MICROSTRIP BANDPASS FILTER DESIGN (COMBLINE)

MOHAMAD HAMIZAN BIN TUAH

This report is submitted in partial fulfilment of the requirements for the award of

Bachelor of Electronic Engineering (Telecommunication Electronics) With Honours

Faculty of Electronic and Computer Engineering

Universiti Teknikal Malaysia Melaka

May 2009

UNIVERSTI TEKNIKAL MALAYSIA MELAKA

FAKULTI KEJURUTERAAN ELEKTRONIK DAN KEJURUTERAAN KOMPUTER

BORANG PENGESAHAN STATUS LAPORAN

PROJEK SARJANA MUDA II

Tajuk Projek : Microstrip Bandpass Filter Design (Combline)

Sesi

Pengajian : 2008/2009

Saya MOHAMAD HAMIZAN BIN TUAH

mengaku membenarkan Laporan Projek Sarjana Muda ini disimpan di Perpustakaan dengan syarat-

syarat kegunaan seperti berikut:

1. Laporan adalah hakmilik Universiti Teknikal Malaysia Melaka.

2. Perpustakaan dibenarkan membuat salinan untuk tujuan pengajian sahaja.

3. Perpustakaan dibenarkan membuat salinan laporan ini sebagai bahan pertukaran antara institusi

pengajian tinggi.

4. Sila tandakan ( √ ) :

SULIT*

(Mengandungi maklumat yang berdarjah keselamatan atau

kepentingan Malaysia seperti yang termaktub di dalam AKTA

RAHSIA RASMI 1972)

TERHAD* (Mengandungi maklumat terhad yang telah ditentukan oleh

organisasi/badan di mana penyelidikan dijalankan)

TIDAK TERHAD

Disahkan oleh:

__________________________ ___________________________________

(TANDATANGAN PENULIS) (COP DAN TANDATANGAN PENYELIA)

Alamat Tetap: ……………………………......

……………………………......

Tarikh: ……………………….. Tarikh: ………………………..

√

iii

“I hereby declare that this report is result of my own effort except for the quotes

as cited in the references”

Signature : ................................................................

Name : MOHAMAD HAMIZAN BIN TUAH

Date : ................................................................

iv

“I hereby declare that I have read this report and in my opinion this report is

sufficient in terms of the scope and quality for the award of Bachelor of

Electronic Engineering (Telecommunication Electronics) With Honors.”

Signature : ....................................................................

Name : MR. ABD SHUKUR BIN JA’AFAR

Date : ....................................................................

v

” To my beloved Mother and Family”

vi

ACKNOWLEDGEMENT

First of all, I would like to thank Allah SWT for giving me the strength to

endure and overcome the obstacles and difficulties throughout the process of

completing the projects and the report of my final year project.

I also want to express my gratitude towards Mr Abd Shukur Bin Ja’afar for

being very supportive and helpful in helping me with my project. It is with his

guidance and supervision that I have managed to complete this project in time.

To my beloved mother, thank you for being such a good example and a friend

to me. Your advice and action gives me strength to endure the challenges i have

faced. To my whole family and friends, thank you for your support, suggestions and

for all the help.

Last but not least, I would like to acknowledge any individual that has

contributed their energy and knowledge for my project.

vii

ABSTRACT

This project is about designing a wide bandpass microstrip filter that will

operate at the industrial, scientific and medical (ISM) band. The ISM band was an

unlicenced band which is originally reserved internationally for the use of RF

electromagnetic field for industrial, scientific and medical purposes other than

telecommunication. A bandpass microstrip filter working at the ISM band will reject

other frequency outside the ISM band. The design process will be started by

obtaining the order of the filter base on the specification given. Then, lumped

element bandpass filter will be designed. Next is the realization to microstrip

structure process. The dimension of microstrip such as the length, width and spacing

of the microstrip filter will be determined by using filter realization method.

