A Low-Pass Filter Design

using Microstrip

By

SARVAJEET HALDER SOURAV SARKAR

A Project thesis Submitted to Haldia Institute of Technology

In Partial Fulfilment of the Requirements for Degree of B. Tech in Electronics & Communication Engineering,

West Bengal University of Technology

UNDER THE SUPERVISION OF

Asst. Prof. S. Mukherjee

Department of Electronics and Communication Engineering, Haldia Institute of Technology

DEPARTMENT OF

ELECTRONICS & COMMUNICATION ENGINEERING HALDIA INSTITUTE OF TECHNOLOGY

2012

2

Certificate

This is to certify that the thesis entitled “Low-Pass Filter Design using Microstrip Filter” submitted by Sarvajeet

Halder and Sourav Sarkar, is absolutely based upon their

work under the supervision of Asst. Prof. S. Mukherjee, Department of Electronics & Communication Engineering, Haldia Institute of Technology, and that neither this thesis nor any part of it has been submitted for any degree/diploma or any other academic award anywhere before.

Dr. Sunandan Bhunia Head of the Department

Electronics & Communication Engineering Haldia Institute of Technology

Asst. Prof. S.Mukherjee

Department of Electronics & Communication Engineering

Haldia Institute of Technology

3

Certificate of Approval

The foregoing thesis entitled “A Low-Pass Filter Design using Microstrip” is hereby approved as a creditable study of an engineering

subject carried out and presented in a manner satisfactory to warrants its

acceptance as a pre-requisite for the degree for which it has been submitted. It is understood that by this approval the undersigned do not necessararily

endorse or approve any statement made, opinion expressed or conclusion drawn therein but approve the thesis only for the purpose for which it is submitted.

Dr. Sunandan Bhunia Head of the Department Electronics & Communication Engineering Haldia Institute of Technology

4

Acknowledgement

We would lie to convey our thanks to our project guide Asst. Prof. S. Mukherjee, Department of Electronics & Communication Engineering, Haldia Institute of Technology for assisting and guiding us in every step of our project. The project could not be progressed without your help and guidance. We would also like to thank Dr Sunandan Bhunia, Head of the Department, Electronics and Communication Engineering, Haldia Institute of Technology for allowing us to use the laboratory of the Department.

Sincerely

SARVAJEET HALDER

SOURAV SARKAR

5

Contents.

ABSTRACT 1

1. Introduction 2

2. Objective 3

3. Fabrication 3

1. Copper-clad boards 3

2. Thin-film technology 4

3. Thick-film fabrication 4

4. Working Principle 5

1. Richard’s Transformation 5

2. Kuroda Identity 5

5. Microstrip Design 7

6. Realisation of Low-pass Microstrip Filter 11

7. Use of Microstrip Line Calculator Software 14

8. Why microstrips? 15

6

9. Applications 15

10. Conclusion 16

Bibliography 17

7

ABSTRACT

Microstrip Technology utilizes printed metallic strips and other varying configurations, depending on the circuit itself, on substrates of various

thicknesses and materials. Our vision is to develop a Low pass filter using a

Microstrip. The report for this semester is concentrated mainly on Microstrip development and technology. The knowledge of microstrip will let us know

about the working principle of microstrip low-pass filters.

8

1. Introduction

Microstrip is a type of electrical transmission linewhich can be fabricated

using printed circuit board 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. In such a technology

reciprocal and nonreciprocal passive components are obtained by varying

the configuration of the printed metallic strips, while interconnections can

be made on the substrate. Microwave components such as antennas,

couplers, filters , power dividersetc. can be formed from microstrip, the

entire device existing as the pattern of metallization on the substrate.

Microstrip is thus much less expensive than traditional waveguide

technology, as well as being far lighter and more compact. Microstrip was

developed by ITT laboratories as a competitor to stripline (first published by

Grieg and Engelmann in the December 1952 IRE proceedings).

The disadvantages of microstrip compared with waveguide are the generally

lower power handling capacity, and higher losses. Also, unlike waveguide,

microstrip is not enclosed, and is therefore susceptible to cross-talk and

unintentional radiation. For lowest cost, microstrip devices may be built on

an ordinary FR-4(standard PCB) substrate. However it is often found that

the dielectric losses in FR4 are too high at microwave frequencies, and that

the dielectric constantis not sufficiently tightly controlled. For these

reasons, an alumina substrate is commonly used.

