Triple-Mode Microwave Filters With Arbitrary Prescribed Transmission Zeros

Mustafa S Bakr

doi:10.1109/access.2021.3052059

Abstract

This article presents a new filter design for arbitrary placement of real frequency transmission zeros in triple-mode filters, allowing for either asymmetric or symmetric responses. Triple-mode filters can be viewed, in general, as a series combination of coupled resonators with non-adjacent couplings, allowing for the realisation of transmission zeros. The strength and phase of couplings dictate the transmission zeros locations. The arbitrary placement of transmission zeros requires controlling all amplitude and phase rations, i.e., the ability to realise both negative and positive couplings. In general, this necessitates the use of capacitive and inductive probe structures complicating the construction and hence increasing the filter cost. The method presented in this article enables arbitrary placement of transmission zeros using only inductive or only capacitive couplings structures. In addition, broadband filtering characteristics can be realised using triple-mode filters. Examples of several narrow and broadband bandpass filters are given indicating that this method is valid for most triple-mode structures.

Highlights

High performance microwave filters are key devices in many microwave systems, which are used to separate different communications signals according to their electrical frequency
Further size reduction can be achieved by using multiple degenerate modes which increases the efficiency of volume per resonance for a given Q factor
Let’s us consider a broadband filter having a symmetric response with the following specifications: Passband: 1.927-2.0603 GHz; Return Loss: 20 dB

Summary

INTRODUCTION

High performance microwave filters are key devices in many microwave systems, which are used to separate different communications signals according to their electrical frequency. Several papers reported the realisation of triple-mode dielectric resonator filters using non-degenerate modes [5]–[8] They do not explain how to control real-frequency transmission zeros placement. The input and output coupling structures perturb the resonant modes such that the field distrubtion inside the metal cavity undergoes a set of rotation in a 3-D fashion such that the maxima and minima are not always located on the xy plane. Such behaviour will be shown in the subsections. For this configuration only one real solution exists which is physical and are of practical interest

PHYSICAL DIMENSIONING

BROADBAND BANDPASS FILTERS WITH TRANSMISSION ZEROS ON BOTH SIDE