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A simple amplitude modulator
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An amplifier was built to have variable gain. The idea was to make the gain fluctuate at voice-frequency rates up to 3 kHz or so. Vacuum tubes were used as amplifiers back then, because solid-state components hadn t been invented yet. But the principle of amplitude modulation (AM) is the same, whether the active devices are tubes, bipolar transistors, or FETs. If bipolar transistors had been around in 1920, the first amplitude modulator would have resembled the circuit shown in Fig. 26-5. This circuit is simply a class-A RF amplifier, whose gain is varied in step with a voice signal coupled into the emitter circuit. The voice signal affects the instantaneous voltage between the emitter and base, varying the instantaneous bias. The result is that the instantaneous RF output increases and 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 that the audio input isn t too great. If the AF is excessive, overmodulation will occur. This will result in a distorted signal.
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The AM transmitter
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Two 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 amplification stages 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 final amplifier. As long the PA is modulated correctly, the output will be a clean AM signal; RF linearity is of no concern. The extent of modulation is expressed as a percentage, from 0 percent, representing an unmodulated carrier, to 100 percent, representing full modulation. Increasing the modulation past 100 percent will cause distortion of the signal, and will degrade, not enhance, the effectiveness of data transmission. In an AM signal that is modulated 100 percent, only 1/3 of the power is actually used 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 of available transmitter power. Perhaps the most widely used is single sideband (SSB), which you ll learn about shortly.
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Amplitude modulation for voice 479
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26-5 An AM modulator.
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Bandwidth of an AM voice signal
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Suppose you could get a graphic display of an AM signal, with frequency on the horizontal axis and amplitude on the vertical axis. This is in fact done using an instrument called a spectrum analyzer. In Fig. 26-7, the spectral display for an AM voice radio signal at 1340 kHz is illustrated. On a spectrum analyzer, unmodulated radio carriers look like vertical lines, or pips, of various heights depending on how strong they are. The carrier wave at 1340 kHz shows up in Fig. 26-7 as a strong pip. The horizontal scale of the display in Fig. 26-7 is calibrated in increments of 1 kHz per division. This is an ideal scale for looking at an AM signal. The vertical scale is calibrated in decibels below the signal level that produces 1 mW at the input terminals. Each vertical division represents 3 dB. Decibels relative to 1 mW are abbreviated dBm by engineers. Thus, in Fig. 26-7, the top horizontal line is 0 dBm; the first line below it is 3 dBm; the second line is 6 dBm and so on. The audio components of the voice signal show up as sidebands on either side of the carrier. All of the voice energy in this example is at audio below 3 kHz. This results in sidebands within the range 1340 kHz plus or minus 3 kHz, or 1337 to 1343 kHz. The frequencies between 1337 and 1340 kHz are the lower sideband (LSB); those from 1340 to 1343 kHz are the upper sideband (USB). The bandwidth of the RF signal is the difference between the maximum and minimum sideband frequencies. In this case it is 1343 to 1337 kHz, or 6 kHz.
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