# RFIC - Microstrip Transmission Line Design

2016-10-04

Microstrip transmission line is one of the basic type of transmission line in RF integrated circuit. Here is a basic/simple design example of it. This is a homework for course ECE810 RF Integrated Circuits in MSU.

# To begin with

Microstrip is one kind of transmission line in the circuit. We use them to achieve impedance matching or some other application. Here are some simple design examples.

# Problems

Design following transmission lines (use simplifying assumptions):

• A 50-$\Omega$ microstrip line for a substrate height of 128 $\mu$m and $\varepsilon_r$ of 3 (RT Duroid)

• A 50-$\Omega$ microstrip line for a substrate height of 10 $\mu$m and $\varepsilon_r$ of 4.1 (CMOS)

• A 50-$\Omega$ microstrip line for a substrate height of 5 $\mu$m and $\varepsilon_r$ of 4.1 (CMOS)

• For the CMOS case, assuming the minimum allowable line width and spacing is 2 $\mu$m on the top metal, what is the highest characteristic impedance we can achieve?

• What is the lowest characteristic impedance we can achieve? What would be the limiting factor?

# Solution

## Problem 1

For RT Duroid material, use the $w/h1$ formula to recalculate. i.e.

$Z_0=\frac{120\pi}{\sqrt{\varepsilon_{\rm eff}}\big[\frac{w}{128}+1.393+0.667\ln(\frac{w}{128}+1.44)\big]}$

This time I substitute the $w=400 \rm{\mu m}$ (another assumption) in the $\varepsilon_{\rm eff}$ formula. I choose this value to get rid of the recursive calculation to find the exact value of $w$.

$\begin{eqnarray} \varepsilon_{\rm eff} &\approx& \frac{\varepsilon_r+1}{2}+\frac{\varepsilon_r-1}{2}\Big[\frac{1}{\sqrt{1+12h/w}}\Big] \\ &=& \frac{3+1}{2}+\frac{3-1}{2}[\frac{1}{\sqrt{1+12\times128/400}}] =2.4545 \end{eqnarray}$

Denote $C = \frac{120\pi}{Z_0\sqrt{\varepsilon_{\rm eff}}}-1.393$, we obtain

$\begin{eqnarray} f(w) &=& \frac{w}{128}+0.667\ln(\frac{w}{128}+1.44)-C=0 \\ \implies f(x) &=& x+0.667\ln(x+1.44)-C \\ f'(x) &=& 1+\frac{0.667}{x+1.44}-C \end{eqnarray}$

Use Newton-Raphson method to find the value $w$.

$x_{i+1}=x_{i}-\frac{f(x_i)}{f'(x_i)}$

and finally $\frac{w}{128}=2.9520$, thus $w=2.9520\times 128=320.5948 \rm{~\mu m}$.

## Problem 2

When it comes to CMOS cases, the substrate height $h$ is usually very small, but I also assume $w/h>1$. Take $w=15 {~\mu m}$ to calculate the efficient $\varepsilon$.

$\begin{eqnarray} \varepsilon_{\rm eff} &\approx& \frac{\varepsilon_r+1}{2}+\frac{\varepsilon_r-1}{2}\Big[\frac{1}{\sqrt{1+12h/w}}\Big] \\ &=& \frac{4.1+1}{2}+\frac{4.1-1}{2}[\frac{1}{\sqrt{1+12\times10/15}}]=3.0667 \end{eqnarray}$

Go through the similar process above, it is not hard to calculate that $C_1=2.9125$, and $\frac{w}{h}=2.0746$, which returns $w_1=20.7459 \rm{~\mu m}$. This result is close to the value we used to calculate $\varepsilon_{\rm eff}$, so there is no need to redesign again.

## Problem 3

For the CMOS case with substrate height $h=5 \rm{~\mu m}$, similarly first calculate the efficient $\varepsilon_{\rm eff}$.

$\begin{eqnarray} \varepsilon_{\rm eff} &\approx& \frac{\varepsilon_r+1}{2}+\frac{\varepsilon_r-1}{2}\Big[\frac{1}{\sqrt{1+12h/w}}\Big] \\ &=& \frac{4.1+1}{2}+\frac{4.1-1}{2}[\frac{1}{\sqrt{1+12\times5/8}}]=3.0816 \end{eqnarray}$

And $C_2=2.9021$, $\frac{w}{h}=2.0658\implies w = 10.3289\rm{~\mu m}$.

## Problem 4

$Z_0$ increases with $h$ and decreases with $w$ and $\varepsilon_r$. When the minimum allowable $w_{\min}=2\rm{~\mu m}$,

$\begin{eqnarray} \varepsilon_{\rm eff} &\approx& \frac{\varepsilon_r+1}{2}+\frac{\varepsilon_r-1}{2}\Big[\frac{1}{\sqrt{1+12h/w}}\Big] \\ &=& \frac{4.1+1}{2}+\frac{4.1-1}{2}[\frac{1}{\sqrt{1+12\times\frac{1}{2}}}]=2.980 \end{eqnarray}$

When $h=10\rm{~\mu m}$,

$\begin{eqnarray} Z_{0,\max} &=& \frac{120\pi}{\sqrt{\varepsilon_{\rm eff}}\big[\frac{w}{h}+1.393+0.667\ln(\frac{w}{h}+1.44)\big]} \\ &=& \frac{120\pi}{\sqrt{2.98}\big[\frac{2}{10}+1.393+0.667\ln(\frac{2}{10}+1.44)\big]} \\ &=& 113.5691~\Omega. \end{eqnarray}$

When $h=5\rm{~\mu m}$,

$Z_{0,\max}=\frac{120\pi}{\sqrt{2.98}\big[\frac{2}{5}+1.393+0.667\ln(\frac{2}{5}+1.44)\big]}=96.7805~\Omega$

That is the highest characteristic impedance we can achieve with the CMOS.

## Problem 5

As demonstrated above, $Z_0$ increases with $h$ and decreases with $w$ and $\varepsilon_r$. We need to lower the substrate height $h$ and increase microstrip width $w$ and $\varepsilon_r$. Let’s choose a substrate material, say GaAs, and $\varepsilon_r=12.7$.

$\begin{eqnarray} \varepsilon_{\rm eff} &\approx& \frac{\varepsilon_r+1}{2}+\frac{\varepsilon_r-1}{2}\Big[\frac{1}{\sqrt{1+12h/w}}\Big] \\ &=& \frac{12.7+1}{2}+\frac{12.7-1}{2}[\frac{1}{\sqrt{1+12\times\frac{1}{2}}}]=9.0611 \end{eqnarray}$

More detailedly, it is the ratio $w/h$ that matters. For here, I choose $w/h=2$.

$Z_{0,\max}=\frac{120\pi}{\sqrt{9.0611}\big[\frac{2}{1}+1.393+0.667\ln(\frac{2}{1}+1.44)\big]}=29.6983~\Omega$

Further, if I increase the ratio to $w/h=5$, we can calculate that $Z_0=16.4027~\Omega$. This is lower than the previous case.

I don’t know whether there is a theoretical limitation for the $w/h$ ratio, but in practical application when $w$ is too high, there will be less space for other components. Also, a rule of thumb is that we need to limit the thickness of the microstrip substrate to 10% of a wavelength.

# Simulation result

• $w_1 = 305.872 \mu\rm{m}$
• $w_2 = 18.5694 \mu\rm{m}$
• $w_3 = 8.71934 \mu\rm{m}$