A VHF tunable lumped-element filter with mixed electric-magnetic couplings

Abstract

A lumped-element varactor-tuned bandpass filter (BPF) with variable center frequency and constant absolute bandwidth (BW) is presented in this paper. Mixed electric/magnetic inter-resonator coupling schemes—implemented with parallel LC resonators—facilitate the actualization of an almost constant BW passband over a wide frequency tuning range (1.6∶1). The linearity performance of the varactor-tuned BPF is studied and an investigation to reduce the RF swing on the filters' tuning elements is presented. A third-order lumped-element BPF was designed, built and measured. It exhibited a tunable center frequency between 230–370 MHz with constant 3-dB bandwidth of 56.3 ± 0.2 MHz across the entire frequency band. Within this band, the insertion loss was measured between 0.9–1.4 dB and the third-order intermodulation intercept point (IIP3) was measured between 28.1–41.4 dBm.