Lec.6 Impedance Matching

RF & Microwave EngineeringBETE-Spring 2009

Department of Electrical EngineeringAir University

Impedance Matching

Design of Matching Networks

Lecture No. 5

RF & Microwave EngineeringBETE-Fall 2009

Basit Ali ZebDepartment of Electrical Engineering, AU

Why Impedance Matching?

• Impedance matching i.e., transforming one impedance value to another, is an immensely important part of microwave component or system design engineering.

• Why impedance matching is necessary?– Maximum power is delivered to the load when the TL is matched at both the load and source ends

– Increased sensitivity of the device– Reflected power can be harmful to some microwave devices and demands extra protection circuitry

– To avoid multiple reflections which cause amplitude and phase distortions.



Introduction



Impedance Matching and Tuning



Matching a TL Circuit

Impedance matching using discrete components

Impedance matching using distributed components



Features

Matching network should ideally be lossless to avoid

unnecessary loss of generator power

The simplest design of matching network that satisfies the

required specification is generally the most preferable.



General Rule- Impedance Matching

In order to match a complex load impedance to a

TL of Z0, the real part of the input impedance

looking into the matching network must be Z0

while the imaginary part must be zero.

This implies that a general matching network

must have at least two degrees of freedom.



Matching Network

Narrowband Matching

Wideband Matching



Types of Solutions

• Solve analytically to calculate the values of required elements

In designing an impedance matching circuit, we

have two broad approaches at our disposal:

• To rely on Smith Chart as a graphical

design tool

Both methods have advantages and disadvantages



Matching Techniques

• L Networks

• Single Stub Tuner

• Quarter Wave Transformer

Discrete component

implementation

Microstrip or stripline

Implementation

We will discuss three methods for impedance matching

in this course:



Matching With Lumped Elements



L-Networks

At a single frequency, any positive real complex impedance

can be matched to any other positive real complex impedance using no more than two reactive elements.

There are 8 possible L-matching circuits:



Matching With L-Network



The Technique

Imagine that yL =gL +jbL is at the point marked Load in the circuit for gL< 1. By adding jb we will move on the constant

g=gL circle. We select the value of b such that we land at one

of the two intersections of the g=gL circle with the r = 1 circle.



The Technique

At this point, the impedance is 1 + jx, and the series-tuning element can be used to remove the reactive part. The

solution that demands the smallest b should normally be

chosen, because it gives the largest bandwidth.



Smith Chart Solution



Example - Case RL< Z0



Solution Procedure

1. Mark the normalized load impedance point on smith chart



Solution on Smith Chart



Impedance Movement-Smith Chart

A general rule of thumb concerning the direction of rotation is that whenever an inductor is involved we move the impedance in the upper half of the chart while capacitance results in the movement towards lower half.

The addition of series reactance with load results in motion along constant- resistance circle.

A shunt connection produces motion along the constant-conductance circle.



L-Matching Networks

zs

Four possible

matching networks

zL*



Mixed Design Matching

No. of transmission line sections connected in series with

capacitors placed in parallel configuration.

Inductors are usually avoided because they tend to have

higher resistive losses than capacitors.

Placement and the component value of capacitor gives

greater tuning flexibility.



Microstrip Line Matching Networks

• Increasing frequency means decreasing wavelength, and the effect of parasitics in discrete components becomes noticeable.

• This makes design more complicated and often the distributed elements are used.

• In mid-GHz range, matching networks usually employ combined discrete and distributed components



Quarter Wave Transformer



Quarter Wave Transformer- Matching

±∞===

l

radl

l

1

1

1

tan

2

4

βπβλ



QWT Design

We can adjust Z1 such that Zin = Z0

In other words, a quarter wavelength TL section with this

particular characteristic Impedance, will present a perfect match (Γ=0) to the left hand side of TL



Broadband QWT



Taper Design



Microstrip Broadband QWT

Multi-section QWT

Tapered Line QWT



QWT Matching

DISADVANTAGES



Adjusting TL Z0



STUB Matching

Parallel or Series connection

SINGLE STUB TUNER

DOUBLE STUB TUNER



Single Stub Tuner (SST)

This is an example of parallel SST. The shunt-connected

section is called the STUB

Yin = Y0 + j Bs

Yin = Y0 + j 0

We require:

At input:



Adjustable Parameters

1. Length of the stub line section “ls”

2. Location of the stub line from the load “d”

Although not necessary, all the TL sections will be

assumed to have same

Two degrees of design freedom



For Proper Impedance Match



SST Design Using Smith Chart

Normalized values

Procedure:



SST Design Example

Solution:

Question:



The Solution

Normalized

admittance

coordinates

Two Solutions

are possible:



Solution Contd-

Rotate wavelengths

towards generator to:

FINAL 2 SOLUTIONS:



Short or Open Stubs?

In stripline and microstrip form, open circuited stubs are much

more popular than short circuited stubs, because they are simpler to make.

Good short circuits are difficult to make in microstrip and stripline,

but they are easy to make in coaxial line.

Series stubs cannot be made in stripline or microstrip. In coaxial

line it is possible, but mechanically difficult and therefore

expensive.



2 GHz Power Amplifier for a cellular phone



Printed Circuit Board Layout



Study

• Article 5.1, 5.2, 5.3, 5.4 from the text book

• Next topic of Discussion

“Microwave Network Analysis”

Lec.6 Impedance Matching

Documents

Transcript of Lec.6 Impedance Matching