Passive enhancement of resonator Q in microwave notch filters

The lumped-element bridged-T notch filter concept is extended to reciprocal distributed-element microwave filters. A prototype microstrip bridged-T notch filter employing single-mode resonators is described that enhances the effective resonator Qu by a factor of 325, while a different microstrip filter topology with a triple-mode resonator is described that enhances the effective resonator Qu by a factor of 89. The new notch filters can be either partly reflective or fully absorptive within the stopband. Additional enhanced-Qu notch filter topologies, such as a hybrid-coupler notch filter, that include at least two resonances and at least two signal paths are suggested.