DESIGN AND SIMULATION OF TWO, FOUR AND SIX ELEMENT BROADBAND

MICROSTRIP BALUN CIRCUITS AT 5 GHZ FOR WIRELESS APPLICATIONS

A Project

Presented to the faculty of the Department of Electrical and Electronic Engineering

California State University, Sacramento

Submitted in partial satisfaction of the requirements for the degree of

MASTER OF SCIENCE

in

Electrical and Electronic Engineering

by

Suvarna Mahale

FALL 2015

ii

DESIGN AND SIMULATION OF TWO, FOUR AND SIX ELEMENT BROADBAND

MICROSTRIP BALUN CIRCUITS AT 5 GHZ FOR WIRELESS APPLICATIONS

A Project

by

Suvarna Mahale

Approved by:

______________________________, Committee Chair Dr. Preetham B. Kumar ______________________________, Second Reader Dr. Fethi Belkhouche ______________________________ Date

iii

Student: Suvarna Mahale

This is to certify that this student has met the requirements for format contained in the

University format manual, and that this project is suitable for shelving in the Library and

credit is to be awarded for the project.

__________________________, Graduate Coordinator ________________, Dr. Preetham B. Kumar, Date Department of Electrical and Electronic Engineering

iv

Abstract

of

DESIGN AND SIMULATION OF TWO, FOUR AND SIX ELEMENT BROADBAND

MICROSTRIP BALUN CIRCUITS AT 5 GHZ FOR WIRELESS APPLICATIONS

by

Suvarna Mahale

The broad goal of this project is to improve the efficiency of a broadband microstrip

balun circuit at a center frequency of 5 GHz, for possible wireless applications. To obtain

this result, the design optimization, simulation, fabrication and testing of the balun are

done with stricter requirements as compared with traditional coupler design. . Starting

with a conventional four-port coupler design, key modifications in the design have been

implemented to obtain the desired magnitude and phase values at the two output ports.

These modifications include center-tapping the output line of an even order multi-stage

coupler, and including appropriate termination at the through port. The final outcome of

this effort resulted in three designs: 2, 4 and 6 section balun networks, which show good

amplitude and phase balance.

The fabrication of the balun networks that were evolved in this work will be carried out

in the next continuing project of another graduate student.

____________________________, Committee Chair Dr. Preetham B. Kumar ______________________ Date

v

ACKNOWLEDGEMENTS

I would like to take this opportunity to thank everyone who is responsible for me

to complete the project work. My sincere acknowledgements to the Department of

Electrical and Electronics engineering for providing the technical resources and

permitting me to work on this project. My special thanks to Dr. Preetham Kumar,

Graduate Coordinator at Sacramento State University, who has mentored me throughout

the length and breadth of this project. Without his support and motivation, I could not

have achieved the desired success in this project. I would also like to thank my second

reader Dr. Fethi Belkhouche for his time, guidance and patience on reviewing the project

report document.

I am thankful to my project partner Praveen Kumar Veeranki Krishna who

worked with me to complete this project and who is going to continue work forward on

the design. Also not to forget the Office of Graduate Studies, its staff has been extremely

helpful in coordinating my project plan.

Last but not the least; I would like to thank my family and friends for being supportive

under all circumstances.

v

Table of Contents

List of Figures .............................................................................................................. vii

List of Tables ................................................................................................................ ix

