1. Introduction

With the growth in technology in the field of telecommunication, the market demands and governmental regulations push the invention and development of new applications in wireless communication which not only provide services but also deal with the coverage, capacity and the quality of services (QoS) which guarantee the quality of the transmission of data from the transmitter to the receiver with no error. But a strategy would be to open certain frequency regions for new applications or systems, in order to provide additional transmission capacity. Today Wi4 (Worldwide interoperability version-4) is believed as a key application for solving many actual problems related to telecommunication. A microstrip patch antenna consists of a conducting patch of any planar or non-planar geometry on one side of a dielectric substrate with a ground plane on other side. It is a printed resonant antenna that is very popularly required for wireless links of narrowband microwave because of its semi-hemispherical coverage [1]. Microstrip Patch antennas are low cost, low profile, light weight, mechanically robust, easy to fabricate and analyses. Compact size, radiation pattern and selective range of resonance frequency draw major attractions. The microstrip antenna radiates a relatively broad beam broadside to the plane of the substrate. Thus the microstrip

*Corresponding author.

© Elsevier Publications 2014.

Bandwidth Improvement in BPF using Microstrip Couple lines

Tasher Ali Sheikh*, Janmoni Borah, Sahadev Roy

Department of Computer Science and EngineeringNational Institute of Technology, Yupia, Arunachal Pradesh, India-791112

Email: tasher372@gmail.com

ABSTRACT: This paper comprises a brief idea related to designing of a bandpass filter (BPF) using microstrip parallel coupled line structure and some calculations based on distributed components like inductors and capacitors. As Bandpass filters play a significant role in wireless communication systems related to transmitted and received signals, so filtering at a certain center frequency with a specific bandwidth is a great concern. The center frequency with 5.84GHz is mainly used in Wi4 (Worldwide interoperability version-4) for high-speed wireless broadband solution that is configured to transport data, voice, and IP video which requires more bandwidth. So designing a filter of such property is our concern. The design filter consists of three sections: two coupled lines separated by a non-uniform line resonator. The layout is designed to have a centre frequency at 5.84GHz and bandwidth of 0.0745 and also the length of each line is tuned such that the impedance of filter is 50Ω. The paper presents a novel approach to improve low insertion loss and high selectivity. The simulative analysis is performed using FEKO software.

Keywords: Wi4; FBW; Microstrip; Insertion loss; Return loss ; Resonators; Dielectric substrate.

antenna has a very low profile, and can be fabricated using printed circuit (photolithographic) techniques. This implies that the antenna can be made conformable, and potentially at low cost [2].

Realization of system like Wi4 needs a complete new transmitter and receiver. Bandpass filter is a passive component found in the transmitter or receiver which is able to select signals inside a specific bandwidth at a certain center frequency and reject signals in another frequency region, especially in frequency regions, which have the potential to interfere the information signals. Designing a Bandpass filter requires knowledge related to the maximal loss inside the pass region, and the minimal attenuation in the reject/stop regions, and characteristics of the filter in transition regions [2].

Some strategies are taken into consideration while designing in order to fulfill above requirements, for example, the choice of waveguide technology for the filter is preferred in respect to the minimal transmission loss (insertion loss) [3]. In this work we would like to give a way to conceive, design and fabricate Bandpass filter for the Wi4 application at the frequency 5.84GHz, (C-band) with parallel-coupled microstrip as opposed to the one which designed filter for WLAN 5.75GHz and designed with composite resonators and stepped impedance resonators for filter realization [4].

International Conference on Signal and Speech Processing (ICSSP-14) (105–109)

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2. Realization of Filter Using LC Components

The figure.1 below provides an idea of circuit implementation of the filter using components namely inductors (L) and capacitors (C), for the even and odd filter degree (n).

The values of the components used in fig.1 above can be calculated as:

g0 = gn+1 = 1 (1)

gini =

−( )

( )22 12

2sin .............. (2)

For, i=1 to n, Where ‘n’ denotes order of filter..

3. Transformation to Bandpass Filter

The above calculation was done for analyzing the low pass filter. But to have a bandpass characteristic of the signal, a transformation is required such that the component L (Inductor) transform into serial combinations of Ls and Cs, whereas C (Capacitor) becomes parallel combination of Lp and Cp. Also, the center frequency (w0) and the relative frequency bandwidth (FBW) can be calculated using the lower and upper cut-off frequencies ω1 and ω2 respectively as:

0 1 2= (3)

FBW = − 1 2

0 (4)

And the Transformation values are:For the serial combination,

LFBW

Z gs =

1

00..

(5)

C FBWZ gs =

0 0

1.. (6)

And for parallel combination,

CFBW

gZp =

1

0 0. (7)

L FBW Zgp =

0

0 (8)

Where, Z0 is the load impedance, generally set to 50Ω.

