MODULATION SYSTEMS

To complete your understanding of AM modulation, we are now going to analyze the operation of a typical plate modulator. Detailed circuit descriptions will be used to give you an understanding of a basic AM plate modulator. In addition, we will cover basic circuit descriptions for cathode and grid electron-tube modulators and for base, emitter, and collector transistor modulators in this chapter.

Plate Modulator

Figure 1-45 is a basic plate-modulator circuit. Plate modulation permits the transmitter to operate with high efficiency. It is the simplest of the modulators available and is also the easiest to adjust for proper operation. The modulator is coupled to the plate circuit of the final rf amplifier through the modulation transformer. For 100-percent modulation, the modulator must supply enough power to cause the plate voltage of the final rf amplifier to vary between 0 and twice the dc operating plate voltage. The modulator tube (V2) is a power amplifier biased so that it operates class A. The final rf power amplifier (V1) is biased in the nonlinear portion of its operating range (class C). This provides for efficient operation of V1 and produces the necessary heterodyning action between the rf carrier and the af modulating frequencies.

Figure 1-45. - Plate-modulation circuit.

PLATE MODULATOR CIRCUIT OPERATION. - Figure 1-46, views (A) through (E), shows the waveforms associated with the plate-modulator circuit shown in figure 1-45. Refer to these two figures throughout the following discussion.

Figure 1-46. - Plate-modulator waveforms.

The rf power amplifier (V1) acts as a class C amplifier when no modulation is present in the plate circuit. V2 is the modulator which transfers the modulating voltage to the plate circuit of V1. Let's see how this circuit produces a modulated rf output.

View (A) of figure 1-46 shows the plate supply voltage for V1 as a constant dc value (E_b) at time 1 with no modulating signal applied. V1 is biased at cutoff at this time. The incoming rf carrier [view (A)] is applied to the grid of V1 by transformer T1 and causes the plate circuit current to PULSE (SURGE) 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 _b) 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 (E_b), 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.

Collector-Injection Modulator

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.

Figure 1-47. - Collector-injection modulator.

In figure 1-47, the rf carrier is applied to the base of modulator Q1. The modulating signal is applied to the collector in series with the collector supply voltage through T3. The output is then taken from the secondary of T2. With no modulating signal, Q1 acts as an rf amplifier for the carrier frequency. When the modulation signal is applied, it adds to or subtracts from the collector supply voltage. This causes the rf current pulses of the collector to vary in amplitude with the collector supply voltage. These collector current pulses cause oscillations in the tank circuit (C4 and the primary of T2). The tank circuit is tuned to the carrier frequency. During periods when the collector current is high, the tank circuit oscillates strongly. At times when the collector current is small, or entirely absent, little or no energy is supplied to the tank and oscillations become weak or die out. Thus, the modulation envelope is developed as it was in a plate modulator.

As transistor technology continues to develop, higher power applications of transistor collector-injection modulation will be employed. Plate and collector-injection modulation are the most commonly used types of modulation because the modulating signal can be applied in the final stages of rf amplification. This allows the majority of the rf amplifier stages to be operated class C for maximum efficiency. The plate and collector-injection modulators also require large amounts of af modulating power since the modulator stage must supply the power contained in the sidebands.

Q.40 For what class of operation is the final rf power amplifier of a plate-modulator circuit biased?
Q.41 The modulator is required to be what kind of a circuit stage in a plate modulator?
Q.42 How much must the fpa plate current vary to produce 100-percent modulation in a plate modulator?
Q.43 The collector-injection modulator is similar to what type of tube modulator?