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26-6 At A, transmitter using low-level AM. At B, a transmitter using high-level AM.
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In an AM signal, the bandwidth is twice the highest audio modulating frequency. In the example of Fig. 26-7, the voice energy is all below 3 kHz, and the bandwidth of the complete RF signal is 6 kHz. At 3 kHz above and below the carrier, the frequency cutoffs are abrupt. This transmitter uses an audio lowpass filter that cuts out the audio above 3 kHz. Audio above 3 kHz contributes nothing to the intelligibility of a human voice. It s important to keep the bandwidth of a signal as narrow as possible, so there will be room for many signals in a given band of frequencies.
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As mentioned previously, AM is not efficient. Most of the power is used up by the carrier; only 33 percent of it carries data. Besides that, the two sidebands are mirror-image duplicates. An AM signal is redundant, as well as inefficient, for voice transmission.
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Amplitude Modulation for Voice 481
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26-7 Spectral display of a typical voice AM signal.
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During the fifties, engineers began to work on an alternative to conventional AM. They mused, Suppose all of the transmitter power could go into the voice, and none be taken up by the carrier That would be a threefold effective increase in transmitter power! And what if the bandwidth could be cut to 3 kHz rather than 6 kHz, by getting rid of the sideband redundancy That would put all the energy into half the spectrum space, so that twice as many voice signals could fit in a band. Spectrum-space conservation was getting to be a big deal. The airwaves were starting to become overcrowded. These improvements were realized by means of circuits that cancel out, or suppress, the carrier in the modulator circuit, and that filter out, or phase out, one of the two sidebands. The remaining voice signal has a spectrum display that looks like the graph of Fig. 26-8. Either LSB or USB can be used, and either mode works as well as the other.
The SSB transmitter
The heart of an SSB transmitter is a balanced modulator. This circuit works like an ordinary AM modulator, except that the carrier wave is phased out. This leaves only the LSB and USB. One of the sidebands is removed by a filter that passes only the RF within a 3-kHz-wide band. A block diagram of an SSB transmitter is shown in Fig. 26-9. High-level modulation won t work for SSB. The balanced modulator is in a low-power part of the transmitter. Therefore, the RF amplifiers after the modulator must all be linear. They usually work in class A except for the PA, which is class AB or class B. If class-C amplification is used with an SSB signal, or if any of the RF amplifiers aren t linear for any reason, the signal envelope (waveform) will be distorted. This will degrade the quality of the signal. It can also cause the bandwidth to exceed the nominal 3 kHz,
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26-8 Spectral display of a typical voice SSB signal. In this case, it is the lower sideband.
26-9 An SSB transmitter.
resulting in interference to other stations using the band. Engineers and technicians refer to this as splatter.
Frequency and phase modulation
Both AM and SSB work by varying the signal strength. This can be a disadvantage when there is sferic noise caused by thundershowers in the vicinity. Sferics are recognizable as crashes or crackles in an AM or SSB receiver. Ignition noise can also be a problem;
Frequency and phase modulation 483 this sounds like a buzz or whine. Sferics and ignition noise are both predominantly amplitude-modulated. In frequency modulation (FM), the amplitude of the signal remains constant, and the instantaneous frequency or phase is made to change. Because the carrier is always full-on, class-C power amplifiers can be used. Linearity is of no concern when the signal level does not change.
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