Inline comb filters with one or two transmission zeros

This work describes a design method for introducing one or two transmission zeros in classical direct coupled resonators filters without cross couplings. It is shown that this goal can be achieved by placing input and/or output in correspondence of the second and/or last-but-one resonators; in this way the first and last resonators act as rejection resonators and up to two transmission zeros can be introduced in the filter response. The design method is based on a low pass prototype, which is obtained by applying the matrix rotation technique, followed by a sequence of suitable circuit transformations. The proposed design technique has been applied for introducing two transmission zeros in classical in-line comb filters; the measured results obtained from a test device are presented and compared with simulations.