Compact tunable 2.1–2.9 GHz band-pass filter using a novel vertical stepped-impedance resonator

A novel vertical stepped-impedance resonator (SIR) is proposed based on the conventional multi-layer printed-circuit board (PCB), exhibiting high unloaded quality factor (Q = 185), compact size and low electromagnetic (EM) coupling to nearby RF circuits. The proposed resonator is used in a 2–3 GHz 4-pole tunable bandpass filter. The resulting filter occupies a footprint of 13 mm × 15 mm and covers a frequency range of 2.1 – 2.9 GHz. In addition, to improve the filter selectivity, multiple transmission zeroes are obtained in each tuning state by employing source-to-load coupling and cross-coupling between non-adjacent resonators. To our knowledge, this represents a new way to fabricate compact high-performance shielded filters in low-cost PCB technology.