High power broadband cancellation system

Abstract : The objective of this report is to improve the broadband cancellation technology associated with adaptive UHF (225-400 MHz) Interference Cancellation Systems (ICS) employed with tunable or hopping transmitters. More specifically, the objective is to cancel a narrowband transmitter waveform centered at a frequency f sub o plus its noise and modulation sidebands that extend over plus or minus 0.02 % of f sub o. This report describes the concept for a two-channel ICS which provides improved broadband cancellation compared to that achievable with a single-channel ICS. The input to the second-channel weight is spectrally pre-shaped by an adaptive notch filter which automatically tracks the frequency of the interference. Control of the second-channel weight utilizes a novel pilot-directed control system. A functioning unit was constructed to implement the design concept. Experimental measurements of the unit's performance across the 225 to 400 MHz band are presented which show f sub o cancellation averaging about 60 dB, and sideband cancellation (at f sub o plus or minus 85 KHz) averaging almost 55 dB. The settling time is less than 20 milliseconds. Suggestions for design refinements and for more extensive testing of the unit are discussed.