Amplitude-Modulated Radio Transmitters

High-Level Amplitude Modulation. A sinusoidal radio-frequency carrier wave is generated in the crystal-controlled oscillator (page 304), which may also include a buffer-amplifier stage. The carrier wave is amplified in a class C radio-frequency power amplifier (page 295) until it is of sufficient strength to be impressed on the modulator. The speech or program signal to be transmitted is impressed on the audiofrequency power amplifier, and the signal strength is increased until it is sufficient to be impressed on the modulator. As is evident in Fig. 1, the output of the modulator is impressed on the antenna. Thus, the power level at modulation approximates the output of the system, and the term high-level modulation applies.

Low-Level Amplitude Modulation. The carrier wave is generated as before and is then amplified, but not to so high a level as in the system just described. The speech or program signal to be transmitted is amplified in the audio-frequency power amplifier, but, again, the power level is much less than with high-level modulation. After modulation, the carrier and sidebands are amplified in a class B radio-frequency power amplifier (page 295). This raises the power level to that required for radiation by the antenna (Fig. 1).

Figure 1. Basic types of amplitude-modulation radio transmitters.