Relations of Carrier and Side-Bands in Radio Transmission

The modulation of a carrier wave by a signal wave and the subsequent reproduction of the signal wave are discussed in terms of the sustained sinusoidal components into which the various waves may be resolved. The conception of a modulated wave as being made up of a carrier component and two side-bands is explained. The saving in frequency range resulting from transmitting a single side-band is emphasized. Carrier suppression and homodyne reception are shown to save sending power, increase receiving sensitiveness and aid in suppressing interference from both stations and static. Equations are derived for the distortion of the reproduced wave in terms of that of the modulated wave. In general, both the amplitude and phase of each component of the signal depend on both the amplitudes and phases of the corresponding side-band components and of the carrier. The effect on the signal of various typical distortions of the radio wave is examined for both single and double side-band transmission, as is also that of altering the phase of the locally supplied carrier and of altering its frequency. The resulting distortion of the signal is found, in general, to be more serious for telephony when both side-bands are used and for telegraphy when only one is used. It is also shown that the reproduced signal has twice the power when both side-bands are transmitted as when the same sending power is concentrated in one. The signal to static ratio is, however, the same in both cases.