a design and realization of miniaturized low pass filter using defected ground structure technique...

8/13/2019 A DESIGN AND REALIZATION OF MINIATURIZED LOW PASS FILTER USING DEFECTED GROUND STRUCTURE TECHNIQ

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International Journal of Electronics,

Communication & Instrumentation Engineering

Research and Development (IJECIERD)

ISSN(P): 2249-684X; ISSN(E): 2249-7951

Vol. 4, Issue 1, Feb 2014, 97-108

TJPRC Pvt. Ltd.

A DESIGN AND REALIZATION OF MINIATURIZED LOW PASS FILTER USING

DEFECTED GROUND STRUCTURE TECHNIQUE WITH WIDE STOPBAND

INDER PAL SINGH1, PRAVEEN BHATT

2& AJAY S. YADAV

3

1Shinas College of Technology, Shinas, Oman, India

2SGI, Panipat, India

3SRM University, Ghaziabad, Uttar Pradesh, India

ABSTRACT

A novel fifth order Chebyshev stepped impedance low pass filter is designed, simulated and fabricated.

Parameters of circuit are extracted from the EM design equations and then values are optimized. Defected ground

structures (DGS) have been developed to enhance different characteristics like wide stop band, frequency response, flat

pass band and minimum ripples etc. of many microwave devices. In this paper we designed and fabricated fifth order low

pass filter and the characteristic of this low pass filter is improved when this filter is designed with dumbbell Shaped Slot

Defected Ground Structure (DGS). The response of the filter is analyzed with respect to variation in dimension of the GS

unit. The variation of dimensions of defects studied with their corresponding change in capacitance, inductance as well as

frequency response. The defects dimensions are modeled with respect to the frequency. Chebyshev stepped impedance five

pole DGS is adopted to increase the bandwidth of stopband and decrease the size of the filter. This proposed low pass filter

achieves a wide stop band with overall 38-dB attenuation up-to 10 GHz. The frequency response of the micro strip filter is

modeled with respect to the variation in dimension of DGS also using CST microwave studio

KEYWORDS: Filters, Defected Ground Structures, CST Microwave Studio, Wide Stop Band, Stepped Impedance,

Wide Stop Band, Electromagnetic Band-Gap (EBG)

INTRODUCTION

A microwave filter is a two- port network used to control the frequency response at a certain point in a microwave

system by providing transmission at frequencies within the pass band of the filter and attenuation in the stop band of the

filter. Defected ground structures (DGS) are recently one of the hottest topics which are researched in microwave domain,

which developed from the photonics band gap (PBG) structures [1].

In modern telecommunication systems, high-performance microwave circuits with compact size, low insertion

loss, steep rejection, and wide stop band are widely required. In [2], a PBG for micro strip lines is proposed for achieving

steep and wide stop band in the photonic frequency band. The conventional PBG structures have constraints in the wide

stop band performance and high pass band ripples caused by the periodicity of PBG patterns, thus non uniform dimensions

of PBG patterns are investigated to improve the pass band ripples [3][4]. In [5], fractal EBG is introduced to achieve wide

band gap characteristic, but the structure is sti ll large as nine period patterns are employed. Therefore, it is imperative for

investigating a method to miniaturize the EBG structure with good performance. Recently, defected ground structures

(DGSs) have been gaining interests for its planar structure and easy fabrication with photolithographic technique. DGS is

an etched lattice which makes one or a few of PBG etched ground elements in the ground plane. Periodic or non periodic

DGSs show property of rejecting microwave in some frequency, so it has a potentially great applicability to restrain

spurious response by rejecting harmonic in the microwave circuits. Any defect such as dumbbell elliptical, square etc.


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98 Inder Pal Singh, Praveen Bhatt & Ajay S. Yadav

etched in the ground plane of the micro strip line disturbs its current distribution and can give rise to increasing effective

capacitance and inductance. A broader area attached to simple slot heads etched in the ground plane of a micros trip line

increases its effective length [67]. DGS can achieve high-performance which cannot be obtained by conventional

technology. By etching DGS on the ground plane it is possible for the designer to increase the equivalent inductance Lhighly and to decrease the equivalent capacitance C at the same time, and finally to raise the impedance of the micro strip

line to a level more than 200 [8].

