Tunable lumped-element notch filter with constant bandwidth

A varactor-tuned first-order (two-resonator) lumped-element absorptive notch filter with tunable operating frequency and constant absolute bandwidth has been introduced, along with an approach to its design. To reduce signal voltage across individual varactors, it is suggested to add additional pairs of stacked varactors in parallel, rather than in series. A cascade of three first-order subcircuits is used to make a third-order filter with useful levels of frequency selectivity and stopband bandwidth and attenuation, with a wide tuning range, and with constant -30dB and -3dB bandwidths. Filters of this type are expected to find use in miniature low-power receivers that must function in dynamic operating environments that may have relatively large narrowband interference.