Abstract: A radio frequency amplifier uses an offset phase locked loop to produce a radio frequency signal from a baseband signal. In order to allow the phase locked loop to process the signal as one of constant amplitude, the signal is amplitude limited prior to application to the phase comparator, and envelope restoration is achieved by envelope-modulating the output in the RF power amplifier stage.

Abstract: A cartesian amplifier system for pre-distorting a signal to be amplified to take account of non-linearities and other non-ideal characteristics of the amplifier itself. A signal to be amplified is divided into quadrature and in-phase signal components in a digital signal processor 1. The processor 1 also pre-distorts the resolved quadrature and in-phase components using various pre-distortion factors. These pre-distortion factors are derived on the basis of a feedback signal from a measurement of the output signal power from the amplifier and on the basis of the signal levels of the pre-distorted resolved components themselves. These pre-distortion factors are constantly updated, thus taking into account any shifts in the non-ideal characteristics of the amplifier.