Filter Design Using Double Extracted Zeros

Abstract

In [1] a single transmission zero was used to enhance the lower stopband of an all-pole bandpass filter, using a capacitively loaded coaxial stub inductively coupled to the first and/or last resonator of the bandpass circuit. An improvement was presented in recent publications [2], [3], [4], showing that it is possible to produce in-line bandpass designs with a transmission zero near the passband, using the same physical resonator structure as employed in the bandpass portion. This has been accomplished using shunt resonators weakly coupled (using a virtual inductive coupling) to the first and/or last bandpass resonator, resulting in a configuration that locates the zero near a passband edge, with the concomitant pole located far from the passband. Both the original and the improved circuits have thus been called “Extracted Zero”. Use of the same physical resonator structure simplifies the physical implementation. In this paper, the “Double Extracted Zero” is introduced. Similar in physical structure to the single zero in [1], the double zero is used to provide additional depth or width to the desired zero frequency region, and can be located at either or both ends of a given passband.