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A Design of Microstrip Bandpass Filter with Narrow Bandwidth using DGS/DMS for WLAN

Arjun Kumar Department of Electronics and Computer Engineering

Indian Institute of Technology Roorkee Roorkee, India

akdec.iitr@gmail.com

M.V. Kartikeyan Department of Electronics and Computer Engineering

Indian Institute of Technology Roorkee Roorkee, India

kartik@ieee.org

Abstract—In this paper, a compact microstrip bandpass filter is designed using defected ground structure (DGS) with narrow bandwidth. Here a 50Ω quarter wave microstrip line is used for designing the bandpass filter. A circular head dumbbell shaped DGS in the ground plane of a microstrip line is used which provides the bandstop characteristics. Two series gap slot is introduced for achieving the bandpass characteristics in the conducting strip. These slots in conducting strip are also called defected microstrip structure (DMS). This arrangement provides better coupling in the pass band. In this paper no stubs and via are used. The bandwidth of the filter is 500 MHz and insertion loss less than 0.5 dB in passband at 5.4 GHz. The measured insertion loss (S21) is 0.6 dB at the center frequency 5.4 GHz with a bandwidth of 500 MHz which is good in agreement with measured results after the fabrications)

Keywords-Bandpass Filters, DGS/DMS.

I. INTRODUCTION The trend of integrating WLAN communication system into the mobile electronic products leads to a great demand in developing filters with compact size. The various technologies of size reductions are used by the researchers such as photonic band gap structure (PBG), frequency selective surface (FSS) etc. [2]. The DGS introduces a slow wave effect or change the current path length in the ground plane and due to this slow wave effect the impedance of the line will change and to compensate the impedance to 50 Ohm, the length of the filter will get reduced. In this paper, a narrow bandpass filter of 500 MHz bandwidth at center frequency 5.4 GHz is proposed with reduced size of more than 60 % of the conventional type of narrow bandpass filters [3]. A two slot in conducting line with the circular head defected ground structure is proposed for achieving the bandpass characteristics with low insertion loss in the passband and high attenuation in the stopband at center frequency 5.4 GHz[4]. The DGS provides better coupling. Gap discontinuities in the conducting strip introduce the gap capacitance (gap coupling) that provides the bandpass characteristics [5] [9-10]. In this paper, first a conventional bandpass filter is designed using parallel coupled lines and then using same design goal a

proposed bandpass filter is also designed using DGS which is very compact in size with the conventional bandpass filter. The design goals are shown in Table-I.

TABLE- I. DESIGN GOALS OF THE FILTER

Frequency 5.4 GHz Order of the Filter (N) 5

Bandwidth 500 MHz Dielectric Constant 3.38 Substrate Thickness 1.524 mm

Fractional Bandwidth 10% at 5.4 GHz Dielectric loss(tanδ) 0.0025

Strip Thickness 0.07 mm

II. CONVENTIONAL PARALLEL COUPLED LINE BANDPASS FILTER

Fig.1 gives the circuit implementation of the filter by means of concentrated components like inductors (L) and capacitors (C), for the even and odd filter degree (n). g0, g1, .., gn can be taken from [2].

Fig.1. Realization of the filter using LC components. Fig.2 shows the conventional filter structure implemented in this work. This filter type is known as parallel-coupled filter. The size of this conventional filter is 60mm x 60mm = 1200mm2.

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Fig.2. Top view of the conventional filter at 5.4 GHz []

The following equations for designing the parallel-coupled filter are used [5]:

( ) ( )01 0 0 1[ / 2 ]J Y FBW g gπ= (1)

[ ], 1 0 1/ 2j j j jJ Y FBW g gπ+ +⎡ ⎤= ⎣ ⎦ (2)

For j=1 to n=1

( ) ( ), 1 0 1/ 2n n n nJ Y FBW g gπ+ += ⎡ ⎤⎣ ⎦ (3)

Fractional bandwidth (FBW) is the relative bandwidth. Jj, j+1 is the characteristic admittance of J inverter and Yo is the characteristic admittance of the connecting transmission line. With the data of characteristic admittance of the inverter, the characteristic impedances of even-mode and odd-mode of the parallel-coupled microstrip transmission line can be calculated [6] - [7] as:

( ) ( ) ( ) ( )2

, 1 , 1, 11 1 / /oe o j j o j j oj j

Z Y J Y J Y+ ++⎡ ⎤= + +⎢ ⎥⎣ ⎦ (4)

For j = 0 to n and

( ) ( ) ( ) ( )2

, 1 , 1, 11 1 / /oo o j j o j j oj j

Z Y J Y J Y+ ++⎡ ⎤= − +⎢ ⎥⎣ ⎦

(5)

The distributed model bandpass filter values are calculated using the formulas given below [7]:

( ) ( )1 1/ 2oZ J gπ= Δ⎡ ⎤⎣ ⎦ (6)

( )1/ 2o n n nZ J g gπ += Δ (7)

For n = 1,2….N

( ) ( )1 1/ 2o N N NZ J g gπ+ += Δ⎡ ⎤⎣ ⎦ (8)

Where ∆ = (ω2 – ω1)/ω0

Z0 = characteristic impedance = 50 Ω

The even and odd mode impedance is calculated by [7]

Z0e = Z0[ 1+ jZ0 + (jZ0)2] (9)

Z0o = Z0[ 1- jZ0 + (jZ0)2] (10)

From these values, width, length and spacing of the parallel coupled line are calculated and are shown in Table-II [11].

