A Novel Diplexer Integrated With a Shielding Case Using High $Q$ -Factor Hybrid Resonator Bandpass Filters

This letter presents a new approach for designing a high-selectivity diplexer, which is based on compact and high unloaded quality factor hybrid resonator (HR) bandpass filters. The proposed HR is composed of a microstrip line and a short-circuit coaxial line, which has great saving of space in the shielding case of RF module. Additional transmission zeros on both sides of the passband response, which are created by the source-load (S-L) coupling and mixed electromagnetic coupling scheme, can easily be realized to enhance the out-of-band rejection of the filters. A diplexer prototype at center frequencies 2.4/2.7 GHz with 3-dB bandwidth of 80/102 MHz, respectively is designed. The measured insertion losses are better than 1.5 dB for the two channels with greater than 40 dB in output isolation, which shows good agreement with simulation results.