An intermediate frequency predistortion linearizer for an earth station traveling-wave tube amplifier

In multicarrler single-channel-per-carrier (SCPC) communication system design, intermodulation distortion is often a limiting factor. The intermodulation distortion caused by the AM/AM and AM/PM conversion effect of the microwave transmit high power amplifier (HPA) can be reduced by applying linearization techniques. Various linearization techniques are reviewed and an appropriate scheme is selected for use with a SCPC earth station traveling-wave tube amplifier (TWTA). A nonlinear model based on the third-order distortion phase of a TWTA is used to develop a new linearizer design model. The amplitude and phase transfer characteristics of a 14 GHz TWTA were measured and the results used in the design of an intermediate frequency (IF) predistortion linearizer. The predistortion linearizer developed uses an antiparallel diode distortion generator in a parallel branch network. The linearizer circuit operates at an IF of 280 MHz preceding an upconverter stage and the 14 GHz TWTA. The linearizer was tested with a 20 watt 14 GHz TWTA under two-tone, multicarrler, and noise loading conditions. The experimental linearizer circuit effectively reduced the intermodulation distortion over a considerable range of normal drive levels, providing a 3 dB decrease in output backoff while maintaining a third-order IMD performance of 25 dBc. The linearizer compares favourably with published results of similar predistortion linearizers designed for satellite applications, and shows potential for practical use in low cost Ku band SCPC earth stations.Applied Science, Faculty ofElectrical and Computer Engineering, Department ofGraduat