A Design Technique for Dual - Mode Suspended - Substrate Stripline Filter

A new design technique of dual-mode ring-resonator suspended-substrate stripline filter is reported in this paper 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 resonation is one wavelength long. By using 90ソ input and output port arrangement, all transmission zeros of the dual-mode filter can be well controlled and synthesized, thus 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 dual-mode response of the filter can be synthesized. Measurement results of the first dual-mode filter prototype showed a return loss and an insertion loss of better than 16.42 dB and 0.926dB, respectively, whilst an outof-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.