1. INTRODUCTION ................................................................................................... 1

1.1. Principal of Balun Circuit: ............................................................................... 2

1.2 Workflow of Balun design Project: .................................................................. 3

1.3 Organization of the project report .................................................................... 3

2. MICROSTRIP LINES AND BALUNS .................................................................. 5

2.1 Implementation of Microstrip Balun Transformer ........................................... 7

2.2 Fundamental Balun Design .............................................................................. 7

3. DESIGN OF MICROSTRIP BALUN CIRCUIT .................................................. 11

3.1 Design requirements of wideband balun design ............................................ 11

3.2 Design approach to development of broadband microstrip balun.................. 12

3.2.1 Center-tapped transformer principle to obtain phase balance ................. 12

3.2.2 Multi-section Coupled line couplers ....................................................... 13

3.2.3 Calculation of multi-section coupler structure impedances .................... 14

3.3 Schematic of 2-section Balun Design ............................................................ 16

3.3.1 Design of the 2 section balun with short circuit ..................................... 16

3.3.2 Schematic of 2-section Balun Design with open circuit termination ..... 17

3.4 Comparison of the amplitude balance of 2-section balun designs ................. 17

3.4.1 ADS simulation of prototype balun circuit with short circuit termination18

3.4.2 ADS simulation of prototype balun circuit with open circuit termination19

4. DESIGN AND SIMULATION OF INCREASED SECTION BALUNS DERIVED FROM PROTOTYPE STRUCTURE ........................................................................... 22

3.5 Schematic of 4-section Balun Design ............................................................ 22

3.6 Comparison of the amplitude balance of 4-section balun design: ................. 23

3.7 Comparison of phase balance of 4-section Balun design:.............................. 24

vi

4.4 Schematic of 6-section Balun Design ............................................................ 25

4.5 Comparison of the amplitude balance of 6-section balun design: ................. 27

4.6 Comparison of phase balance of 6-section Balun design:.............................. 28

5. CONCLUSION AND RECOMMENDATIONS .................................................. 29

References: .................................................................................................................... 30

vii

List of Figures

Figure 1: Representation of a 2 Port Balun Circuit ........................................................ 2

Figure 2: Microstrip line layout ...................................................................................... 5

Figure 3: Diagram of L-C Lumped Balun ..................................................................... 8

Figure 4: Coaxial Balun ................................................................................................. 8

Figure 5: Simple Coupled Line Balun ........................................................................... 9

Figure 6: Simple Coupled Line Balun, using Broadside Coupler Structure .................. 9

Figure 7: Multi-section binomial transformer design principle .................................... 13

Figure 8: N- Section coupled line coupler ................................................................... 13

Figure 9: Design of 2-section Balun Circuit with short circuit termination ................. 17

Figure 10: Design of 2-section Balun Circuit with open circuit termination ............... 17

Figure 11: S – Parameters magnitude balance comparison (2-section) (Short circuit

termination) ................................................................................................................... 18

Figure 12: S-Parameters phase balance comparison (2-section) (Short circuit termination)

....................................................................................................................................... 19

Figure 13: S–Parameters magnitude balance comparison (2-section)(Open circuit

termination) .................................................................................................................. 20

Figure 14: S – Parameters phase balance comparison (2-section) (Open circuit

termination) ................................................................................................................... 21

Figure 15: Design of 4-section Balun Circuit ............................................................... 23

Figure 16: S – Parameters magnitude balance comparison (4-section) ....................... 24

viii

Figure 17: S-Parameters phase balance comparison (4-section) .................................. 25

Figure 18: Design of 6-section Balun Circuit ............................................................... 26

Figure 19: S – Parameters magnitude balance comparison (6-section) ....................... 27

Figure 20: S-Parameters phase balance comparison (6-section) .................................. 28

ix

List of Tables

Page

Table 1: Common Applications of Balun Circuit 2

1

1. INTRODUCTION

The origin of the word balun is “balanced to unbalanced” transformer. The function of an

electrical balun circuit is to convert signals from a single-ended input to a balanced

output mode, having two signals of equal amplitude but 180 degrees out of phase, over

the specified frequency range with minimum loss and low voltage standing wave ratio.

The circuit has one input port and two output ports. Transformer baluns can be used to

connect lines of differing impedance. Baluns can also be used to generation

communication signals such as Single Sideband (SSB) signals, and in modern cellular

devices to couple across different subnetworks. [1]

These principles of balun circuits are used in modern day communication systems

over a very wide range of interfacing products. Baluns are used in balanced mixers, push

pull amplifiers, balance frequency multipliers, phase shifters, dipole antenna nodes,

balanced modulators and several other components where the requirement would be to

transmit the signal with a phase difference of 180 degrees and an equal magnitude. [1]

Balun transformers can be used between various parts of a wireless or cable

communications system. Table 1 below lists some common applications.

