Design Principles of Amplitude-Modulated Subcarrier Telemeter Systems

Of the many multichannel radio telemeter systems that are being used for the instrumentation of airplanes and guided missiles, one of the simplest employs a separate amplitude-modulated audio subcarrier for each channel. In this paper a rational basis for the design of such systems is presented. The problems of multichannel overload, cross talk from adjacent channels, filter design criteria, and signal-to-noise ratio are discussed. The principal contributions of the paper are (1) a new criterion for multichannel overload which is easy to use and is simply related to single-signal overload; and (2) a demonstration that, contrary to general opinion, nothing is gained by spacing filter midband frequencies in such a way that harmonics of lower subcarriers fall outside the pass bands of higher-frequency channel filters. This fact is basic for successful design of an amplitude-modulated subcarrier telemeter because it permits subcarrier frequencies to be spaced much more closely than would otherwise be possible, and so results in improved signal-to-noise ratio.