Graduation Year

2004

Document Type

Thesis

Degree

M.S.E.E.

Degree Granting Department

Electrical Engineering

Major Professor

Sankar, Ravi

Keywords

non-linear, AM-AM, AM-PM, IMD, simulation

Abstract

The existing UHF Satellite Communications (SATCOM) transponders used for military applications use efficient, saturated power amplifiers, which provide one earth-coverage antenna beam. The amplifier is dedicated to small frequency band and only handles a few carriers simultaneously. The communications capacity needed to support future military forces on the move will require satellite payload power amplifiers to support hundreds of channels simultaneously, with the channels spread over the entire military UHF SATCOM band. To meet the capacity requirements and simultaneously meet the out-of-band emission, power amplifiers will have to be highly linear. The high-efficiency, ultra-linear power amplifier architecture proposed to support the requirements can only be met by use of linearity improvement techniques. The literature search revealed many power amplifier linearity improvement techniques.

Each technique was reviewed to determine its suitability for the proposed power amplifier architecture. The adaptive digital predistortion technique was found to be the most suitable in terms of bandwidth, correction achievable, and complication. A discussion on common linearization techniques is presented, followed by analysis of the adaptive digital predistortion technique. A SIMULINK simulation model of an adaptive digital predistorter was developed. The simulation results show that adaptive digital predistortion was able to significantly reduce the Inter-Modulation Distortion (IMD) terms generated by a memory-less power amplifier operating in the 240 MHz to 270 MHz range. An actual hardware implementation of adaptive digital predistorter was constructed and the test results show that there was a large reduction in IMD terms generated by a memory-less power amplifier.

In the contrary, the results show there is only moderate improvement in IMD performance if the power amplifier has memory. The electrical memory in the power amplifier with memory was minimized, but this resulted only a modest improvement in the IMD performance. Therefore, it was concluded the majority of the memory effect was due to thermal memory.

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