Positive Feedback Pilot System for Second Loop Control in a Feedforward Power Amplifier

Abstract

A novel pilot generation and detection system for aligning the second loop of an adaptive feedforward compensated power amplifier (FFPA) is described. A frequency-selective nonlinear positive feedback loop embedded within the FFPA generates a pilot tone and IF detection measures the residual pilot power after the second loop cancellation. The proposed system differs from other pilot approaches because the self-oscillating feedback structure uses the gain of the main amplifier and the cancellation of the second feedforward loop to generate the pilot. The latter causes the amplitude of the generated pilot to drop abruptly (turn off automatically) when the second loop alignment converges, eliminating spurs in the output spectrum associated with residual pilot feedthrough. Self-oscillation allows the power-based detection to, optionally, be extended into synchronous detection by measuring the frequency of the generated pilot. A theoretical analysis of the positive feedback pilot circuit is developed and verified using experimental results. Contours obtained from two-dimensional sweeps of the available alignment settings verify that the pilot system measurements are suitable for descent-based search algorithms using either the power or synchronous detection modes. Improved convergence is achieved using a proposed algorithm based on synchronous detection.