Methods of Amplitude Modulation

Modulated Class C Amplifier, Modulating Wave Injected in Plate Circuit. A simplified circuit is shown in Fig. 2, and the equivalent circuit in Fig. 3. The magnitude of the instantaneous radio-frequency output voltage depends on the magnitude of the direct plate-to-cathode voltage (page 295). For perfect modulation this relation should be linear. Then, as indicated in Fig. 3, when the audio voltage from the modulating amplifier varies in accordance with the signal to be transmitted, the plate voltage of the modulator, or modulated amplifier, will vary as shown by e_b of Fig. 3(b), and the plate current will vary as shown in (c). The parallel resonant circuit C-L is tuned to the carrier frequency (and sidebands), and the alternating current through coil L and the alternating voltage across the L-C circuit will vary as in (d). This is an amplitude-modulated wave, and this output circuit is coupled to the antenna, the arrangement shown being for high-level modulation.

Figure 2. Simplified circuit of a push-pull audio-frequency modulating amplifier (left) driving a modulated class C amplifier, or modulator, (right). The modulated output wave of Fig, 3 (d) appears in the circuit L-C which is coupled to the antenna in a high-level modulation circuit. The neutralizing circuit (page 299) is omitted.

For conditions of operation commonly used, the peak value of the carrier voltage impressed on the grid of the modulated class C amplifier drives the control grid positive. With the grid bias value known (page 295) the necessary carrier voltage can be estimated. Since the plate voltage varies at an audio signal rate, the cutoff point varies and the carrier power input varies. For a triode, the carrier power input required is approximately 10 percent of the modulated plate-power output. The audio-frequency modulating power amplifier of Fig. 2 is often class B push-pull (page 294). In amplitude modulation, the magnitude of the power output of the carrier-frequency component is the same before and during modulation. The power that goes into the carrier wave output of the modulated amplifier comes from the plate power supply E_bb. The power that goes into the sidebands comes from the audio-frequency modulating amplifier. For 100 percent modulation, the power in the two sidebands is 50 per cent of that in the carrier.

Modulated Class C Amplifier, Modulating Wave Injected in Grid Circuit. A simplified circuit is shown in Fig. 4(a) which includes a neutralizing capacitor C_N. The grid is biased considerably beyond cutoff as Fig. 4(b) indicates. With the carrier voltage only impressed_? the instantaneous grid voltage and plate current are as shown by A-B. When the modulating voltage E_s from the modulating amplifier is impressed in series with the carrier voltage, the instantaneous grid voltage and plate current are as shown in B-C. When this current flows through the tuned circuit at the right in Fig. 4(a), the alternating voltage across this circuit and the alternating current through the inductor will be as shown by Fig. 3(d). Usually the antenna or the feeder is inductively coupled to this inductor.

The grid of the tube in Fig. 4 is driven positive; therefore, the class C amplifier, associated with the crystal in the lower diagram of Fig. 1, and the audio-frequency modulating amplifier (of Fig. 1) must have power output capacities sufficient to supply the losses of the grid circuit of the modulated class C tube. But the audio-modulating amplifier does not have to supply the power for the sidebands. Each amplifier must supply about 10 percent of the modulated output power. The circuit of Fig. 4 is for low-level modulation (Fig. 1).

Figure 3. Equivalent circuit for a modulated class C amplifier, and curves showing the operation, (a) Plate current with carrier only impressed. (6) Direct voltage Eb on the plate, and total voltage eb when modulating signal is impressed, (c) Plate current when carrier voltage and modulating voltage both are impressed, (d) Voltage drop across tuned parallel circuit and current in tuning coil and condenser.

Other Methods of Amplitude Modulation. The balanced modulator of page 421 has been used in small radio broadcast transmitters.⁴ The circuit was arranged to suppress the audio signal and transmit the carrier component. Another circuit sometimes used is called cathode modulation in which the carrier wave is impressed in the grid circuit, and the modulating wave simultaneously is impressed between cathode and ground.

Figure 4. Simplified circuit (a) for a modulated class C amplifier with modulating wave injected in the grid circuit. This is often called grid-bias modulation, or grid modulation, and is commonly used in low-level modulation. In (b) is shown how this modulator operates. When both the carrier and the modulating signal Es are injected, the instantaneous grid-voltage variations and the instantaneous plate-current variations are shown by B-C. These plate-current changes produce the conventional amplitude-modulated wave in the plate circuit.

The tubes indicated in the modulated amplifiers of Figs. 2 and 4 are triodes, but beam-power tubes or pentodes are often used for this purpose. When pentodes are used, suppressor-grid modulation is possible. With this arrangement the carrier wave is impressed, or injected, in the control-grid circuit, and the audio-modulating wave is injected between the suppressor grid and cathode.⁵