But the problem arises, as there is no fixed mathematical model in order to relate the frequency response with

respect to the change in dimension of DGS Unit cell. In order to derive the equivalent circuit parameters of DGS unit at the

reference plane, the S-parameters vs. frequency should be calculated by full-wave electromagnetic (EM)-simulation to

explain the cut off and attenuation pole characteristics of the DGS section.

The full-wave analysis does not give any physical insight of the operating principle of DGS. The flow chart in

Figure.1 shows the conventional design and analysis methods of DGSs. [12]. The size of DGS is determined by accuratecurve-fitting results for equivalent-circuit elements to correspond exactly to the required inductance. In this paper Iterative

method is applied to achieve the optimized results.

Figure 1: Conventional Design and Analysis Method of Dumbbell DGS

STRUCTURE DESIGN AND ANALYSIS

EBG Structure

Figure 4 shows the schematic of the one-dimensional EBG structure. The planar EBG structure consists of an

array of uniform square patterns etched on the micro strip line. In Figure 2, d1is the distance between two adjacent EBG

cells, represents the side length of square pattern. According to the photonic crystal theory, the planar EBG structure

exhibits a band gap characteristic while the following Bragg reflection condition is satisfied. [9]

1.d (1)

g

2 (2)

eff

gf

c

.0


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A Design and Realization of Miniaturized Low Pass Filter Using Defected Ground Structure Technique with Wide Stop band 99

Where is the guided wave number of the substrate, g is the guided wavelength, 0f represents the central

frequency of the stop band, c is the speed of light in free space, eff is the effective permittivity that can be estimated from

[10].

wh

rreff

1212

)1(

2

)1(

(3)

Where r and h are the relative dielectric constant and the thickness of the substrate, respectively, and w is the

width of the micro strip line. Substituting (2) into (1) yields

2..

2

1

0

1

g

efff

cd

(4)

The filling factor

1d

aplays an important role in the attenuation performance [9]. Bandwidth and suppression level

of the stop band are mainly influenced by a. As shown in Figure.2, the transmission responses of the stop band can be

improved significantly as a increases. With increasing of a, the capacitance of the square pattern will become larger.

This will result in the decrease of central frequency and increase of bandwidth within the stop band. However, the ripple in

the Pass band increases simultaneously with the larger filling factor. In present paper

1d

ais optimized to be 0.35 in case of

conventional low pass filter and 0.45 when DGS low pass filter is realized which is very close to 0.5.

Figure 2: Simulated 21S Parameters of the Fifth-Order EBG Structure for Different Values of a

Figure 3: Simulated 21S Parameters of the EBG Structure for Different Orders

Figure.3 shows the transmission responses of the planar EBG structures for different orders

(number of etched square patterns). The attenuation level and selectivity of the transmission responses become better with

increasing order of EBG patterns, whereas the dimension of structure will be larger accordingly. Moreover, as can be seen


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from Figure 2, the ripples in the pass band near the cut off frequencies are large, which is caused by the periodicity of EBG

structure. Many methods have been presented to reduce the size of EBG structures [3], [11], [5]. However, so far a large

number of cells of EBG structure are still required to improve the stop band performance. Therefore, an effective way to

modify the propagation behaviour without increasing the structure size must be taken into consideration.

Figure 4: Schematic of the One-Dimensional EBG Structure

Figure 5: Micro Strip Transmission Line with Single DGS on the Ground Plane

ModellingEquivalent Circuit of DGS

Figure 6: Shows the Micro Strip Configurations Consisting of a 50 Ohm

Transmission Line and a Single DGS Underneath the Micro Strip Line

Figure 7: Equivalent Circuit Models of (a) A Prototype LPF (N=1) and (b) A Single DSS

Once that frequency information is found, the equivalent inductance and capacitance of the DGS can be calculated

by the general insertion loss method LPF design equation and a resonator theory. Figure 5 shows the micro strip

configurations consisting of a 50 ohm transmission line and a single DSS underneath the micro strip.