2

Balanced Unbalanced

Television receiver Coaxial cable network

Television receiver Coaxial antenna system

FM broadcast receiver Coaxial antenna system

Dipole antenna Coaxial transmission line

Parallel-wire transmission line Coaxial transmitter output

Parallel-wire transmission line Coaxial receiver input

Parallel-wire transmission line Coaxial transmission line

Table 1: Common applications of Balun Circuit [3]

1.1. Principal of Balun Circuit:

A typical balun Circuit is shown below:

Figure 1: Representation of a 2 Port Balun Circuit [2]

As seen above, this circuit consists of a single input port and two output ports. The signal

which needs a transition is fed at the input which gets modified to a signal with a

different phase at the other end.

3

1.2 Workflow of Balun design Project:

In this project, we are essentially trying to create an optimized broadband balun network,

at a center frequency of 5 GHz, with about 50 percent bandwidth. Additionally, the balun

network is required to provide an output signal whose magnitude and phase difference

has to match or be near to the theoretical specifications of a balun circuit output.

The project will be executed in the following phases:

a. Design the broadband balun network, starting from standard multi-section coupler design.

b. Implement the structural changes, including center tap in the output line, and appropriate

termination at the through port.

c. Simulate the balun circuit, with 2, 4 and 6 section on the Advanced Design System

(ADS) tool.

As mentioned earlier, fabrication of these designs will be part of the next student project on

this topic. After the fabricated circuit is created, theoretical and practical results will be

compared, to test the validity of the design.

1.3 Organization of the project report

The report is organized as follows:

Chapter 1 gives a brief introduction of the project work. Chapter 2 explains the

fundamental components used in the project such as the micro strip line and how the

balun circuit will be implemented using microstrip lines.

Chapter 3 of the report describes the new balun designs that were evolved from the basic

multi-section coupler network. Chapter 4 describes the computer simulations and

4

optimization needed to obtain the final form of the wideband balun circuits. Chapter 5 of

the report describes the conclusions of the project and the direction of future work.

Finally the report gives list of relevant references.

5

2. MICROSTRIP LINES AND BALUNS

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

circuit board technology, and is used to carry microwave-frequency signals. It consists of

a conducting strip separated from a ground plane by a dielectric layer known as substrate.

Microwave components such as antennas, couplers, filters, power dividers and

transformers can be built from microstrip material, with 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. [3]

Figure 2 below shows the structure of a typical microstrip line:

Figure 2: Microstrip line layout

As seen above, the microstrip consists of conductive strip separated from the ground

plane by an insulated substrate. The strips can be fabricated using a PCB routing machine

or by a chemical etching process. Microstrip transmission lines consist of a conductive

6

strip of width W and thickness t and a wider ground plane, separated by a dielectric layer

called the substrate of thickness H. Microstrip is by far the most popular microwave

transmission line, especially for microwave integrated circuits and MMICs. The major

advantage of microstrip over stripline is that all active components can be mounted on

top of the board. [3]

There are important reasons why microstrip lines are used in the final design,

some are mentioned below:

a. The strip lines can be easily fabricated and accommodated using the latest PCB routing

or chemical etching techniques which can produce circuits in bulk. Hence, network

interconnections can be easily accommodated on the surface.

b. It is considered as the most convenient form of transmission line structure for

obtaining reliable voltage, current and other RF Circuit Parameters.

c. Because of their proximity in getting developed as a fine piece within a PCB,

microstrip lines can be easily used in integrated semiconductor form with

interconnections done to microwave integrated circuits. [4]

7

2.1 Implementation of Microstrip Balun Transformer

Keeping some of the above basis points, our broadband balun circuit will eventually be

designed and tested on a PCB using a microstrip line. Towards this end, we will start with

the required specifications, and then initiate the design process using ADS; finally, the

design will be simulated and built for its best results to obtain a good transformer. The

simulation gives various characteristics of the circuits, mainly being the magnitude and

phase differences of the signals fed into and received out of our circuit.

Ultimately, the design layouts that are obtained and simulated with the ADS

software, will be fabricated using a PCB routing technique to obtain the microstrip balun

circuits designed to work at a center frequency of 5 GHz. These circuits will then be

experimentally characterized on the network analyzer and practical output characteristics

compared with the theoretical values obtained from ADS.

