Relations of Carrier and Side-Bands in Radio Transmission1

Abstract

This paper discusses generally the characteristics of carrier transmission as applied in radio and in carrier current communication over wires and analyses the factors which affect the faithfulness with which such systems reproduce the signals imparted to them. Modulation is shown to generate two side bands which, with respect to frequency, lie just above and just below the carrier frequency, the frequency width of each side band being the same as the frequency width of the original signals. Upon detection, currents of frequencies corresponding to the difference frequencies between all the possible pairs of component frequencies of the side bands and carrier are produced and, in general, are all found in the received message. It is therefore impossible to transmit messages, either telephone or telegraph, by carrier which will be absolutely free of distortion, but since the amplitude of any particular difference frequency is proportional to the product of the amplitudes of its two generating frequencies the distortion can be reduced below a troublesome value by maintaining the amplitude of the original carrier sufficiently large with respect to the amplitudes of the signal components. The distortion which arises from phase shifts between the component frequencies of the transmitted message and carrier is also considered. The paper discusses single side-band transmission and carrier suppression with homodyne detection and their various merits are pointed out. Single side band transmission reduces the width of frequency band required for each message. Carrier suppression results in a saving of power, or a more economical expenditure of power, it having been determined that for proper freedom from distortion the power of the carrier component alone, when transmitted, should be rather larger than the peak power in a carrier suppression system. The use of local carrier in homodyne radio telephony assists in frequency selection in the same way as does the heterodyne wave in radio telegraph reception. The same applies also to static interference, and, as the object of high power stations is to make the signals large compared with static, there is a gain in concentrating the power in side bands rather than in carrier. Consideration of distortion arising from phase shifts shows that in homodyne telegraphy distortion can most readily be avoided by transmitting both side-bands, while in telephony these factors favor the transmission of only one side-band. The power of the reproduced signals is twice as great with two side-bands as with one, but there is no choice between one and two side-bands on the basis of the ratio of signals to interference. The result of using a local detecting frequency which is not exactly equal to the original carrier frequency is discussed, and a balanced detector is described by means of which the distorting effect of the received carrier may be very much reduced. Considering a local carrier which is out of synchronism with the original carrier, it is again found that single side-band transmission is most favorable in telephony, and the transmission of both side-bands is best in telegraphy. — Editor.