On a smaller scale, microstrip transmission lines are also built into

monolithic microwave integrated circuits. Microstrip lines are also used in

high-speed digital PCB designs, where signals need to be routed from one

part of the assembly to another with minimal distortion, and avoiding high

cross-talk and radiation.

Microstrip is very similar to striplineand coplanar waveguide, and it is

possible to integrate all three on the same substrate. Microstripis a type of

electrical transmission line which can be fabricated using printed circuit

board 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.

9

2. Objective

The objective of the project for this semester is limited to the basics which

would give us the concepts required for designing a microstrip low pass

filter,

1. Microstrips

2. Determine the impedance and width of the microstrip to be used in

low pass filter

3. Implementation in low pass filter design

3. Fabrication

Materials and fabrication technologies

Broadly speaking there are 3 main technologies for fabricating microstrip

circuits:-

Copper-clad boards

Thick-film fabrication, on ceramic substrates

Thin-film fabrication, on ceramic substrates, other substrates(e.g.

Quartz), and integrated circuits(GaAs, InP, Si, etc)

Copper-clad boards

Here,copper is put on large fibre-glass or other woven or PTFE-based

boards, using electrodeposition or rolling.photoresist is usually

applied by laminating a prepared film onto the substrate. It might also

be applied by dipping in a tank,or by spinning(for small circuits). The

photoresistst is then exposed to uv via a mask,and developed.the

copper is then etched away where it is not covered by photoresist.

10

Thick-film fabrication

In this technology,metal and dielectric pastes are applied to a ceramic

base substrate using screen printing.the screen is a fine metal wire

mesh,and it has a photographic emulsion applied.the circuit pattern

is reproduced onto this emulsion layer.during printing,the paste is

squeezed through the mesh where there are emulsion openings onto

the substrate.the paste is then dried and fired at around 850 deg

c.successive layers can be printed to from multilayer circuits.

Thin-film technology

In this technology metel deposition technique such as sputting and

evaporation are used,possibly with electroplating as well for increase

metel thickness. The equipment used is relatively expensive,and the

substrate must be in a vacuum in mass production,the need to wait

for the chamber pressure to drop down,and the limited substrate

size,are significant drawbacks.however,thin-film technology gives the

best pattern definition and highest performance if suitable meterials

are used.

11

4. Working Principle

RICHARD’S TRANSFORMATION

To accomplish the conversion from lumped and distributed circuit designs,

Richards proposed a special transformation that allows open and short

circuit transmission line segments to emulate the inductive and capacitive

behavior of the discrete components.

The input impedance of a short circuit transmission line of

characteristicimpedance Zo is purely reactive.

Zin= j Zo

tan (βl) = j Zo

tan Θ

Here the electric length Θ can be rewritten in such a way as to make the

frequency behavior explicit. If we pick the line length to be λo/8 at a

particular reference frequency

fo = Vp/λo

the electric length becomes

Θ = (П/4)Ω

On substituting we get

jωL = j Zo

tan ((П/4) Ω) = SZo

similarly jωC = j Yo

tan ((П/4) Ω) = SYo

here S= j tan ((П/4) Ω) is Richards transform

Richards transformation allows us to replace lumped inductors with short

circuit stubs and capacitors with open circuit stubs of characteristic

impedance Zo= 1/ C.

KURODA IDENTITY

Kuroda Identities (aka Kuroda Transforms) are used to convert a section of

transmission line with an open parallel stub into an electrically equivalent

section of transmission line with a shorted series stub. As a result, an

identical S-parameter matrix is produced that performs the same function.

The technique is handy when designing distributed element circuits where

one configuration is possible and the other is not. Filters are a good

12

example, because in the physical layout open parallel stubs are difficult (or

impossible) to realize whereas series shorted series stubs are.

For all four transforms, use n2 = 1 + Z2/Z1, and the rectangular boxes are

λ/8 transmission line sections with the indicated characteristic impedances.

Kuroda Identity (Transform) Parallel Capacitor Input

Kuroda Identity (Transform) Series Inductor Input

Kuroda Identity (Transform) Parallel Inductor Input

Kuroda Identity (Transform) Series Capacitor Input

13

5. Microstrip Design

The Microstrip line it has become the best known and most widely used

planar transmission line for RF and Microwave circuits. This popularity and

widespread use are due to its planar nature, ease of fabrication using

various processes, easy integration with solid-state devices, good heat

sinking, and good mechanical support.