2.2 Fundamental Balun Design

There are different types of balun designs: Lumped L-C baluns, and distributed

Transmission line and Microstrip designs.

• L-C balun design as shown below in Figure 3, is also known as a “lattice-type” balun.

It is essentially a bridge. It has two capacitors and two inductors, which produce the

+/- 90 degree phase shifts.

8

Figure 3: Diagram of L-C Lumped Balun [14]

The main application for this circuit is on the output of a push-pull amplifier, which

provides a balanced signal and with the need of convert to a single un-balanced output.

1) A coaxial line consists of an outer shield and a center conductor separated by a

dielectric material. Coax transmission lines are more commonly used due to their reduced

vulnerability to interference. Coaxial balun realized from a quarter length (λ/4) of coaxial

cable, and gives a 1:1 impedance transformation [4]

Figure 4 below shows a coaxial balun.

Figure 4: Coaxial Balun [14]

9

2) Microstrip design is the main design focus for this project. There is a wide-range of

printed/micro-strip balun topologies which have the advantage of being inexpensive,

realized as they are on the Printed Circuit board (PCB) or Microwave Integrated Circuit

(MIC) substrate. An example of simple coupled line balun is shown below in Figure 5

while Figure 6 shows a coupled line balun with broadside coupler structure. [10]

Figure 5: Simple Coupled Line Balun [14]

Figure 6: Simple Coupled Line Balun, using Broadside Coupler Structure [14]

10

The next chapter describes the changes and the steps that were taken to design

miniaturized broadband balun to operate at a center frequency of 5 GHz.

11

3. DESIGN OF MICROSTRIP BALUN CIRCUIT

This chapter details the design approach implemented for the development of

broadband balun coupler circuit, involving the following steps:

• Design of appropriate termination at the different ports of the coupler to obtain the

desired amplitude balance over the frequency range from 1-10 GHz, centered

around 5 GHz.

• Design of the center-tapped output line in the coupler circuit to obtain the desired

phase balance over the frequency range from 1-10 GHz, centered around 5 GHz.

• Evolution from two-section to six-section coupler design to obtain improved

performance with increased number of sections.

Since there were many approaches tried before for the design of a multisection balun

coupler, we decided to start from basic design of a standard multisection coupler, and

then evolve to the broadband balun with the desired specifications. We stated with a two-

section balun, which gave an initial promising result for both amplitude and phase

balance. Then we progressed on to four-section and six-section designs, which gave

successively superior broadband results, as compared to the two-section design.

3.1 Design requirements of wideband balun design

The requirements for the desired balun represented in the report are as follows:

• The wideband frequency range should be between 1 GHz – 10 GHz, with a center

frequency of ~ 5 GHz.

12

• The amplitude balance should be maintained at both the output ports between 0 to

20dB in wideband frequency range.

• The phase balance /S21-/S31 should be precise at ~180 degrees over the band.

• The design should be completely distributed design, built in microstrip without any

lumped elements to be soldered on to the circuit.

• The balun design should be very small in size, in the range of approximately 200-300

mils, to conform to wireless applications.

In order to meet the above mentioned parameters and get an optimized balun circuit, we

considered the standard coupler design as a starting point, and then made appropriate

modifications in the design to obtain desired balun specifications. The next section

describes the changes and the steps that were taken to design miniaturized broadband

balun. [8]

3.2 Design approach to development of broadband microstrip balun

There are three important steps in the broadband balun design which are described below:

3.2.1 Center-tapped transformer principle to obtain phase balance

The principle behind a standard balun design is the center-tapped transformer as shown in

Figure 7. The transformer uses a coupling element for a balanced output and taps are used

for coupling of the signals to generate balun outputs. In the new design, we use short

circuit or open circuit to get equal amplitude balance. [5]

13

Figure 7: Multi-section binomial transformer design principle [5]

3.2.2 Multi-section Coupled line couplers

The standard design procedure required to obtain the final microstrip balun design is a

multi-section approach, which is represented by Figure 8 below [7]

Figure 8: N- Section coupled line coupler [7]

14

We typically design the coupler such that it is symmetric, i.e.: C1=CN, C2=CN-1,

C3=CN-2…. where N is odd. The coupled output can be written as [7]

3.2.3 Calculation of multi-section coupler structure impedances

The default microstrip dimensions are calculated by using standard procedures as

detailed below [7]. The example shown below is the theoretical approach; however

modern tools like LINECALC can be utilized to obtain the impedances of the

coupled lines in the multisection structure.

