High selectivity wideband bandpass filter based on stepped impedance open stubs loaded ring resonator

Abstract

A high selectivity wideband bandpass filter (BPF) is proposed based on a stepped impedance open-stub loaded ring resonator. The resonant modes can be adjusted by the parameters of the stepped impedance stubs and eventually to tune the bandwidth conveniently. The even resonant modes can be tuned lower than the first odd resonant mode that is excited by the half wavelength resonator. Due to the analysis of the electric field distributions on the resonator, three adjacent resonant modes can be employed to form a wide bandwidth by electric coupling between input/output feeding line and the resonator. In addition, by introducing two more resonant modes in the passband using the parallel-coupled feeding open stubs, the selectivity is further improved. The related coupling topology is theoretically analyzed and experimentally verified. As the design example, a wideband BPF centered at 3.24 GHz with a 3-dB fractional bandwidth of 58.3% is fabricated and measured. The experimental results fit the simulations well.

Introduction

Since microstrip components have the advantages of low cost, small size, light weight, and easy fabrication, more and more researches have studied microstrip filters for different requirements, in which high performance microstrip filters play important roles in the wireless communication devices. As one class of them, wideband filters with high selectivity are required in certain applications, such as in radar warning receivers. The conventional method for realizing wideband BPFs with high selectivity is to cascade multiple resonators or introducing cross coupling or source-load coupling, which usually results in large size or complex coupling topology [1], [2], [3], [4], [5], [6]. In order to achieve more compact size, one popular approach is to employ multi-mode resonators to reduce the number of resonators required [7], [8]. For example, dual resonant modes produced by the shorted stub loaded resonator are used to design wideband BPFs in [7]. In [8], a triple-mode cross-shaped resonator is employed to design a three-pole transversal BPF. The coupling matrix is used for direct synthesis of the filter. Meanwhile, coupled lines are always used to excite multiple resonances [9], [10], [11] to realize wideband filtering characteristic. However, the number and frequencies of the resonances are not easy to tune.

On the other hand, to further reduce the size of the resonators, stepped impedance resonators (SIRs) are applied in multi-mode wideband BPF design [12], [13], [14]. In [22], six short circuited line loaded triangular stub resonators are used to obtain a six-order compact wideband BPFs. The dimension of the triangular stub gets larger, the size of the resonator becomes more compact. Also SIRs can adjust the resonant modes by tuning the length ratio or impedance ratio conveniently. As a result, SIR loaded resonators have been widely used in different kinds of microstrip filters applications.

Meanwhile, ring resonators [15], [16], [17], [18] have been studied in many studies because they can provide two transmission paths for introducing transmission zeros (TZs) by signal interfering, which can improve the filter performance. In [16], the stepped ring resonator provides quad-mode in the passband, and the parallel-coupled lines introduce two more transmission poles. Interdigital coupled lines are always installed at input/output port of the ring resonators for realizing wide passband [18], [20], [21]. In [18], the interdigital coupled lines also introduce two resonances into the passband, and the loaded open stubs are arranged inside the square ring for size compactness. The loaded stub can also control the transmission zeros [20], [21]. However, thus far, little research theoretically illustrates the working principle of the added resonant modes in detail. The selectivity performance can be further enhanced on both sides of the passband by adjusting the coupling coefficients between the resonances.

In this paper, a stepped impedance open-stub loaded ring resonator is presented. The coupling between the input/output port and multiple resonances of the loaded ring resonator are illustrated by the electrical field distributions on the ring. According to the position of the feeding line, triple-mode can be excited in one passband, which are used to realize a wide passband in this design. Meanwhile, two transmission zeros (TZs) introduced by the intrinsic dual-path of the ring are used to enhance the selectivity. In addition, the feeding stubs are also used to add two more resonant modes in the passband, which can further improve the selecting performance of the filter. A synthesized approach is proposed to design a five-order wideband BPF. To the best of the authors’ knowledge, it is the first time to address a coupling topology and the related coupling matrix to explain the working principles of the quint-mode excited in the passband and the TZs. This paper is organized as follows: Section 2 will analyze the resonance characteristics of the proposed resonator. The coupling principles and topology are analyzed in Section 3, and a design example is proposed for the verification. The simulated and measured results demonstrate the principles theoretically and experimentally. Finally, conclusions are given in Section 4.