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Q-43. The collector-injection modulator is similar to what type of tube modulator?
Control-Grid Modulator
In cases when the use of a minimum of af modulator power is desired, a form of low-level
modulation is necessary. The CONTROL-GRID MODULATOR is used widely in portable and mobile
equipment to reduce size and power requirements. It is also used in extremely high-power, wideband
equipment such as television transmitters where high-level or plate modulation is difficult and costly to
achieve. Figure 1-48 is a basic schematic for a typical control-grid modulator.
Figure 1-48.—Control-grid modulator.
The control-grid modulator uses a variation of grid bias (at the frequency of the modulating signal)
to vary the instantaneous plate voltage and current. These variations cause modulation of the carrier
frequency. The carrier frequency is introduced through coupling capacitor Cc. The modulating frequency
is introduced in series with the grid bias through T1. As the modulating signal increases and decreases
(positive and negative), it will add to or subtract from the bias on rf amplifier V1. This change in bias
causes a corresponding change in plate voltage and current. These changes in plate voltage and current
add vectorially to the carrier frequency and provide a modulation envelope in the same fashion as does
the plate modulator. Since changes in the plate circuit of the rf amplifier are controlled by changes in the
grid bias, the gain of the tube requires only a low-level modulating signal. Even when the input signals
are at these low levels, occasional modulation voltage peaks will occur that will cause V1 to saturate. This
creates distortion in the output. Care must be taken to bias the rf amplifier tube for maximum power out