Figure 7 shows a lumped equivalent circuit model of a maximally flat prototype LPF (N=1) and a single DSS in

the ground plane. Equations (5) and (6) can be found from Figure 7.

cL

Zg

jLjjX

01

1 .. (5)


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r

rDGSr

DGS

C

jjX

1. (6)

Since (5) and (6) have the same reactance at =c, the following holds:

DGSL jXjX (7)

Equation (7) results in (8), which gives the equivalent capacitance. Once the capacitance is found, the other

equivalent circuit component, inductance can be easily calculated from (9).

r

c

c

rr

DGS

Zg

C

..

1

01

(8)

DGSr

DGSC

L.

1

2

(9)

In LPF design with a DGS, the cut off frequency of the DGS dimensions must be equal to the specified design cut

off frequency.

Modelling Losses of DGS

As shown in Figure. 8 the anti-resonant peak points of the circuit simulations and measured are different.

The difference of these peak points is caused due to losses of the micro strip transmission line and a loss of slotted ground

section. In practical case, micro strip transmission line has the dielectric, conductor, and radiation losses.

The conductor loss caused by the finite conductivity of the conducting micro strip line and ground plane is

represented by the series resistance, and the dielectric loss caused by the complex permittivity of dielectric material is

represented by the shunt conductance [16]. Normally, a radiation loss is generated due to the impedance mismatching and

discontinuity of a transmission line. [17]

Thus, if the equivalent circuit simulation of the dumbbell shaped-slotted ground section, which includes a

radiation and mismatch loss, matches the measured data, it proves that this circuit model presents the exact

Figure 8: Circuit Simulation and Measured S-Parameters of a DSS with the Dimensions of

D=5 Mm and G=0.7mm H=30 Mil and =2.2, CDGS=0.096 PF, LDGS=3.4072 NH


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Figure 9: Circuit Simulation of the Complete Equivalent Circuit Model and

Measurement of DGS with theDimensions of D=5mm and G=0.7mm,

LDGS= 3.407nh, CDGS= 0.096pf, RDGS=2142.2 ohm

The impedance of DSS section in the ground can be expressed as equation (10)

DGSDGS

DGS

DGS

DGS

LCj

R

YZ

1

111 (10)

At resonance the imaginary part of (10) is zero. Equation (10) can be expressed as (11) at its resonance.

DGSDGS RZ at 0 (11)

The series resistance DGSR representing the radiation and mismatch loss can be determined through measured or

EM simulated 21S .

21

210 12

S

SZRDGS

(12)

With the definition of the insertion loss in (13), (12) can be shown as (14)

21

0

1log20

SdBIL (13)

20

20

0

0

0

10

1012

dBIL

dBIL

DGS

Z

R (14)

The insertion loss is found as 27dB, i.e.,

21

0

1log20

SdBIL =27dB in Figure. 9, so 21S is determined as

0.0446 by (13). Thus, DGSR is also determined as 2142.2 ohm by (14). Finally, the resulting lumped equivalent circuit

model is shown in Figure. 9 which proves the circuit simulation and measurement results are in agreement

DESIGN OF FILTER AND RESPONSE DUE TO DEFECTED GROUND

The low pass filter configuration having five sections of alternating high and low impedances is shown in the

Figure 10. The order of filter designed is of 5th

order. The Dumbbell Shaped Slot DGS section is fully described by two

parameters the etched lattice dimension and gap distance. The influences of these two parameters on frequency


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characteristics of a micro strip are shown by simulations. All simulations were carried out on CST Microwave studio.

The dimension of DGS slot are given below in Figure. 10 as l, w, g respectively.

