A Phase-Opposition System of Amplitude Modulation

The increasing economic importance of power efficiency for high-power broadcast stations is illustrated in Fig. 1 showing that some 50 per cent saving is possible. Different known modulation schemes are compared as regards necessary input power, including the recent Chireix and Doherty systems. The influence of rates for electric power is stressed. A so-called "phase-opposition" system is discussed starting with simple circuit theory, first for seriesand then for parallel-tuned circuits. Principally the system is based on the co-operation of two generators; i.e., one "carrier" generator constantly excited, and one "side-band" generator whose excitation is changed 180 degrees in phase according to the phase of modulation. Mean input power is calculated for this system, giving about the same input as good systems for about 30 per cent modulation, but 10 to 25 per cent more power at 100 per cent modulation. The paper is concluded with a consideration of tube characteristics and possible means for obtaining linearity are shown.