Rigorous Analysis of Effect of Input-output Direct Coupling on Triple-mode Third-order Chebyshev Bandpass Filter

An equivalent circuit for the triple-mode third-order Chebyshev response filter is proposed which includes the direct-coupling between the input and output ports. Based on this structure, the existence of a transmission zero for the filter is established through mathematical expressions derived for the computation of the zero. To verify the accuracy of the theoretical transmission zero, a triple-mode Chebyshev filter with a centre frequency at 1.713 GHz and bandwidth of 60 MHz, is designed and implemented, using a dielectric loaded cubical cavity resonator. Very close agreement between the computed and measured results of the filter is achieved.