Figure 10: Design of Low Pass Filter Figure 11: Design of Low Pass Filter with Defects

When the single dumbbell shaped slot is placed at the centre, it provides inductance and hence by placing the

DGS in the structure, effective inductance increases and the cut off frequency decreases. The line width is chosen to be the

characteristic impedance of 50 micro strip line for simulations. Three DGS unit circuits were simulated with the differentdimensions. In order to investigate the influence of the square lattice dimension, the etched gap, which is related with the

gap capacitance, was kept constant to 0.1 mm for all three cases and the etched square area was varied. The substrate with

0.762 mm thick and a dielectric constant of 3.2 and thickness of metal 0.01mm is used for all simulations. We observed

that employing the proposed etched lattice increases the series inductance to the micro strip line. This effective series

inductance introduces the cut off characteristic at certain frequency. As the etched area of the unit lattice is increased, the

effective series inductance increases, and increasing the series inductance gives rise to a lower cut off frequency, as seen in

Table 1 The capacitance values are identical for all cases due to the identical gap distance. However, the attenuation pole

location, which corresponds to the resonance frequency of the parallel LC circuit, also becomes lower because as the series

inductance increases, the resonance frequency of the equivalent parallel LC circuit decreases. [13].

Table 1: Variation of Length Slot and Gap is Constant (G = 0.1mm) in DGS

Gap g =0.1mm

Slot length (Variable) d =7mm d =8mm d =9mm

Inductance(nH) 5.24 6.39 7.56

Capacitance(pF) 0.70 0.70 0.70

Cut off freq (GHz) 1.70 1.48 1.34

Center freq (GHz) 2.59 2.36 2.21

Table 2: Variation of Gap and Length Slot is Constant (D = 5mm) in DGS

Slot Length d=5mmGap G=0.1 mm G=1 mm G=2 mm

Inductance(nH) 3.58 3.58 3.58

Capacitance(pF) 0.72 0.18 0.08

Cut off freq (GHz) 2.25 3.4 3.52

Center freq (GHz) 3.16 7.14 8.5

The lattice dimension (d) is kept constant to 5 mm for all three cases and the etched gap (g) distance is varied.

Due to the constant lattice dimensions, we can expect that the effective series inductances are also constant for all cases.

There is no change in cut off frequency despite the variation of the gap distance. This means that the gap distance does not

affect the effective series inductance of a micro strip. Variation of the effective capacitance only affects the attenuation

pole location [1]. As the etched gap distance increases, the effective capacitance decreases so that the attenuation pole

location moves up to higher frequency, seen in Table 2.


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Table 3: Variation in Cut off Frequency with Respect to the Change in d

S. No. D(mm) Cut Off Frequency (GHZ) Slope(DB/GHZ)

1 6.3 2.4214 7.4808

2 6.1 2.4434 7.3361

3 5.9 2.4787 7.13

Table 3 shows the effect of change in d.

According to the Quasistatic Theory of DGS depicted in [13] the electric and magnetic fields are mostly confined

under the micro strip line. The return current on the ground plane is the mirror image of the current distribution occurred at

the strip line. The maximum surface current lies over the ground plane and the width of side filament arm which contribute

to the inductance of DGS [13]. The gap is represented by the equivalent capacitances, the inductances and capacitances are

derived from the physical dimensions using quasi-static expressions for micros trip crosses, lines and gaps given in [14].

The electrical equivalent model of DGS is given below in Figure 12 [13, 15].

Figure 12: Equivalent Circuit of DGS

Figure 13: (a) Fabricated Defected Ground Low Pass Filter

Figure 13: (b). Fabricated Dumb Bell Shape DGS

Low Pass Filter (Back Side of Filter)

SIMULATION AND MEASURED RESULTS

To validate the above analysis and design, the prototypes as shown in Figure 10 and Figure 11 is fabricated with

the standard PCB technology. In Figure 10 two EBG rectangular patterns are alternatively connected with three high

impedance lines and at the ends the 50 lines are connected.