4. Filter Realization with Microstrip Techonology

A. Transmission Line Designing

In most of the Radio frequency (RF) applications, microstrips are the mostly used planar transmission line [5]. Microstrip can also be used for designing certain components, like filter, coupler, transformer or power divider. If a microstrip transmission line, as depicted in Fig.2 below, is used for transport of wave with relative low frequency, the wave type propagating in this transmission line is a quasi-TEM wave. This is the fundamental mode in the microstrip transmission line. The width of the strip W together with the dielectric constant and the thickness of the substrate determine the characteristic impedance Zo of the line.

The planar configuration can be achieved by several ways, for example with the photolithography process or thin-film and thick film technology. The components exploited in designing microstrip transmission line include:

1) Metallic strip 2) Metallic ground 3) Dielectrics substrate

B. Designing Bandpass Filter

In our Parallel-coupled filter designed the strips are arranged parallelly,which are close to each other, so that they are coupled with certain coupling factors[6]. In this designing we used following equations.

Figure 1. Analysis of filter using LC Circuit.

Figure 2. Microstrip transmission line.

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UY

FBWg g

01

0 0 12= (9)

UY

FBWg g

j j

j j

, +

+

=1

0 121

(10)

For j=1 to n=1

UY

FBWg g

n n

n n

, +

+

=1

0 12

(11)

Where g0,g1,…,gn are taken for distributed components used in bandpass designing, FBW is the relative bandwidth that is explained in section I. Y0 is the transmission line characteristic admittance and Uj,j+1 is the characteristics admittance of U inverter.

In the above fig.3, the two circles indicates two resonators formed by the combination of parallel-coupled microstrip used in the design and so we can have more two resonators for other successive microstrips.Table 1. Parameters and obtained results of couple line BPF.

Design ParametersDimension used(in mm)

A B C DS1 0.5 0.6 0.5 0.9S2 1.5 1.5 2.2 1.5w 0.6 0.6 0.5 0.5w1 0.75 0.75 0.75 0.75w2 0.8 0.8 0.8 0.8l 4 4 4 3l1 9.3 9.3 9.3 9.3l2 9.09 9.09 9.09 9.09

Obtained resultsfc (center frequency) in GHz 5.8448 5.8893 5.7683 5.8841

BW in GHz 0.0745 0.0567 0.0206 0.0197FBW (Relative frequency bandwidth)

0.0127 0.0096 0.0035 0.0033

Figure 4. Bandwidth vs Frequency plot for Table.1

The table.1 shown above describes a comparison of results obtained for different dimension used in four models A, B, C and D, during simulation. The characteristic impedances of even-mode and odd-mode of the parallel-coupled microstrip transmission line using the characteristic admittance of the inverter, can be calculate as follows [7, 8]

for j = 0 to n,

ZY

UY

UYe j j

j j j j0 1

0

1

0

1

0

21 1( ) = + +

+

+ +,

, , (12)

for j = 0 to n.

ZY

UY

UYo j j

j j j j0 1

0

1

0

1

0

21 1( ) = − +

+

+ +,

, ,

(13)

5. Result and Analysis

We have designed the filter keeping in mind the parametric dimension of R.T.Duriod raw material with thickness 0.786 mm (0.03 inch) such that we could have the same during PCB fabrication. The dielectric substrate used in our design has a relative permittivity of 2.32 and tangent loss of 0.0022. In order to obtain the wave impedance of 50 ohms in PCB fabrication, we designed the microstrip line whose strip width is 0.786 mm.

As depicted from the fig.4 and table.1 given in section IV, it was found that model A provides a better result related to Bandwidth and Relative frequency bandwidth. So, to designed parallel couple microstrip bandpass filter we have taken the center frequency of 5.84 GHz with a bandwidth of 0.0745GHz (-3dB), and relative bandwidth, FBW = 0.0745GHz /5.84GHz = 0.0126. The order of filter used is n =3.

Using eq. (1) and (2) we get g0=g4 =1; g1=1; g2 =2; and g3=1.

Figure 3. Top view of designed filter

Tasher Ali Sheikh et.al.

