Design of wideband bandpass filter using mode-exciting technology

This article presents a simple method to design a new compact wideband bandpass filter (BPF) using the mode-exciting technology. The basic resonator used in the design was composed of an open-circuited stub and a pair of symmetrical parallel coupled-lines. By properly tuning the electrical length ratio of the stub and parallel coupled-lines, a controllable resonant mode was excited and two transmission zeros were created in accordance with the results of imaginary-admittance analysis. With the help of the high-impedance coupled microstrip lines, tapped on the Input/Output (I/O) ports, additional modes were excited to provide a wide bandpass response. The designed wideband BPF had an advantage of controllable bandwidth due to a variation of created transmission zeros. A filter example with a compact circuit size was verified to have the measured results including a wideband response from 3.4 to 8 GHz with a low insertion loss of 0.4 dB.