Simulation was done by using Microwave Office software to observe the S11 and S21

of the filter. The next process is to fabricate the designed microstrip bandpass filter

on the FR4 boar and measurement will be done on the fabricated filter by using

network analyzer. The procedure, results and discussion were discussed in this

report.

viii

ABSTRAK

Projek ini adalah berkenaan dengan mereka bentuk sebuah penapis mikrojalur

lulus jalur lebar yang beroperasi pada jalur frekuensi Industrial, Scientific and

Medical (ISM). Jalur frekuensi ISM adalah jalur frekuensi yang tidak memerlukan

lesen dan dikhaskan secara global untuk kegunaan medan electromagnet frekuensi

radio untuk kegunaan industri, saintifik dan perubatan selain untuk kegunaan

telekomunikasi. Penapis mikrojalur lulus jalur yang beroperasi pada frekuensi ISM

akan bertindak menghalang frekuensi-frekuensi selain daripada frekuensi ISM.

Proses mereka bentuk sebuah penapis lulus jalur bermula dengan menentukan

bilangan peringkat penapis berdasarkan tentuan yang diberikan. Kemudian penapis

lulus jalur elemen gumpalan akan direka. Seterusnya adalah proses untuk merealisasi

kepada struktur mikrojalur di mana dimensi mikrojalur seperti panjang, lebar dan

jarak untuk penapis lulus jalur mikrojalur akan ditentukan dengan menggunakan

realisasi Simulasi dijalankan menggunakan perisian Microwave Office dimana nilai

S11 dan S21 akan diperhatikan. Proses seterusnya adalah proses fabrikasi penapis

mikrojalur lulus jalur diatas papan FR4 dan pengukuran akan dijalankan keatas

penapis yang telah siap dibina dengan menggunakan alat penganalisis rangkaian..

Tatacara, dapatan dan diskusi akan dibincangkan di dalam laporan ini.

