Bandpass Filter Using Discriminating Coupling for Extended Out-of-Band Suppression

This letter presents two bandpass filters with wide out-of-band suppression using multiple different resonators and discriminating coupling. By utilizing half- and quarter-wavelength resonators, the second harmonic can be eliminated. Discriminating coupling is used to obtain zero coupling coefficient at the third harmonic, whereas the coupling in the passband can be tuned to desired values. The proposed filter could obtain 30 dB rejection level in the stopband from 1.2f₀ to 3.8f₀. Meanwhile, the filter is simple to design and easy to fabricate. The theoretical analysis of the proposed filter is given and two demonstration filters are fabricated and measured. The simulated and measured results are presented and good agreements are observed, which confirm the proposed method.