Symmetric Stripline Impedance

PCB Symmetric Stripline Impedance Calculator

Symmetric Stripline Impedance Calculator

Inputs

Trace Thickness
T
Substrate Height
H
Trace Width
W
Substrate Dielectric
Er

Outputs

Impedance (Z):

Introduction

The symmetric stripline is reliable method for creating a transmission line. The stripline is a TEM (transverse electromagnetic) transmission line. Modeling approximation can be used to design the microstrip trace. By understanding the stripline transmission line, designers can properly build these structures to meet their needs.

Description

A stripline is constructed with a flat conductor suspended between two ground planes. The conductor and ground planes are separated by a dielectric. One advantage of the stripline is that there is an improve isolation between adjacent traces when compared with the microstrip.

Stripline Transmission Line Models

Models have been created to approximate the characteristics of the microstrip transmission line.

\Large m =\frac{6h}{3h+t}

\Large w_{eff} =w+\frac{t}{\pi}\cdot\ln{\left(\frac{e}{\sqrt{\left(\frac{t}{4h+t}\right)^2+\left(\frac{\pi t}{4\cdot(w+1.1\cdot t)}\right)^m}}\right)}

\Large b=2\cdot h+t

\Large D=\frac{W}{2}\cdot \left ( 1+\frac{t}{\pi w}\cdot \left ( 1+\ln \left ( \frac{4\pi w}{t} \right ) \right )+.551\left ( \frac{t}{w} \right )^{2} \right )

\Large zo_{sst2}=\frac{60}{\sqrt{er}}\cdot \ln \left ( \frac{4b}{\pi D} \right )

\large when

\Large \left ( \frac{w}{b}< .35 \right ) or \left ( \frac{t}{b}\leq .25 \right ) or \left (\frac{t}{w}\leq .11 \right )

\Large zo_{ss}=\frac{\eta o}{2\pi \sqrt{er}}\ln \left ( 1+\frac{8h}{\pi \cdot w_{eff}}\cdot \left ( \frac{16h}{\pi \cdot w_{eff}}+\sqrt{\left ( \frac{16h}{\pi \cdot w_{eff}} \right )^{2}+6.27} \right ) \right )

\large else

\Large zo_{ss}=\frac{94.15}{\left ( \frac{\frac{w}{b}}{\left ( 1-\frac{t}{b} \right )} +\frac{\theta }{\pi }\right )}

The source for these formulas are found in the IPC-2141A (2004) “Design Guide for High-Speed Controlled Impedance Circuit Boards”