ix

TABLE OF CONTENT

CHAPTER CONTENT PAGE

PROJECT TITLE i

STATUS REPORT FORM ii

STUDENT DECLARATION iii

SUPERVISOR DECLARATION iv

DEDICATION v

ACKNOWLEDGEMENT vi

ABSTRACT vii

ABSTRAK viii

TABLE OF CONTENTS ix

LIST OF TABLES xiii

LIST OF FIGURE xiv

LIST OF ABBREVIATION xv

LIST OF APPENDIXES xvi

x

I INTRODUCTION 1

1.1 Background Studies 2

1.2 Problem Statement 2

1.3 Objectives 3

1.4 Scope of Work 3

II LITERATURE REVIEW 5

2.1 The Microstrip Filter Concept 5

2.1.1 Transfer Function

2.1.1.1 General Definition 6

2.1.1.2 Butterworth Response 6

2.1.1.3 Chebyshev Response 7

2.1.2 LPP Filters & Elements 8

2.1.2.1 Butterworth LPP Filters 9

2.1.2.2 Chebyshev LPP Filters 10

2.1.3 Frequency & Element Tranformation 11

2.1.3.1 Lowpass Transformation 12

2.1.3.2 Highpass Transformation 13

2.1.3.3 Bandpass Transformation 13

xi

2.1.3.4 Bandstop Transformation 15

2.2 Network Analysis 16

2.2.1 Network Variable 16

2.2.2 Scattering Matrix 17

2.2.3 ABCD Parameter 18

2.3 FR-4 board 19

2.3.1 Specification 20

2.3.2 Advantages & Disadvantages 20

2.4 Transmission Line 21

2.4.1 Microstrip Lines 22

2.4.2 Coaxial Cables 23

2.4.3 Coupled Line 24

2.4.3.1 Theory of Coupled Line Couplers 25

2.5 Microstrip Filter Implementation 26

2.5.1 End-coupled half-wavelength resonator 26

2.5.2 Parallel-coupled half-wavelength resonator 27

2.5.3 Hairpin Filter 27

2.5.4 Interdigital Filter 28

2.5.5 Combline Filters 29

2.6 Coupled Rectangular Bars 33

III METHODOLOGY 34

xii

3.1 Block Diagram

3.1.1 Filter Specification 35

3.1.2 Lowpass Prototypes 36

3.1.3 Lumped Elements 36

3.1.4 Filter Realization 37

3.1.4.1 Filter Dimension 38

3.1.5 Simulation 41

3.1.6 Fabrication 41

3.2 Project Flow Chart 42

IV RESULTS & DISCUSSION

4.1 Filter Specification 43

4.2 Combline Filter Design 44

4.2.1 Lowpass Prototype Values 44

4.2.2 Determining The Dimension 44

4.2.3 Simulation 46

4.2.4 Analysis of result 48

V CONCLUSION & SUGGESTION 50

5.1 Conclusion 50

5.2 Suggestion and Recommendation 51

REFERENCE

APPENDIX

xiii

LIST OF TABLES

NO TITLE PAGE

2.1 Element values for Butterworth lowpass prototype 9

2.2 Transformation Table 12

3.1 g value for n=4 39

3.2 Transformation TableValues spacing and quality factor 39

3.3 Values spacing and coupling coefficient 40

4.1 g value for n=4 44

4.2 Qe and Mij values 44

4.3 Spacing for n=4 combline filter 46

4.4 S11 & S21 simulated values 48

4.5 -3dB value for simulated result 49

xiv

LIST OF FIGURE

NO TITLE PAGE

2.1 Chebyshev and Butterworth Response 7

2.2 LPP for all-pole filter 8

2.3 Microstrip line 22

2.4 Coaxial Cable 23

2.5 Coupled-line 24

2.6 Couple Line Coupler 25

2.7 End-Coupled half-wavelength resonator 26

2.8 Parallel-Coupled half-wavelength resonator 27

2.9 Hairpin Filter 27

2.10 Interdigital Filter 28

2.11 Combline Filter 29

2.12 Coupled Rectangular Bars Centered Between Parallel Bars 33

3.1 Block Diagram 34

3.2 Lowpass Prototype Filter 36

3.3 Lumped Element Bandpass Filter 37

3.4 Resonators Configuration to Determined Spacing 38

3.5 S21(deg) Vs Frequency 38

3.6 Qe Vs Spacing 39

3.7 S21(dB) Vs Spacing 40

xv

3,8 M Vs Spacing 40

3,9 Project Flow Chart 42

4.1 Qe Vs Spacing 44

4.2 M Vs Spacing 44

4.3 EM structure of n=4 combline filter 46

4.4 S21 and S11 simulated values 48

4.5 S21 cutoff values 49

xvi

LIST OF ABBREVIATION

LPP- Lowpass Prototypes

LPF- Lowpass Filter

HPF- Highpass Filter

BPF- Bandpass Filter

BSF- Bandstop Filter

S-parameter- Scattering Parameter

RF- Radio Frequency

FR-4 board- Frequency Redundant 4 board

ISM band- Industrial, Scientific and Medical Band

ADS- Advanced Design System

xvii

LIST OF APPENDIXES

NO CONTENT

A Element value for Chebyshev lowpass prototype

B Graph of Attenuation Vs normalized Frequency

C Element Values for Chebyshev

D Lumped Element Calculation

E Quality Factor and Coupling Coefficient Calculation

F Design of Microwave Filter

G Characteristic of Coupled Microstriplines

H Gantt Chart

I Technical Report

CHAPTER 1

INTRODUCTION

Filters play important roles in many RF/microwave applications including

cellular radio, satellite communication, and radar. They are used to separate or

combine different frequencies. The electromagnetic spectrum is limited and has to be

shared. Therefore, filters are used to select and confine the RF/microwave signals

within assigned spectral limits.

The first chapter of this report is the introduction part of the report. In this

chapter, the background studies, the problem statements, the objectives and the scope

of work will be discussed

2

1.1 Background Studies

The development of applications such as wireless communication s continue

to challenge RF/microwave filters with even more stringent requirement such as

higher performance, smaller size, lighter weight and lower cost. Depending on the on

the requirements and specifications and specifications, RF/microwave filters maybe

designed as lumped elements or distributed elements circuits and can be realized in

various transmission line structures, such as waveguide, coaxial line and microstrip.

Microstrip is a type of electrical transmission line which can be fabricated

using printed circuit board [PCB] technology, and is used to convey microwave-

frequency signals. It consists of a conducting strip separated from a ground plane by

a dielectric layer known as the substrate. Microwave components such as antennas,

couplers, filters, and power dividers can be formed from microstrip, the entire device

existing as the pattern of metallization on the substrate. Microstrip is thus far cheaper

than traditional waveguide technology, as well as being far lighter and more

compact.

1.2 Problem statement

The industrial, scientific and medical (ISM) band was originally reserved

internationally for the use of RF electromagnetic fields for industrial, scientific and

medical purposes other than communications. Therefore, a device is needed to filter

signals so that it will operate in this ISM band. Therefore, does microstrip filter fulfil

this requirement? Theoretically, microstrip filter can operate at high frequency and

microstrip filter is easy to fabricate. Therefore, the behaviour of microstrip filter will

be studied.