In simple terms, Microstrip is the printed circuit version of a wire over a

ground plane, and thus it tends to radiate asthe spacing betweenthe ground

plane and the strip increases. A substrate thickness of a few percent of a

wavelength (or less) minimizes radiation without forcingthe strip widthto

betoo narrow.

In contrast to Stripline, the two-media nature (substrate discontinuity)

of Microstrip causes its dominant mode to be hybrid (Quasi-TEM) not

TEM, with the result that the phase velocity, characteristic

impedance, and field variation in the guide cross section all become

mildly frequency dependent.

The Microstrip line is dispersive. With increasing frequency,the

effective dielectric constant gradually climbs towards that of the

substrate, so that the phase velocity gradually decreases. This is true

even with a non-dispersive substrate material (the substrate dielectric

constant will usually fall with increasing frequency).

In Microstrip development a new concept of Effective Dielectric

Constant εeff was introduced, which takes into account that most of

the electric fields are constrained within the substrate, but a fraction

of the total energy exists within the air above the board.

14

The Effective Dielectric Constant εeff varies with the free-space

wavelength λ0. The dispersion becomes more pronounced with the

decreasing ratio of strip width to substrate thickness, W/h.

Dispersion is less pronounced as the strip width becomes relatively

wider, and the Microstrip line physically starts to approach an ideal

parallel-plate capacitor. In this case we get: εr ~ εeff· The Effective

Dielectric Constant εeff is expected to be greater than the dielectric

constant of air(ε = 1) and less than that of the dielectric substrate.

In this expression shielding is assumed to be far enough from the

Microstrip line.

Effective Dielectric εeff can be obtained by static capacitance

measurements.

If the static capacitance per unit length is C with partial

dielectric filing, and Co with dielectric removed, we get εeff =

C/Co.

Guided Wavelength in Microstrips is given by:

λ0 / √ εeff where λ0 is the wavelength in free space

The same as in Stripline case, in Microstrip fundamental mode the hot

conductor is equipotential (every point in it is at the same potential).

A simple but accurate equation for Microstrip Charateristic Impedance

is:

The characteristic impedance of the Microstrip line changes slightly

with frequency (even with a non-dispersive substrate material). The

characteristic impedance of non-TEM modes is not uniquely defined,

and depending on the precise definition used, the impedance of

Microstrip either rises, falls, or falls then rises with increasing

frequency. The low-frequency limit of the characteristic impedance is

15

referred to as the Quasistatic Characteristic Impedance, and is the

same for all definitions of characteristic impedance.

Microstrip frequency limitation is given mainly by the lowest order

transverse resonance, which occurs when width of the line (plus

fringing field component) approaches a half-wavelength in the

dielectric. Have to avoid using wide lines.

For very wide lines, the fields are almost all in the substrate, while

narrower lines will have proportionally more field energy in air.

Propagation Delay for a given length in a Microstrip line is only

function of εr:

Any practical Microstrip line has following Sources of Attenuation, due

to:

a. Finite conductibility of the line conductors.

b. Finite resistivity of the substrate and its dumping phenomena.

c. Radiation effects.

d. Magnetic loss plays a role only for magnetic substrates, such as

ferrites.

Waveguides and Striplines have no radiation losses, while in

Microstrip case (since the Microstrip is an open transmission line)

radiation effects are present at any discontinuity section.

For Microstrip using high dielectric materials εr and accurate

conductor shape and matching, conductor and dielectric losses are

predominant in relation to the radiation losses. In practice, it has

been found that the Microstrip impedance with finite ground plane

width (Zo) is practically equal to the impedance value with infinite

width ground plane (Zi), if the ground width Wg is at least greater

than 3*W.

16

Microstrip’s primary advantages of low cost and compact size are

offset by its tendency to be more lossy than Coaxial Line, Waveguide,

and Stripline.

Radiation Losses depend on the dielectric constant, substrate

thickness, and the circuit geometry.

The lower the dielectric constant, the less the concentration of energy

in the substrate region, and, hence, the greater the radiation losses.

The real benefit in having a higher dielectric constant is not only

reducing radiation losses but also that the package size decreases by

approximately the square root of the dielectric constant.