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From (9) and (10), we get

UY

UY

01

0

3 4

0

0 1412= =. , (14)

and

UY

UY

1 2

0

2 3

0

0 0141, ,.= = (15)

The characteristic impedance for the even-mode and odd-mode of the parallel couple microstrip line can be determine from the equation (12) and (13)

Z Ze e0 0 1 0 3 458 0568( ) = = ( ), ,. Ω (16)

Z Zo o0 0 1 0 3 443 9368( ) = = ( ), ,. Ω

(17)

Z Ze e0 1 2 0 2 350 7149( ) = = ( ), ,. Ω (18)

Z Zo o0 1 2 0 2 349 3049( ) = = ( ), ,. Ω (19)

The width of the parallel couple microstrip lines ‘w’ and separation between them ‘S’ can be calculated according to the rule describe in [7, 9, 10]. A set of two parallel couple microstrip lines with a specific width and separation distance gives even-mode and odd-mode characteristic impedances. According to the procedure explain in [7], if the separation between two parallel couple microstrips transmission lines is small than the even-mode impedance becomes high and the odd-mode impedance small. As in (16) and (17), We use search algorithm to attained perfect value of width (w) and the separation between them (s) of the parallel couple microstrip lines to get the even-mode impedance (Z0e)0,1=58.0568Ω and odd-mode impedance (Z0o)0,1 =43.9368Ω.

The fig.5 and 6 above is a simulated result showing the

return loss (S11) and insertion loss (S21) obtained for model A with parametric dimension as given in Table.1. From fig.6 below, we can see that a resonant at desired frequency of 5.844 GHz with smaller return and insertion loss.

The Smith’s chart shown below in fig.8 shows the measured reflection factor in complex plane. It is seen that at the resonant frequency of 5.84 GHz, indicated by black box, is closer to the centre of the Smith’s chart

6. Conclusion

The designed bandpass filter using Microstrip coupled lines and distributed components, i.e. inductors and capacitors give a better filter characteristic at the center frequency of 5.84 GHz with frequency bandwidth of 74.5 MHz, compared to other parametric dimension used in this research.

Acknowledgement

We would like to express our sincere thanks to Prof. (Dr.).C.T.Bhunia, Director of NIT, Arunachal Pradesh and S.K.Chakraborty Assistant Professor of CSE, NIT, AP for their valuable feedback and supporting us to carry this research works.Figure 5. Return Loss (S11)

Figure 6. Insertion Loss (S21)

Figure 7. Reflection (green) and transmission factor (blue) of the filter

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References

[1] Vivek Hanumante, Panchatapa Bhattacharjee, Sahadev Roy, Pinaki Chakraborty and Santanu Maity,“Performance Analysis of Rectangular Patch Antenna for Different Substrate Heights” in International Journal of Innovative Research in Electrical, Electronics, Instrumentation and Control Engineering, Vol. 2, Issue 1, pp. 515–518, Jan. 2014.

[2] Amit Kumar et al., “Bandwidth Enhancing technique in the Designing of Wireless Microstrip Patch Antenna”, International Journal of Review in Electronics & Communication Engineering, Volume 1 - Issue 2, pp. 28–31, June 2013.

[3] Pyatak, N.I.; Klochko, T.V.; Chumachenko, S.V., “The calculation of the matrix of a scattering wave-guide dielectric resonator of a complex structure”, The Fifth International Kharkov Symposium on Physics and Engineering of Microwaves, Millimeter, and Submillimeter Waves, 2004. MSMW 04, pp. 722–724, Vol.2, 21–26 June 2004.

[4] Abdel-Rahman, A.B.; El Dein, A.Z.; Hamed, H. F A; Ibrahim, A.A., “Design of small size coupled resonator band pass filters with capacitor loaded slot Using FDTD method”, Radio Science Conference (NRSC), 2011 28th National , pp.1–8, 26–28 April 2011.

[5] Ganguly, A.K.; Krowne, C.M., “Characteristics of microstrip transmission lines with high-dielectric-constant substrates”, IEEE Transactions on Microwave Theory and Techniques, vol.39, no.8, pp. 1329–1337, Aug 1991.

[6] Chen-Mao Rao; Tzu-Jung Wong; Min-Hua Ho, “A Parallel Doubly Coupled Dual-Band Bandpass Filter”, IEEE MTT-S International on Microwave Symposium Digest, 2006, pp. 511–514, 11–16 June 2006.

[7] Mudrik Alaydrus, “Designing Microstrip Band pass Filter at 3.2 GHz”, International Journal on Electrical Engineering and Informatics, pp. 71–83, Vol.2, No. 2, 2010.

[8] Bhattacharjee, P. S.; Das, S.; Chowdhury, S. K., “Characteristics impedance of coupled microstrip lines”, IEEE International Symposium on Electromagnetic Compatibility, 1995. Symposium Record, 1995, pp.137–138, 14–18 Aug 1995.

[9] M. Kirschning, R.H. Jansen, and N.H.L. Koster, “Accurate model for open end effect of microstrip lines”, Electronics Letters 17, pp. 123–125, Feb. 1981.

[10] Wensong Wang; Shuhui Yang; Yinchao Chen, “The Research and Design of 5.5 GHz Bandpass Filter for WLAN”, International Conference on Multimedia Technology (ICMT), 2010, pp.1–5, 29–31 Oct. 2010.