Transmitter Signal Coupling: DC Versus AC

Abstract

Radiofrequency telemetry systems using digitized data and FM transmitters in missile development tests result in serious loss of data at the receiving stations when the receivers have narrow bandwidth. This problem is related to the capacitive coupling network between data input and the transmitter exciter. The relative merits of capacitive coupling versus DC coupling are discussed in this paper on the basis of Fourier analysis of a series of pulses. It may be observed that the digitized data in the systems studied always manifest a DC component due to uneven distribution of binary digits 1 and 0. This DC component is eliminated by a capacitor coupling, but is preserved with a DC coupling. This paper assesses the effects of the elimination of the DC component on the relative position of the spectrum to the carrier. The discussion draws on the results of a test situation of an actual telemetry system. In this test, two identical telemetry transmitters were used; the only variable was the type of coupling exciter employed. A DC coupling is recommended for the transmission of binary information on an FM telemetry transmitter as a result of this test and analysis. Conditions for the proper use of such a coupling are stated.