Ultra-compact high-performance filters based on TM dual-mode dielectric-loaded cavities

Abstract

Ultra-compact band-pass filters with Nth-order pseudoelliptic response are presented allowing dramatic volume and mass savings (up to about 90%) compared to conventional filters at the price of a modest Q-factor reduction. TM dual-mode cavities loaded with high-permittivity (εr > 30) ceramic cylinders are employed, while non-resonating modes are used to create N transmission zeros symmetrically or non-symmetrically located in the pass-band. A single cavity behaves as a generalized doublet generating two transmission poles and two transmission zeros and is used as a basic building block for N-pole and N-zero filters with symmetric or asymmetric response. To prove the effectiveness of the approach, a very compact fourth-order filter with 4.35 GHz center frequency, four transmission zeros has been designed, fabricated, and tested showing an unloaded Q close to 2000. It is finally shown that the filter response can be shifted in frequency by simply replacing the dielectric material with another having different εr, so that the same metal enclosure can be used to realize various filters operating at different frequency bands.