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
