A Balanced Filtering Directional Coupler with Wide Common-Mode Suppression Based on Slotline Structure

Luyan Qiao,Yong Yang,Rui Li,Nankai Wu,Feng Wei,Xiaoning Yang,Ying Han

doi:10.3390/electronics10182254

Luyan Qiao, Yong Yang + Show 5 more

Open Access

https://doi.org/10.3390/electronics10182254

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Journal: Electronics	Publication Date: Sep 14, 2021
Citations: 3	License type: CC BY 4.0

Affiliation: China Academy of Space Technology, Xidian University

Abstract

In this paper, a balanced-to-balanced filtering directional coupler (FDC) that can realize a 3 dB coupling degree directional coupler with high isolation and directivity is proposed. The design of the proposed FDC is primarily based on microstrip/slotline transition structures, resonance structures, and odd–even mode phase velocity compensation structures. A U-type microstrip feed line integrated with a stepped-impedance slotline resonator is adopted at the input and output ports, which makes the differential-mode (DM) responses independent of the common-mode (CM) ones, and brings superior DM transmission and CM suppression. In addition, by loading the microstrip stub-loaded resonators (SLRs), a DM passband with sharp filtering performance is realized, and transmission zeros (TZs) can be added into the design, which makes it more selective. Moreover, phase compensating slotlines are added into the coupling structure to enhance the isolation. In order to verify the feasibility of the proposed design method, an FDC prototype circuit was made and tested. The simulation results are in good agreement with the measured results. The designed coupler’s DM operating band covers 2.65 GHz to 3 GHz (FBW = 12.4%), and the insertion and return losses are 4.6 dB and 20 dB, respectively. The isolation degree is better than 15 dB, and the CM suppression is more than 55 dB. The total coupler size is about 67.7 mm × 63.8 mm. The designed balanced-to-balanced FDC can be widely used in S-band wireless communication systems.

Highlights

In recent years, with the development of small-scale integration, RF and microwave systems require more integrated circuits and devices
stub-loaded resonators (SLRs) consists of a half-wavelength microstrip resonator and an open-circuit sub-loaded in the center
It can be seen from the figure that after adding the multi-U-type parity mode phase velocity compensation structure, the isolation of the balanced-to-balanced filtering directional coupler (FDC) is significantly improved

Summary

Introduction

With the development of small-scale integration, RF and microwave systems require more integrated circuits and devices. The coupler adopts a U-type microstrip feed line integrated with a stepped-impedance slotline resonator, as shown in Figure 1a (1), which can separate CM and DM, and effectively improve CM suppression. When the RF signal is excited from the balanced input port, the DM signal is transferred into the slotline through the U-type microstrip feeder, and reaches the filtering part through the underlying L-junction gap structure. The odd mode with compensated phase is coupled from the slotline to the microstrip line, and energy is the output from the coupling end. The balance microstrip transition gap structure can effectively save the size, under the condition of ensuring the function

Filter Resonant Structure

L-Junction Slotlines and Transmission Zeros

Design Procedure

Simulation and Measurement Results

Conclusions