Investigation of Compact Balun-Bandpass Filter Using Folded Open-Loop Ring Resonators and Microstrip Lines

Chia-Mao Chen,Cheng-Fu Yang,Shoou-Jinn Chang,Sung-Mao Wu,Yuan-Tai Hsieh

doi:10.1155/2014/679538

Abstract

A balun-bandpass filter was proposed by using two folded open-loop ring resonators (OLRRs) to couple three microstrip lines. By tuning the size of the OLRR, the operating frequency of the balun-bandpass filter could be tuned to the needed value. By tuning the size of open stub at the end of microstrip lines, the balanced impedance of the balun-bandpass filter could also be tuned. The fabricated balun-bandpass filter had a wide bandwidth and a low insertion loss at center frequency of the passband. The balun-bandpass filter presented an excellent in-band balanced performance with common-mode rejection ratio more than 20 dB in the passbands. An advanced design methodology had been adopted based on EM simulation for making these designed parameters of OLRRs and microstrip lines. Good correlation was seen between simulation and measurement, and the result was that first run pass had been achieved in the majority of our designs.

Highlights

A mass of RF/microwave modules is designed for portable terminals such as handsets, e-readers, and tablet PCs
All kinds of tunable Bandpass filters (BPFs) have been under intensive developments [5], but relatively only few researches have been done on the tunable balun BPFs [6]
To obtain the maximum magnetic coupling, the center valley of the open-loop ring resonators (OLRRs) must be positioned in the proper location along the microstrip line with the maximum magnetic field intensity, which can be determined by studying wave motions on a microstrip line

Summary

Introduction

A mass of RF/microwave modules is designed for portable terminals such as handsets, e-readers, and tablet PCs. To cater for dual-band wireless systems, plenty of researches focus on the balun BPFs with two passbands [2]. To achieve the requirement of the compact and low-cost RF module for the modern wireless communication system, the balun BPFs with the balance-to-unbalance conversion are highly desired. The low-loss balun BPF was using two folded open-loop ring resonators (OLRRs) with equal physical dimensions to couple three microstrip lines, as Figure 1 shows. Each OLRR is placed between two microstrip lines and has a perimeter of about a half wavelength of the designed resonant frequency. The proposed balun BPF has low insertion loss, a wide tunable range of passband, transmission zeros, and simple design. By tuning the size of open stub at the end of microstrip line, the balanced impedance of the balun BPF can be tuned. Port 3, balanced Figure 3: Simulation of current distribution and coupling paths oscillating at 2.6 GHz

Design Methodology

Design of Balun-Bandpass Filters

Conclusions