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This paper reports the use of MMIC power amplifier modules in an X-Band power amplifier for the replacement of Travelling Wave Tubes (TWTs) in terrestrial point to point digital communication links. The amplifier modules use MMIC MESFET technology to ensure high reliability and repeatability in manufacture. The use of MMICs results in an easily manufacturable power module, requiring no tuning or phase matching, at low cost. The complete amplifier uses optimum power combiners designed using state of the art electromagnetic simulation software. The solid state amplifier reports improved linearity and increased lifetime compared to the equivalent TWT. The solid state amplifier achieves a P-ldB of greater than 16W CW and at a power output of 6W results in Intermodulation Products of less than -42dBc in the transmitter system. The amplifier and its associated power supply are also lower cost than a replacement Travelling Wave Tube only.' COUPLER ISOLATOR IF in RF out PREDISTORTER POWER BIT DETECTOR UPCONVERTER AMPLIFIER Figure 1 SEL transmitter sub-rack block diagram.different power levels. The low power system operates at 36dBm, and the high power system at 38dBm. The system must produce Intermodulation Products of less than or equal to42dBc at the required output power level. Figure 1 shows a block diagram of the SEL transmitter sub-rack. AMPLIFIER REQUIREMENTS INTRODUCTIONGaAs MMIC Amplifier technology is now sufficiently mature to challenge Travelling Wave Tubes in microwave Power amplifier applications. Microwave Solid State Power Amplifiers (SSPAs) can offer similar performance, efficiency and costs to TWTs with considerably higher reliability. SSPAs also offer improved AM and FM noise levels. These advantages make SSPAs very attractive alternatives to TWTs in communication applications. An SSPA has been designed to meet the specific requirements of a digital point to point terrestrial communication system. The SSPA is a form-fit-function replacement for a TWT and the associated power supply. The SSPA consists of a variable gain amplifier for gain control and temperature compensation, and three driver stages for gain. A six-way low loss splitter then feeds six 2W power amplifier modules. The modules are combined in a six-way power combiner to produce a final output power of over 16W CW.The power supply is a separate unit containing switchmode DC-DC convertors. DIGITAL MICROWAVE SYSTEMThe SEL DRS140/11200 is a digital microwave system operating in X-Band, capable of carrying a 14OMBiUs digital data stream. The system uses 16 QAM, and therefore, to improve linearity, the system uses a predistorter to compensate for the distortion in the power amplifier stage. The power amplifier is also operated about 5dB below the saturated power output. There are two versions of the SEL system offering It was decided for logistical reasons to build one amplifier to satisfy both the high and low power requirements. The solid state amplifier would need to have a P-ldB power output at least 4dB higher than the power requirement of ...

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