Ultracompact Bandpass Filter With Multiple Transmission Zeros and Low Insertion Loss

Two ultracompact bandpass (UCB) filters have been proposed. To the best of the authors’ knowledge, the filter I is the minimum size microwave bandpass filter ever reported. The proposed circuits successfully integrate an integrated passive device (IPD) technique-based high-Q capacitor and energy coupling circuit with a printed circuit board (PCB) substrate. By creating both separate electric and magnetic coupling path (SEMCP) and source-load coupling path, two transmission zeros (TZs) on the stopband are generated. A wide stopband suppression and good passband selectivity are achieved by this technique. New energy coupling circuits are designed to reduce energy coupling losses of the compact filters; therefore, both compact size and low insertion loss (IL) can be achieved by the filters. Two particular examples of the filter are implemented and experimentally demonstrated. The results show a filter with a size of $0.55\times 1.38$ mm2 ( $5.2\times 10^{-4}~\lambda _{g}^{2}$ at 3.7 GHz), an IL of 2.2 dB at center frequency and two TZs on the upper stopband. The other filter demonstrated with a size of $2.1\times 1.32$ mm2 ( $2.7\times 10^{-3}~\lambda _{g}^{2}$ at 4.5 GHz), an IL of 1.81 dB at the center frequency and two controllable TZs on the stopband.