20-WATT X-BAND SOLID-STATE TWTA REPLACEMENT

Abstract

In the past, traveling wave tubes have dominated the power amplifier field in space flight transmitter applications. TWTAs however, are expensive, and high-power TWTAs may be relatively unreliable when considering end-of-mission operating requirements of up to 10 years and longer. Recent improvements in reliable solid-state designs including Gallium Arsenide FETs have resulted in efficient X-band devices which operate at multi-watt levels. A number of these devices would be combined in parallel to achieve the power output desired. This paper discusses the development of an efficient 20 watt X-band solid-state power amplifier to be used as a TWTA replacement for space flight applications. The 20-watt assembly shown in Figure 1 consists of two stand-alone, 8-channel, 11-watt assemblies operating in parallel through a 2-way switched power combiner. Particular emphasis is placed on the power amplifiers and a one-step, 8-way power divider/combiner pair. Each power amplifier channel has 3 stages of gain and develops 1.6 watts of RF output with 18 dB of compressed gain. A driver amplifier module provides additional system gain and a 0.5-watt drive level for the high power sections. The GaAs FET devices utilized are from the output of a device improvement program which has a goal of a minimum power added efficiency of 40%. The one-step combiner utilizes a novel technique to achieve a combining efficiency of 90%. The full system, including the DC-DC power converter, yields an overall system efficiency of 25%. A thermal sensor in the power conditioner is used to hold amplitude variations to ±0.3 dB from 0 to 50°C and phase variations to ±10° over the same range. The assembly also operates in a low power mode producing 9 watts of RF when only one 11-watt assembly is powered and the switched combiner open circuits the off channel.