Microstrip cascaded trisection filter with multiple pairs of transmission zeros

A novel cascaded trisection bandpass filter with multiple pairs of transmission zeros is proposed. In this circuit, each zero pair at real or imaginary frequencies is generated by a single trisection with resonant cross-coupling, i.e. mixed electric and magnetic coupling between non-adjacent resonators. This type of cross-coupling can be simply modeled by a J-inverter multiplied by the complex frequency variable s. Direct extraction of the element values of the lowpass equivalent circuit from the referred quasi-elliptic transfer function is developed. Microstrip resonators are designed to realize the resonant cross-coupling. Experimental filters are implemented and measured to validate the design.