Novel active RF tracking notch filters for interference suppression in HF, VHF, and UHF frequency hopping receivers

The author describes a novel low-loss, highly selective, fast electronically tunable RF active notch filter with wide passband which has been developed to suppress strong fixed and frequency-agile CW and narrowband interfering signals. The design concept is based on the principle of using maximum realizable L/C ratio for the notch resonator to achieve optimum selectivity with a minimum number of tunable notch resonators, and the synthesized negative resistance of an RF capacitive transistor to overcome the dissipation losses in the passive elements to produce high-Q RF resonators. This approach allows compact lumped-element construction of highly selective notch filters with low passband insertion loss at radio frequencies using low-cost surface mount chip components. Experimental confirmation of the design feasibility was achieved using several developmental HF and VHF active varactor-tuned notch resonators and filters with regard to canceling the effects of parasitic losses in the passive notch resonator, notch resonator selectivity as a function of L/C ratio, peak attenuation, passband bandwidth and passband insertion loss and broadband stability