Rj Wilkinson

Noise performance of a Cartesian loop transmitter

The Cartesian loop transmitter is now a well-known linear transmitter architecture and is finding application in a number of mobile radio systems employing linear modulation technologies. In particular, systems utilizing /spl pi//4 DQPSK require a linear transmitter, and many emerging standards [e.g., trans-European trunked radio (TETRA)] provide applications for the Cartesian feedback linearization (CFL) technique. One problem with the CFL transmitter is that its output-noise performance is no longer dominated by that of the RF power module employed within it (as is the case in more conventional transmitter architectures). The use of significant degrees of attenuation, followed by a high level of gain, within the loop, means that the noise performance of the loop is significantly poorer than that of a conventional transmitter. There are a number of tradeoffs that are available to the designer of a CFL transmitter to aid in the optimization of the output-noise performance. This paper presents a derivation of the noise performance of a Cartesian loop transmitter and highlights the design methods that may be employed in order to optimize its noise performance. It also provides a comparison of the theoretically derived behavior with that of a practical transmitter operating in the TETRA (380-400 MHz) band.

A highly-efficient linear amplifier for satellite and cellular applications

This paper presents a scheme to linearise highly-efficient amplifiers over a broad bandwidth, whilst retaining most of their efficiency. Present wideband linearisation techniques are normally restricted to either feedforward or analogue predistortion. Feedforward has been shown to provide excellent linearity, but only modest efficiency; whilst analogue predistortion has little effect on efficiency, but limited linearity improvement. However, the combined use of both techniques has enormous potential, since the linearity improvement is greater than that attainable with feedforward alone, whilst the efficiency approaches that of the main amplifier. Practical results are presented which confirm the above predictions in terms of overall efficiency and linearity, and illustrate the effectiveness of the technique.