Tunable Lumped-Element Notch Filter with Constant Bandwidth
NAVAL RESEARCH LAB WASHINGTON DC
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Interference can drive a receivers front-end amplifier into compression - distorting, masking, andor 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.
- Electrical and Electronic Equipment