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SINGLE-SIDEBAND TRANSMITTER

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2-7 SINGLE-SIDEBAND TRANSMITTER You should remember the properties of modulation envelopes from your study of NEETS, Module 12, Modulation Principles. A carrier that has been modulated by voice or music is accompanied by two identical sidebands, each carrying the same intelligence. In amplitude-modulated (AM) transmitters, the carrier and both sidebands are transmitted. In a single-sideband transmitter (ssb), only one of the sidebands, the upper or the lower, is transmitted while the remaining sideband and the carrier are suppressed. SUPPRESSION is the elimination of the undesired portions of the signal. Figure 2-7 is the block diagram of a single-sideband transmitter. You can see the audio amplifier increases the amplitude of the incoming signal to a level adequate to operate the ssb generator. Usually the audio amplifier is just a voltage amplifier. Figure 2-7.—Ssb transmitter block diagram. The ssb generator (modulator) combines its audio input and its carrier input to produce the two sidebands. The two sidebands are then fed to a filter that selects the desired sideband and suppresses the other one. By eliminating the carrier and one of the sidebands, intelligence is transmitted at a savings in power and frequency bandwidth. In most cases ssb generators operate at very low frequencies when compared with the normally transmitted frequencies. For that reason, we must convert (or translate) the filter output to the desired frequency. This is the purpose of the mixer stage. A second output is obtained from the frequency generator and fed to a frequency multiplier to obtain a higher carrier frequency for the mixer stage. The output from the mixer is fed to a linear power amplifier to build up the level of the signal for transmission. Suppressed Carrier In ssb the carrier is suppressed (or eliminated) at the transmitter, and the sideband frequencies produced by the carrier are reduced to a minimum. You will probably find this reduction (or elimination) is the most difficult aspect in the understanding of ssb. In a single-sideband suppressed carrier, no carrier is present in the transmitted signal. It is eliminated after modulation is accomplished and is reinserted at


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