Control-Grid Modulator

1-59 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