One way to lower the loss of Microstrip line is to suspend the

substrate over the air:

The air between the bottom of the substrate and the ground plane contains

the bulk of the electromagnetic field. The insertion loss of the Microstrip is

reduced because, air essentially has no dielectric loss compared to standard

circuit board substrates, and in addition, the width of the Microstrip line

increases because of the lower effective dielectric constant. Wider lines have

lower current density, and thus, lower ohmic loss.

Suspending Microstrip means that the separation between the signal

and ground paths increases, and so does the Microstrip’s tendency to

radiate, particularly at discontinuities such as corners. From this

reason, suspended Microstrip mostly is used only up to a few GHz.

In a Microstrip line, conductor losses increase with increasing

characteristic impedance due to the greater resistance of narrow

strips. Conductor losses follow a trend that is opposite to radiation

loss with respect to W/h.

Important to remember, a smaller strip width leads to higher losses.

Very simple method for measuring the Dielectric Attenuation constant

is based on the Comparison Technique.

1. Two Microstrip lines with identical electrical characteristics but

different lengths are used.

2. Their insertion losses are measured.

17

3. The difference between two values of insertion loss is used to evaluate

the dielectric attenuation constant.

4. This procedure avoids the systematic errors caused by radiation and

coaxial-to microstrip transitions.

The Power Handling capacity of a Microstrip is limited by heating

caused because of ohmic and dielectric losses and by dielectric

breakdown. An increase in temperature due to conductor and

dielectric losses limits the Average Power of the Microstrip line, while

the breakdown between the strip conductor and ground plane limits

the Peak Power. A metallic enclosure is normally required for most

Microstrip circuit applications, such as Microstrip Filters. The

presence of conducting top and side walls will affect both, the

characteristic impedance Zc and the effective dielectric constant εeff.

In practice, a rule of thumb may be applied in the Microstrip Filter

design to reduce the effect of metallic enclosure: the height up to the

cover should be more than eight times the substrate thickness, and

the distance to walls more than five times the substrate thickness.

6. Realisation of Low-pass Microstrip Filter

Design a low pass filter for fabrication using microstrip lines.

The specifications are:

cutoff frequency=4GHz of impedance of 50 ohm.

18

Using Richard’s Transformation, we have

ZoL= L=3.3487 and Zoc=1/ C=1/0.7117=1.405

Using Kuroda identity to convert S.C series stub to O.C shunt stub.

We have

And

Thus,

Substitute again, we have

And

3487.3/ 2

1 oLZnZ 1/ 22 oZnZ

3487.3

1

1

2 Z

Z

299.13487.3

111

1

22 Z

Zn

35.43487.3299.121 oLZnZ 299.1299.112

2 nZZ o

19

d d d

S.C series

stub

O.C shunt

stub

Z1

Z2/n2=Z

o

n2=1+Z2/Z

1

Z1/n2=Z

oL

Z2

/8

/8/8

/8

/8

Zo=50

Z2=4.35x50

=217.5

Z1=1.299x50

=64.9Zoc=1.405x50

=70.3

ZL=50

Z1=1.299x50

=64.9

Z2=4.35x50

=217.5

50

217.5

64.9 70.3

/8

64.9/8

/8

217.5

50

20

7. Use of Microstrip Line Calculator Software

This is a software for the calculation of width of the microstrip line.

For:

Zo=50Ω, W=0.2532mm

Zo=70.3Ω, W=0.1106mm

Zo=64.9Ω, W=0.1387mm

21

8. Why microstrips?

They have the following advantages:

(a) Compatibility with the microwave active devicesthat can be very easily

mounted on the substrate (b) Enormous reduction in volume and weight

(c) Increase in reliability

(d) Reduction in cost

(e) Mass production

9. Applications

Microstrip circuits find extensive applications in radar systems, microwave

communication links, satellite communication systems, wireless and mobile

communication systems, medical equipment, etc.

22

10. Conclusion

For this semester, we emphasised more on:

1. Microstrips

2. Working principle of microstrip low-pass filter

3. Determination of unknown impedance

4. Determination of width of microstrip filter

But we are optimistic that we would be able to simulate and design a low-

pass filter using a microstrip.

This project not only helped us for understanding filters theoretically but

also helped us to understand a new way of implementation of filters using

microstrips. It would be quite challenging and interesting to see whether we

would be able to imply our theoretical knowledge and design a low-pass

filter.

23

Bibliography