Example

Design a three-section 20 dB coupled line coupler with a binomial (maximally

flat) response, a system impedance of 50 Ohm, and a center frequency of 3 GHz.

We are plotting the coupling and directivity from 1 to 5 GHz.

For a maximally flat response for a three-section (N = 3) coupler, we require that

{ } { }

2/1

30

0

2113

:factor coupling voltage thedefine wefrequency,center At the and odd, N ,2/)1(

]21..........)3(cos)1(cos[)sin(2

πθ

θ

βθ

θθθ

=

−

=

=+=

+−+−=

VVC

ClforNMwhere

CNCNCejVV MjN

15

so

At midband, θ = π/2 and C0 = 20 dB Thus, C = 10−20/20 = 0.1 = C2 − 2C1.

Solving these two equations for C1 and C2 gives

C1 = C3 = 0.0125, C2 = 0.125.

The even- and odd-mode characteristic impedances for each section are

These are the theoretical values of the multisection coupler impedances. As mentioned

earlier, these impedance values can also be obtained by software tools such as

LINECALC. [7]

16

3.3 Schematic of 2-section Balun Design

This section outlines the design procedure of the prototype two-section balun coupler,

which will serve as a basis for larger section couplers, which will more likely deliver the

specified design specifications for magnitude and phase balance.

The design consists primarily of two coupled-line sections, with a center-tap at

the output coupled line. The center-tap is instrumental in achieving the amplitude and

phase balance of the balun circuit. The values of Z0e and Z0o are calculated using the

equations mentioned above.

3.3.1 Design of the 2 section balun with short circuit

We started with the two-section coupler, as shown below in Figure 9. The modifications

introduced in the default four-port coupler design include a center tap to short circuit in

the output line, and a short circuit termination in the through port of the input line.

17

Figure 9: Design of 2-section Balun Circuit with short circuit termination

3.3.2 Schematic of 2-section Balun Design with open circuit termination

Figure 10 below shows an alternative design, The modifications introduced in the default

four-port coupler design include a center tap to short circuit in the output line, and a short

circuit termination in the through port of the input line.

Figure 10: Design of 2-section Balun Circuit with open circuit termination

3.4 Comparison of the amplitude balance of 2-section balun designs

This section outlines the initial simulation results for the two prototype balun designs

explained in the earlier section. The design and simulations were run in Advanced Design

System (ADS).

18

3.4.1 ADS simulation of prototype balun circuit with short circuit termination

S21 and S31 scattering parameters were measured using the ADS software to simulate the

design shown in Figure 9. The amplitude balance and phase balance curves are shown in

Figures 11 and 12 respectively.

As seen in the amplitude and phase balance for the short circuit termination shown

below, the amplitude balance we get is not that good and phase balance is also not good.

The phase balance we get here at the center frequency is zero while it should be 1800,

which doesn’t meet our design requirements for the wideband balun design.

Figure 11: S – Parameters magnitude balance comparison (2-section) (Short circuit termination)

19

Figure 12: S-Parameters phase balance comparison (2-section) (Short circuit termination)

3.4.2 ADS simulation of prototype balun circuit with open circuit termination

As for the open-circuit terminated structure, S21 and S31 scattering parameters were

measured using the ADS software to simulate the design shown in Figure 10.

As seen in the amplitude and phase balance in Figures 13 and 14 respectively for the

open circuit termination shown below, the amplitude balance we get is somewhat better

than the short circuit terminated structure and the phase balance is not that good. The

phase goes to zero at the center frequency but it should be actually 1800.

