate
circuit of the final power amplifier (V1) requires a large amount of audio power. All of the power or
voltage that contains the intelligence must come from the modulator stage. This is why plate modulation
is called high-level modulation.
The heterodyning action in the plate modulator effectively changes an audio frequency to a different
part of the frequency spectrum. This action allows antennas and equipment of practical sizes to be used to
transmit the intelligence. Now, let's look at several other typical modulators.
The COLLECTOR-INJECTION MODULATOR is the transistor equivalent of the electron-tube AM
plate modulator. This transistor modulator can be used for low-level or relatively high-level modulation.
It is referred to as relatively high-level modulation because, at the present time, transistors are limited in
their power-handling capability. As illustrated in figure 1-47, the circuit design for a transistor collector-
injection modulator is very similar to that of a plate modulator. The collector-injection modulator is
capable of 100-percent modulation with medium power-handling capabilities.