A single-sideband controlled-carrier system for aircraft communication

The advantages of applying single-sideband techniques to long-range aircraft communication at high frequencies are discussed and related to the operating conditions of an airborne equipment. The effects of carrier-frequency errors and incomplete sideband suppression are examined, and the requirements for single-channel working are determined. Consideration is given to the problem of receiver carrier-oscillator frequency control, and a transmission system is proposed in which the carrier amplitude is made to vary inversely as the sideband amplitude. This system, which takes full advantage of the limited power output of an airborne transmitter, provides automatic frequency control over a wider range than is usual and avoids the use of a very narrow carrier filter. Methods of sideband suppression are compared, and the design of an experimental transmitter, employing a 4-phase balanced modulator with voice-operated carrier control, is described. The dependence of sideband suppression on the amplitude and phase balance of the modulator is investigated. A receiver using the outphasing method of sideband rejection is described, and it is shown that, for satisfactory rejection of a strong signal on the unwanted sideband, the amplitudes of any harmonic or intermodulation components produced in the demodulators and associated amplifiers must be kept to very low values. This method of sideband rejection has been extended to provide automatic carrier-frequency control, using an electro-mechanical system with a memory, for operation on the intermittent-carrier system. Flight trials with experimental equipment have proved that single-sideband working to aircraft is feasible and that a marked improvement in communication efficiency is obtained.