20

Figure 13: S–Parameters magnitude balance comparison (2-section) (Open circuit termination)

As seen in the Figure 13 above, marker m1 shows the center tap frequency for S21 and

S31. This is a perfect response which accounts to the same magnitude between the signals.

Under theoretical conditions these two should have equal values over the entire range of

frequency. Considering the fact that a series of passive components are being used as part

of the design, a minor variation in the magnitude response can be expected.

21

Figure 14: S – Parameters phase balance comparison (2-section) (Open circuit termination)

Figure 14 represents the phase balance response of the balun circuit measured at various

frequencies. In m2, frequency is measured at 600MHz and the corresponding phase

difference is 180 degrees, while we see that at center frequency 5 GHz phase difference is

close to 0 degrees. There can be improvements on this model to improve the phase

difference.

22

4. DESIGN AND SIMULATION OF INCREASED SECTION BALUNS

DERIVED FROM PROTOTYPE STRUCTURE

This chapter outlines the main thrust of this work, which is the development of practical

broadband balun networks, operating at a center frequency of 5 GHz. The logical

approach was to extend the prototype structure that was discussed in the earlier chapter to

larger and more practical designs. While the prototype design indicates an acceptable

magnitude and phase balance, it could be improved by extending the prototype two-

section coupler to four- and six-section designs. However, these extended designs

maintained the basic principles of center-tapped output line and input line terminated

with open-circuit load. The open-circuit load circuit was preferred over the short-circuit

structure, on account of easier implementation for future fabrication efforts, and also

because the amplitude balance was superior, while phase balance was about the same.

3.5 Schematic of 4-section Balun Design

The design consists primarily of four coupled-line sections, with a center-tap at the output

coupled line, as shown below in Figure 15.

23

Figure 15: Design of 4-section Balun Circuit

As seen in the above design representation, the input impedance at Port 1 and output

impedances at Port 2 and 3 are kept at a constant 50 ohms which follows the principle of

center tap transformer used for coupling, while the through port (Port -4) is open

circuited to reflect back the signal and add to the existing coupling value which

eventually helped us to get good output response.

The first part of the simulation involves comparing the magnitude of the signals

while also the phase difference between the two signals is recorded and checked if there

is a 180 degree phase shift between them.

3.6 Comparison of the amplitude balance of 4-section balun design:

After simulating the design, we obtained the following waveform representation

indicating the magnitude of the signals, as shown below in Figure 16.

24

Figure 16: S – Parameters magnitude balance comparison (4-section)

As seen in the above graph, this is a good response which accounts to the same

magnitude between the signals, and is superior amplitude balance, as compared with the

prototype two-section balun that was reported in the earlier chapter. As we can see that,

the magnitude response is improved over the 2-section balun design and S2,1 and S3,1

are close in the magnitude. The next step would be to check the phase differences. The

corresponding simulated output will be discussed in next section.

3.7 Comparison of phase balance of 4-section Balun design:

Similar process as mentioned in the measurement of amplitude balance is followed to

compare the phase balance of the circuit, where in the circuit is simulated and the

respective values are obtained as a graphical representation in Figure 17 below.

25

Figure 17: S-Parameters phase balance comparison (4-section)

The above graph represents the response of the balun circuit measured at various

frequencies. We can see that phase difference is 180 degrees, while we see that at center

frequency 5 GHz phase difference is close to 0 degrees. However, we should notice that

the magnitude at both output ports shows a null at the center frequency, which does not

give significance to the phase values. We have further improved this model by adding

sections to it.

4.4 Schematic of 6-section Balun Design

The design consists primarily of six coupled-line sections, with a center-tap at the output

coupled line, as shown below in Figure 18.

26

Figure 18: Design of 6-section Balun Circuit

The design consists primarily of six coupled-line sections, with a center-tap at the output

coupled line. As before, the first part of the simulation involves comparing the magnitude

of the signals while also the phase difference between the two signals is recorded and

checked if there is a 180 degree phase shift between them.

As seen in the above design representation, we have used 6 coupled lines and the

input impedance at Port 1 and output impedances at Port 2 and 3 are kept at a constant 50

ohms which follows the principle of center tap transformer used for coupling. As in the

two-section and four-section cases, one of the input port (Port -4) is open circuited to

reflect back the signal which eventually helped us to get good output response.

