Transmission, Single-Sideband

the use of single-sideband modulation to transmit information over a wire or radio communication channel. This method is used to transmit signals in multichannel communication and to send messages on radio channels in the decametric range. It is also used in certain other cases where a communication channel must occupy a very small bandwidth.

The transmitted (modulating) signal has a range of frequencies that extends from Ω₁ to Ω_h, respectively the lowest and highest frequencies of the signal spectrum. By means of a frequency converter (such as a balanced modulator) and a bandpass filter, the frequency spectrum of the signal is shifted into the region of the carrier frequency, and one of the sidebands, ranging from (ω + Ω_t) to (ω + Ω_h), is selected. This single-sideband signal is amplified in the transmitter circuit.

The signal picked up by the receiving apparatus is amplified and fed into a frequency converter that inverts the input signal by using a local oscillator that generates a wave at the carrier frequency ω. If the conversion frequencies for the carrier ω are equal for transmission and reception, the spectrum of frequencies at the output of the low-pass filter will be the same as that of the transmitted signal.

Single-sideband transmission has several advantages over ordinary amplitude modulation. The modulated frequencies occupy only one sideband, which gives a fourfold gain in the transmitting power. Halving of the frequency band occupied by the transmitted signal (and, correspondingly, halving of the receiver passband) reduces interference with radio reception. Single-sideband transmission produces a total gain in transmitter power that amounts to between eight and 16 times the power of ordinary amplitude modulation. However, complicated equipment is required to achieve these advantages.