Using the Pilot Frequency from a Positive Feedback Pilot System to Improve Second Loop Convergence for a Feedforward Amplifier

An extension to the positive feedback pilot generation and detection system, described previously in [1], is proposed to improve convergence of the second loop alignment search used in an adaptive feedforward compensated power amplifier (PA). The original pilot system uses frequency-selective nonlinear positive feedback to generate a pilot tone and IF detection to measure the residual pilot power after the second loop cancellation. It provides information on the magnitude component of the second loop cancellation transfer function. The new system measures the frequency of the generated pilot from which phase information regarding the second loop cancellation transfer function can be inferred. Pilot frequency information converts the original power-based pilot detection into synchronous pilot detection and allows the use of better search algorithms for control of the alignment setting. A novel search algorithm is presented that demonstrates the utility of the pilot frequency measurements. The hardware for the original pilot system exists as part of a commercially available power amplifier. The extension to the pilot system - the measurement of the pilot frequency - is demonstrated using a spectrum analyzer coupled to the RF output. The new algorithm controls the second loop alignment based on the pilot power and frequency measurements. Experimental results show that the search trajectory of the new algorithm is well-damped and converges in a fewer number of iterations than a coordinate descent search