SHF 3-bit Reconfigurable Band Rejection Filter Using Variable-Length λ/4 Anti-Resonators and LC Parallel Coupling

Band Rejection Filters (BRFs) are useful to eliminate unwanted spurious emission and interfering components in the vicinity of desired signals. In the environments where operating carrier frequency changes, it is necessary for the BRF to change its stop band flexibly. In this paper, we propose a novel reconfigurable BRF that employs branch-line switchable variable-length λ/4 anti-resonators. It can digitally change its notch frequency among eight options by controlling the states of RF switches. In order to achieve sharp cut-off and low insertion loss in passband, we introduce LC parallel coupling and optimize the characteristic impedance of inter-stage coupling line. A prototype circuit is designed and fabricated to provide notch frequencies from 3.0 to 4.7 GHz. The measured 6 dB stop bandwidths are from 836 to 940 MHz and insertion loss in lower passband is less than 0.36 dB.