Amplitude modulation for voice

Chapter Amplitude modulation for voice

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • Amplitude modulation for voiceA voice signal is a complex waveform with frequencies mostly in the range 300 Hz to 3kHz. Direct currents can be varied, or modulated, by these waveforms, thereby trans-mitting voice information over wires. This is how early telephones worked.Around 1920, when CW oscillators were developed to replace spark-gap transmit-ters, engineers wondered, “Can a radio wave be modulated with a voice, like dc in a tele-phone?” If so, voices could be sent by “wireless.” Radio communications via Morse codewas being done over thousands of miles, but CW was slow. The idea of sending voiceswas fascinating, and engineers set about to find a way to do it.A simple amplitude modulatorAn amplifier was built to have variable gain. The idea was to make the gain fluctuate atvoice-frequency rates—up to 3 kHz or so. Vacuum tubes were used as amplifiers backthen, because solid-state components hadn’t been invented yet. But the principle ofamplitude modulation (AM) is the same, whether the active devices are tubes, bipo-lar transistors, or FETs.If bipolar transistors had been around in 1920, the first amplitude modulatorwould have resembled the circuit shown in Fig. 26-5. This circuit is simply a class-A RFamplifier, whose gain is varied in step with a voice signal coupled into the emitter cir-cuit. The voice signal affects the instantaneous voltage between the emitter and base,varying the instantaneous bias. The result is that the instantaneous RF output increasesand decreases, in a way that exactly duplicates the waveform of the voice signal.The circuit of Fig. 26-5 will work quite well as an AM voice modulator, provided thatthe audio input isn’t too great. If the AF is excessive, overmodulation will occur. Thiswill result in a distorted signal.The AM transmitterTwo complete AM transmitters are shown in block-diagram form in Fig. 26-6. At A,modulation is done at a low power level. This is low-level AM. All the amplificationstages after the modulator must be linear. That means class AB or class B must be used.If a class-C PA is used, the signal will be distorted.In some broadcast transmitters, AM is done in the final PA, as shown in Fig. 26-6B.This is high-level AM. The PA operates in class C; it is the modulator as well as the fi-nal amplifier. As long the PA is modulated correctly, the output will be a “clean” AM sig-nal; RF linearity is of no concern.The extent of modulation is expressed as a percentage, from 0 percent, represent-ing an unmodulated carrier, to 100 percent, representing full modulation. Increasingthe modulation past 100 percent will cause distortion of the signal, and will degrade, notenhance, the effectiveness of data transmission.In an AM signal that is modulated 100 percent, only 1/3 of the power is actuallyused to convey the data; the other 2/3 is consumed by the carrier wave. For this reason,AM is rather inefficient. There are voice modulation techniques that make better use ofavailable transmitter power. Perhaps the most widely used is single sideband (SSB),which you’ll learn about shortly.478 Data transmission