Three-pole Microwave Bandpass Filters with Non-resonating Nodes and Multiple Transmission Zeroes

Design of microwave filters with transmission zeroes (TZs) at finite frequencies allows increasing filter selectivity for a fixed order (number of resonators), i.e. without a compromise in insertion loss and filter size. With the aid of a cross coupling between non-adjacent resonators, a single TZ can be generated in a three-pole filter or a pair of zeroes can be introduced in a four-pole filter characteristic. Filters with non-resonating nodes (NRNs) which have been recently proposed, can have as many TZs as the filter order is. Design of the three-pole bandpass filter with two or three TZs is presented in the paper. The TZs are obtained from the NRNs which are implemented by means of highly detuned resonators and immittance inverters. The designed microwave filters exhibit higher selectivity compared with a conventional three-pole Chebyshev filter of the same bandwidth and return loss. Besides, the filters with NRNs are less sensitive to parameter variation.