Design and characterisation of dual-mode suspended-substrate stripline filter

A new design technique of a dual-mode ring-resonator suspended-substrate stripline filter is reported here to achieve low passband insertion loss, high quality factor, and good spurious response. A fourth-order bandpass filter was designed and fabricated at the operational frequency of 2.07 GHz with two ring resonators packaged in a metallic cavity where each ring resonator is one wavelength long. The transmission zeros of the dual-mode filter are generated due to the phase cancellation between the two paths in a ring, thus a sharp-skirt selectivity filter response was achieved. Perturbation notch structure was implemented on each ring resonator to provide the electromagnetic coupling of two degenerate modes, thus a dual-mode response of the filter can be synthesised. Measurement results of the first dual-mode filter prototype showed a return loss and an insertion loss of better than 16.42 and 0.926 dB, respectively, while an out-of-band rejection of up to 55 dB was achieved. The novel filter design technique proved that no cross-coupling is required to obtain the transmission zeros of the filter, thus the sharp-skirt response was achievable.