Compact Self-Shielded 2–3 GHz High-Q Coaxial Fixed and Tunable Filters

A novel coaxial stepped-impedance resonator (SIR) filter is proposed based on conventional multi-layer printed-circuit board (PCB) technology. The resonator results in a measured unloaded quality factor (Qu) of 185 at 3.7 GHz, has compact size, is self-shielded and has very low electromagnetic (EM) coupling to nearby RF circuits. The 3-D filter topology easily lends to advanced filter design with multiple transmission zeroes using cross coupling and source-to-load coupling on the top printed-circuit board layer. Two third-order fixed-band bandpass filters, and two fourth-order tunable bandpass filters are demonstrated in this technology. The fixed-band filters have an insertion loss of 1.6 dB with a fractional bandwidth of 6.5% at 3.6 GHz and occupy a size of 9 × 12 mm². The tunable filter covers 2.1-2.9 GHz with a fractional bandwidth of 3.5%, and an insertion loss of 4.2-9.2 dB, limited by the silicon varactors. Multiple transmission zeroes are used in these designs to significantly improve the filter selectivity. Application areas are in fixed and tunable filters with very compact size and high shielding from nearby circuits for advanced communication systems.