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Plate Modulation

Author: J.B. Hoag

Today, the most widely used method for amplitude modulation is known as plate modulation. In this system, radio-frequency or "driver" voltages are applied to the grid of an amplifier (called the "modulated" tube). The audio-frequency or "modulation" voltages are inserted into its plate circuit in series with the B-power supply, as in Fig. 16 F.

Fig. 16 F. Elementary circuit for plate modulation

In order that the a.f. in the plate circuit shall be sufficiently strong, it is necessary to strengthen the microphone voltages with a speech amplifier. The last stage in this amplifier is called the modulator. The secondary of the audio transformer in the plate circuit of the modulator is in series with the B supply of the modulated tube, as in Fig. 16 G.

Fig. 16 G. Further details of plate modulation. More elaborate and practical circuits will be given later

The radio-frequency-choke coil, r.f.c, keeps the high frequency from shunting through the grid-bias battery, and the bypass condenser C keeps it out of the B-battery and speech amplifier.

Fig. 16 H. The principle of plate modulation. The a.f. voltage increases and decreases the total voltage on the plate around the " B battery " value

In Fig. 16 H, we see that when, during a positive half-cycle of the audio frequency, the plate voltage is increased above that of the B supply, the plate current likewise increases. Similarly, when the a.f. is in opposition to the steady B voltage, the output decreases. In order that the rise and fall of the amplitude of the output r.f. shall be a faithful copy of the a.f., the driver's voltage must be large, and the C-bias, the B voltage, and the circuit constants must be properly chosen. In order that the changes in the amplitude of the carrier wave shall be large, the output voltage of the modulator must be nearly equal to that of the B supply.

Last Update: 2009-11-01