Combination of a one-wavelength ring and coupled-lines for compact and selective bandpass filter

This paper presents a wideband bandpass filter topology which is based on a combination of a one-wavelength ring and two identical quarter-wavelength coupled-lines. The 3rd order bandpass response can be controlled by varying the line impedance of the ring and the even- and odd-mode impedances of the coupled lines. A simplified equivalent circuit diagram of the topology is also proposed to open for further analysis of the filter. A realization of the filter is performed using microstrip technology on FR4 epoxy glass substrate and the measurement results are found to be in good agreement with the simulation results.