Interference can drive a receiver's front-end amplifier into compression - distorting, masking, and/or compressing weaker signals of interest - and interferer frequencies can vary with time. Recently, distributed- element absorptive bandstop, or 'notch', filters have been demonstrated that can selectively eliminate such interference using lossy (i.e., small and inexpensive) circuit components. These filters maintain excellent characteristics, even when tuned over broad frequency ranges. To extend this filter technique to lower frequencies, this paper introduces a new lumped- element 'absorptive pair' realization that is able to maintain near constant characteristics while tuning over nearly an octave. As shown in Fig. 1, a 'first-order' absorptive-pair notch filter consists of a pair of resonators, each coupled to a common transmission line and to each other. Due to their relative simplicity, first-order absorptive filters tend to be the most practical for tunable applications, and they can be cascaded to realize wider stopband bandwidths. This paper describes such a varactor-tuned lumped-element notch filter - composed of a cascade of three 'first-order' stages - with a 26 to 50.375 MHz frequency tuning range, a -30 dB stopband bandwidth of 1.625 MHz, and a -3 dB bandwidth of 5.085 +/- 0.185 MHz.
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