TABLE -II. SPECIFICATIONS OF PARALLEL-COUPLED MICROSTRIP LINES

n gn ZoJn Zoe(Ω) Zoo(Ω) W(mm) L(mm) S(mm)1 1.7058 0.3033 69.76 39.43 2.79 7.5 0.5 2 1.2296 0.1084 56 45.17 3.36 7.5 2.06 3 2.5408 0.0888 54.83 45.95 3.39 7.5 2.51 4 1.2296 0.0888 54.83 45.95 3.39 7.5 2.51 5 1.7058 0.1084 56 45.17 3.36 7.5 2.06 6 1.000 0.3033 69.76 39.43 2.79 7.5 0.5

III. SIMULATED S-PARAMETER OF CONVENTION BANDPASS FILTER

Fig.3. Simulation S-parameters of parallel coupled line bandpass filter at 5.39

GHz [11]

Fig.3 shows the simulation response of conventional parallel-coupled microstrip line bandpass filter with center frequency 5.39 GHz which is almost 5.4 GHz with insertion loss of 0.5 dB and the bandwidth of this filter is 500 MHz.

S

L

Parallel coupled

50 ohm line

50 ohm line

Port1

Port2

Top View

W

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IV. PROPOSED DESIGN CONFIGURATION AND SIMULATED RESULTS

Fig. 4: Geometry of proposed Bandpass filter

In this proposed design the height of the substrate is 1.524 mm ,relative permittivity is 3.38, the conductor thickness is 0.070 mm, loss tangent is 0.0025 and width of conducting strip is W = 3.6 mm and 19.5 mm . In this proposed design topology the two slots of spacing S = 0.3 mm in conducting strip and the circular head of radius R = 1.5 mm and the connecting slot g = 0.6 mm in the ground plane of the microstrip line is etched as shown in Fig.4. The size of this proposed filter is 20 x 19.5 mm2. It is well known as R increases the series inductance will increase and the cutoff frequency will be shifted towards lower frequency [8]. The variation in gap (g) also shifts the cutoff frequency. As the gap increases frequency shifts towards the higher frequency. Hence, by the variation of these dimensions, the cutoff and attenuation pole can be changed. The spacing (S) in the conducting strip introduces the gap capacitance and due to this capacitance the bandpass characteristics will appear. The simulated results are shown in the Fig.5, which shows the S21 -0.5dB at the center frequency 5.4 GHz and the upper and lower cutoff frequencies is 5.6 GHz and 5.1 GHz. Hence the total bandwidth of the proposed filter is 500 MHz. All the simulations are carried out in HFSS V 10.

Fig. 5: Simulated S-parameters of proposed filter

V. FABRICATION AND MEASUREMENT

The fabricated design configuration of the proposed design with DGS has been shown in Fig.6. Both the slots have the gap dimension S = 0.3 mm in the conducting strip. The width of line W = 3.6 mm, circular head DGS with radius R = 1.5 mm and connecting head slot gap g = 0.6 mm.

(a) (b)

Fig.6. Fabricated design configuration of proposed bandpass filters: (a) top view (c) bottom view

The Fig.7 shows the measured and simulated S-parameter (S21 in dB) in which measured S-parameter is almost same as simulated S-parameter. The measured insertion loss in 0.6 dB at center frequency 5.4 GHz and the bandwidth of the filter is almost 500 MHz which is good in agreement with simulated and conventional bandpass filter.

Fig. 7: Measured and Simulated S-Parameters

VI. CONCLUSIONS From the simulations, it is quite appreciable that the proposed filter with circular head shaped DGS offers a size reduction >60% with reduced harmonics in the passband. It is clear that in conventional parallel coupled bandpass filter the filter size is 1200mm2 and in proposed design the filter size is 390mm2.In addition, it offers a simple approach to an otherwise difficult problem with low insertion loss of 0.6 dB in the passband and high attenuation of 20 dB in the stopband.

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[11] Arjun Kumar, Kumar Goodwill, and M.V. Kartikeyan, “A Compact Narrow Band Microstrip Bandpass Filter with Defected Ground Structure (DGS),” In Proceeding of National Conference on Communication, IIT Kharagpur, NCC-2012, pp. 1-4, 2012.