High selective bandpass filter with mixed electromagnetic coupling and source–load coupling

High selective bandpass filters with a pair of transmission zeros using right-angled triangular stepped-impedance resonators are proposed and analyzed. The transmission zeros on both sides of passband are attributed to the mixed electromagnetic coupling and capacitive source–load coupling, respectively. The transmission zeros locations from the central frequency can be adjusted by controlling the mixed electromagnetic coupling and source–load coupling. The design is also suitable for dual-band bandpass filter with high frequency selectivity. Single band bandpass filters which center at 2.45 GHz and 3.2 GHz, respectively, and a dual-band bandpass filter which centers at 2.45 GHz and 3.5 GHz have been designed, fabricated and measured, the measurements have demonstrated the prediction. Experimental results show that each passband of the presented filters have a pair of transmission zeros as prediction, which improve the frequency selectivity greatly. The measured filters’ passband insertion losses are less than 2.74 dB at central frequencies.