An improved analytical model for broadside coupled transmission line used on planar circuit

Renbin Tong,Jörgen Olsson,Dragos Dancila

doi:10.1016/j.aeue.2021.153873

Renbin Tong, Jörgen Olsson + Show 1 more

Open Access

https://doi.org/10.1016/j.aeue.2021.153873

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Abstract

Broadside coupled transmission lines have the advantage of more compact size and higher coupling factor compared to the traditional coupled microstrips in the realization of planar circuits such as substrate integrated baluns, directional couplers, and filters on printed circuit boards. A broadside coupled transmission line is a structure with interesting properties composed of two signal lines with strong coupling placed on the top and bottom of a dielectric substrate, surrounded by coplanar ground. This configuration allows the realization of the practical circuit with several hundred ohms impedance for even mode excitation, with the help of lower even mode capacitance. This leads to a high coupling factor and benefits to a high impedance transformation ratio balun or a wide bandwidth filter design. In this paper, an analytical model describing the symmetrical pair of conductors for broadside coupled transmission lines is presented. The analytical model based on the physical geometries of the coupled lines is verified by electromagnetic (EM) simulations and measurements. The results are in very good agreement with each other.

Highlights

When two unshielded transmission lines are in close proximity, power can be coupled from one line to the other due to the interaction of electromagnetic fields
The two signal lines are located on the same side, i.e., the top side, while the ground plane (GND) is located on the opposite side
For the S parameter measurements of these DUTs, for more clarity, the transparent layout of the test fixture without DUT shown in Fig. 5 was designed

Summary

Introduction

When two unshielded transmission lines are in close proximity, power can be coupled from one line to the other due to the interaction of electromagnetic fields. They generally require very tightly coupled lines to pursue the high coupling factor, which is difficult to fabricate for the traditional coupled microstrip lines This feature can be implemented by broadside coupled lines because of its relatively smaller equivalent capacitance under even-mode excita tion. To the best of the author’s knowledge, the circuit design based on this structure completely relies on EM simulations, and there is no systematic inves tigation of this structure and no analytic model yet for the designer to refer to as to guide for engineering the coupled lines [11,12,13] Such a model is developed in this paper using the same methods as for coplanar waveguides [14] and adapting the model for broadside coupled transmission lines to extract the even and odd characteristic.

Theoretical analysis

Even mode

Odd mode

Analytical model verification

Conclusions