3

1.3 Objectives

The objectives of the project are:

i) To design and simulate a microstrip filter that will operate at 2.4 GHz

until 2.5GHz is suitable for the ISM band.

ii) To fabricate a microstrip combline bandpass filter on FR4 board by

using etching technique.

iii) To make a comparison between the result obtained by simulation and

the measured value.

1.4 Scope of Works

The scope of work of the project consists of three major parts which are the

design part, the simulation part, and the fabrication part.

1.4.1 Design

During the designing process, the number of order for the filter will be

determined based on the specification given. When the number of order has been

determined, the lowpass prototype for the filter will be design. Since the objective of

the project is designing a bandpass filter, transformation from lowpass filter to

bandpass filter will be done for the lowpass prototype.

Design Simulation Fabrication

4

1.4.2 Simulation

When the design process had been done, the simulation will be done on the

Advanced Design System (ADS) software and by using the Microwave Office

software. From simulation, the Scattering Matrix and the cut of frequency for the

bandpass filter can be observed.

1.4.3 Fabrication

When the results obtained from the simulation fulfil the requirement of the

filter, fabrication of the filter will be done. For the fabrication, etching technique was

used to fabricate the filter on the FR4 board. When the filter been fabricated, testing

will be done. Testing was done by using the network analyzer. Then comparison will

be made between the simulation and the measurement result obtained from the

network analyzer.

1.5 Report Structure

Chapter Content

I Introduction

II Literature Review

III Methodology

IV Result and Discussion

V Conclusion and Suggestion

CHAPTER 2

LITERITURE REVIEW

This chapter will discuss about the literature review used for my project. For

the project, some study had been done on the microstrip filters concept, the theory

implemented, and the technique used for this project.

2.1 The Microstrip Filters concept.

2.1.1 Transfer Fuctions

A transfer function is a mathematical representation, in terms of spatial or

temporal frequency, of the relation between the input and output of a system.

6

2.1.1.1 General definition

The transfer function of a two-port filter network is a mathematical

description of network response characteristics, namely a mathematical expression of

S21. On many occasions, an amplitude-squared transfer function for a lossless passive

filter network is define as [1].

)(1

1)(

22

2

21

nFjS (2.1)

Where ε is a ripple constant, Fn(Ω) represents a filtering or characteristic function

and Ω is a frequency variable. For convenience purposes, let Ω represent a radian

frequency variable of a lowpass prototype filter that has a cutoff frequency at Ω= Ωc

for Ω=1 (rad/s). Frequency transformations to the usual radian frequency for

practical lowpass, highpass, bandpass, and bandstop will be discussed later. [1]

2.1.1.2 Butterworth (Maximally Flat) response.

The amplitude-squared transfer function for Butterworth filters that have an

insertion loss LAR=3.01dB at the cutoff frequency Ωc=1 is given by

njS

2

2

211

1)( (2.2)

Where n is the degree or the order of the filter, which corresponds to the number of

reactive elements, required in the lowpass prototype filter. This type of response is

also referred to a maximally flat because its amplitude-squared transfer function has

the maximum number of (2n-1) zero derivatives at Ω=0. Therefore a maximally flat

approximation to the ideal lowpass filter in the passband is best at Ω = 0, but

deteriorates as Ω approaches the cutoff frequency Ωc. The Figure 2.1 shows the

typical maximally flat response. [1]

7

2.1.1.3 Chebyshev Response

The Chebyshev response that exhibits the equal-ripple passband and

maximally flat stopband is depicted in Figure 2.1. The amplitude-squared transfer

function that describes these types of response is [1].

)(1

1)(

22

2

21

nTjS (2.3)

Where the ripple constant ε is related to a given passband ripple LAR in dB by Tn(Ω)

is a chebyshev function of first kind of order n, which is defined as

Tn(Ω)=)1coscosh(

)1coscos()(

n

nTn

1

1 (2.4)

Hence, the filters realized from (2.3) are commonly known as Chebyshev filters. [1]

Figure 2.1 Butterworth (maximally flat) and Chebyshev lowpass response [4].