Positive feedback pilot generation and detection for use in feedforward loop control

A new pilot generation and detection system is described for use in a feedforward compensated power amplifier (FFPA). The pilot tone is generated using frequency-selective nonlinear positive feedback within the FFPA and the IF detection circuit. A feedback loop containing the main amplifier, the second cancellation loop, an IF bandpass filter, and a soft limiter generates a low-level oscillation used as the pilot tone. The frequency of the pilot tone is constrained to the bandwidth of the IF detection filter, and is designed to be outside the bandwidth of the linear signal (carrier). When the pilot tone is isolated sufficiently from the linear signal, the detector output is a monotonic function of the second loop misalignment, making it ideal for loop alignment algorithms based on power minimization. The proposed system differs from other pilot approaches because the feedback structure causes the pilot amplitude to turn-off automatically when the second loop is near the optimal alignment (second loop cancellation reduces the feedback gain below the level needed to sustain an oscillation). This eliminates spurs in the output spectrum associated with pilot feed-through. The pilot re-appears automatically if the alignment degrades (due to the increase in the feedback loop gain). A prototype has been built and tested, verifying the correct operation of the circuit. An open loop power sweep measures the characteristics of the detector and the pilot generator. A closed loop test, where the loop attenuation is swept, demonstrates the system's ability to measure the cancellation provided by the second loop of the FFPA. The pilot system is also tested as part of a FFPA.