A Double Sideband-Quadrature Carrier Multiplex Telemetry System

Abstract

A novel FDM telemetry technique was developed consisting of a double sideband-quadrature carrier multiplexing system (DSB-QCM). Each subchannel in the DSB-QCM system carries two completely overlapping DSB data signals, one double-sideband modulated on the subcarrier itself, and the other on a quadrature version of the subcarrier. Demodulation with cophasal and quadrature subcarriers enables simultaneous data extraction from each channel within acceptable distortion levels. The feasibility and practicability of such a DSB-QCM telemetry system is discussed in this paper. Crosstalk levels between the quadrature multiplexed channels were measured and guardband requirements between adjacent channels were assessed for a modem comprised of three pairs of DSB-QCM channels. Crosstalk levels between uniformly loaded DSB -QCM channels were below 2% and guardband requirements equivalent to conventional DSB systems were observed. The DSB-QCM performance was also examined as a function of input SNR with two competing subcarrier synchronization methods. Subcarrier synchronization by means of synthesized reference tones coherently derived from a single pilot was demonstrated to be superior in The presence of noise to a channel reference approach in which each data channel must synchronize its own subcarrier. The major conclusion from this investigation is that DSB-QCM/FM telemetry combines the advantages of both SSB/FM and DSB/FM by accommodating as many data channels as SSB/FM but with low distortion data processing and the dc data response characteristic of DSB/FM.