By R.T. Hsu, Dr. T.M. Straus, P.A. Vogt and J.L. Wrona, Hughes Aircraft Company
Predistortion provides an effective means of improving the linearity of an AM microwave transmitter. Unlike power doubling and feed-forward, predistortion can be implemented with a relatively simple VHF circuit that partly compensates for the nonlinearity of the microwave klystron amplifier utilized in the output of the high power Hughes AML® STX-141 transmitter. The resultant improved overall linearity could permit additional channel loading of an FM band transmitter, the running of either FM or TV channel audio at a higher level than the normally specified -17 dB relative to video without compromising the transmitter C/IM performance, or a 3 dB increase in TV channel transmitter output while maintaining intermodulation distortion and differential gain and phase within specified limits.