Linear Wideband HF Power Amplifier using Adaptive Feedforward Cancellation

To fulfill modern Naval command, control, and communication (C3) requirements a new wideband High Frequency (HF) communications system is being developed that is characterized by the complete removal of high power switching and tuning, and that uses adaptive cancellation techniques in the transmitting subsystem to suppress unwanted radiation of noise and distortion. The paper focuses on the development of an unusually linear 500 watt power amplifier and describes techniques that have been applied to further reduce noise and distortion product generation by means of an iteration of feedforward cancellation stages. Although three stages have been planned, only two have been implemented. To minimize power handling and distortion requirements of the error amplifiers, adaptively controlled weights are used to augment fixed equalizers in the cancellation of fundamental energy for each stage. Performance measurements verifying the concept are presented and various practical limitations are described. Intermodulation distortion to signal ratios of -86 dBc have been achieved.