27

4.5 Comparison of the amplitude balance of 6-section balun design:

After simulating the design, we obtained the following waveform representation

indicating the magnitude of the signals for 6 section design, as seen below in Figure 19

Figure 19: S – Parameters magnitude balance comparison (6-section)

As seen in the above graph, this is much flatter response at both ports which accounts to

the same magnitude between the signals, though an enhanced ripple is indicated in the

output at port 2. As we can see that, the magnitude response is improved over the four-

section balun design and S21 and S31 are close in the magnitude, at least considering the

envelope. With the results obtained above, this design shows the best performance in

magnitude response, considering all the designs discussed in this project.

28

4.6 Comparison of phase balance of 6-section Balun design:

As in the case of the two-section and four-section balun circuits, the measurement of

amplitude balance is followed to compare the phase balance of the circuit, where in the

circuit is simulated and we get the response as seen below in Figure 20.

Figure 20: S-Parameters phase balance comparison (6-section)

As seen in the above graph, this is much better phase response than all our earlier

designs. As we can see that, the phase balance is improving over the four-section balun

design. With the results obtained above, this design shows the best performance in phase

response.

29

5. CONCLUSION AND RECOMMENDATIONS

The microstrip balun design used in this project is aimed for amplitude balance close to

around 15 dB and phase balance of 180 degree at a center frequency of 5 GHz and

bandwidth range of 1-10GHz. A prototype two-section design was developed, starting

from basic coupler principles; however, with key modifications such as center-tapped

with short in the output line, and open-circuit termination at the through port of the

coupler on the input line.

The intent of the project undertaken seems to be fairly achieved by designing and

simulating from the basic 2 section a balun transformer circuit design and then improving

over 4 and 6 sections. We see that as we increase the number of sections we get the good

magnitude and phase balance, which is fairly good for the 6 section design. I am

concluding the experimental analysis of a wideband microstrip balun transformer design

with a much anticipated satisfaction on the work done and results obtained. Overall it was

a very nice experience to get a hand on in designing a microstrip balun circuit.

The future work will be focused on, designing the circuit using microstrip transmission

lines, fabrication using a PCB routing technique under the laboratory conditions and

testing using network analyzer. The measurements using the fabricated circuit will be

compared to the results obtained through simulations.

30

References:

[1] Minicircuits.com, Balun Transformers. Retrieved on Nov 12, 2015 from World

Wide Web: http://minicircuits.com/pages/BalunApplicationNote.html

[2] RFMD.com, an RF205x Family Application Note Matching Circuits and Baluns,

retrieved on Nov 12, 2015 from World Wide Web http://rfmd.com

[3] Wikipedia, Balun Circuits. Retrieved Nov 12, 2015 from World Wide Web:

http://en.wikipedia.org/wiki/Balun

[5] Microwave Transistor Amplifiers: Analysis and Design by Guillermo Gonzalez,

Prentice Hall; 2nd edition (August 30, 1996).

[6] Elizabeth Kelangi and Danny H. Dang, Design, ‘Simulation and Fabrication of

Improved Wideband Microstrip Balun Circuit at 5 GHz’, M.S. Thesis, California State

University, Sacramento, Fall 2010.

[7] Microwave Engineering by David M. Pozar, Wiley; 4th edition (December 1, 2011)

[8] Chung-Hao Tsai, Iat-In Ao Leong, Hung-Chuan Chen, Tzong-Lin Wu “A

Miniaturized and Broadband Balun Using Artificial Coupled Line With Imaginary Even-

Mode Impedance” Microwave Theory and Techniques, IEEE Transactions on Year:

2011, Volume: 59, Issue: 9

31

[9] Melais, S.E., Weller Thomas, Wilhelm M.J. “A low-profile broadband strip-line

balun” Antennas and Propagation Society International Symposium, IEEE Year: 2005,

Volume: 3B, Pages: 369 – 372

[10] Cheng-Yu Lee, Tung-Yi Hsieh, Ching-Wen Tang “Design of the broadband balun

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