The optimized dimensions and other parameters of conventional fifth order low pass filter are as follows Substrate

thickness h= 0.762mm, Dielectric Constant = 3.2, Metal Thickness t = 0.01mm, W1= 0.293 mm, W2= 6.352 mm


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l1= 2.917 mm l2= 7.1323 mm, l3=11.036mm, Lumped Element values L1= 2.05nH, C2=2.1472 pF, L3= 6.634nH, C4=

2.146 pF, L = 2.05nH. Stepped impedance low pass filter of order five with cut off frequency 2.3 GHz, the width W0of

micro strip lines for input and output is 1.5 mm, corresponding to a characteristic impedance of 50 .

Figure 14: Fabricated Low Pass Filter

Figure 15: Response (S11and S21) of Conventional Low Pass Filter, Substrate Thickness

H= 0.762mm, Dielectric Constant r= 3.2, Metal Thickness T = 0.01mm, W1= 0.293 Mm,W2= 6.352 Mm L1= 2.917 Mm L2= 7.1323 Mm L3=11.036mm, W0 =1.5mm

Figure 14 shows the fifth order fabricated conventional low pass filter and Figure 15 shows the simulation result

of this filter , which has 3 dB cut off frequency 2.24 GHz which is very close to designed cut off frequency 2.3 GHz with

0.01 dB ripple level and the return loss is -52dB. The stop band below -20 dB of the simulated planer fifth-order

Chebyshev low pass filter ranges from 4.24 to 7 GHz.

Figure 16: Measured Response of Conventional Low Pass Filter

Figure 16 shows measured results for the fabricated stepped impedance low-pass filter has cut off frequency

2.54 GHz which is 0.24 GHz higher than with calculated cut off frequency with 0.01dB and the return loss is -38 dB. Stop

band of the fabricated structure ranges from 4.6 to 7.65 GHz. Insertion loss -24 dB is observed in simulation and measured

Insertion loss -28dB which higher due to some radiation loss. Measured and simulated result are close to each other.


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In this paper, we proposed a compact DGS low pass filter structure. Wide stop band and compact size are realized

with defected ground structure (DGS). Low pass filter of order five with dumbbell shape defected ground with the

gap g= 0.1mm and length of square patch d=7mm with cut off frequency 2.3 GHz is designed and fabricated and then

compared the simulated and measured result from vector network analyzer. Dimensions and other parameters are Substratethickness h = 0.762mm, Dielectric Constant = 3.2, Metal Thickness t= 0.01mm, W1= 0.293 mm, W2= 6.352 mm,

l1= 2.917 mm, l2= 7.1323 mm l3=7mm, LDGS= 3.87 nH, CDGS=0.7F, RDGS= 1487.3 .

Figure 17: Response of Low Pass Filter with Dumbbell Shaped Defect in the Ground

Figure 18: Measured Response of Defected Ground Low Pass Filter

Figure 17 shows the simulation result of proposed DGS structure, which has 3 dB cut off frequency 2.268 GHz

with 0.01 dB ripple level and the return loss is -47dB .The difference between designed cut off frequency and simulated is

0.032 GHz which is almost close. Response is improved in terms of sharpness because of decrease in the capacitance.

As the area of the slot is kept constant, there is no change in effective inductance and hence the cut off frequency is

constant. When the length of the etched slot is decreased effective inductance is decreased because of which cut off

frequency is increased. Figure.18 shows measured results for the fabricated DGS low-pass filter using the steppedimpedance has the cut off frequency 2.1 GHz which is 0.168GHz less than that the simulated result with 0.01dB and the

return loss is -38 dB. The wide stop band below -20 dB of the planer fifth-order Chebyshev low pass filter ranges from

4 to 9 GHz, while that of the fabricated structure ranges from 3.6 to 10.4 GHz. Moreover, the suppression level is deepened

from 10 to 25 dB.

CONCLUSIONS

The total length of the proposed DGS low pass filter is reduced by 4.036 mm when compared conventional low

pass filter. There is good cut off frequency matching in simulation and measured result. Low pass filter with DGS has wide

stop band which is good achievement with proposed DGS low pass filter.

ACKNOWLEDGEMENTS

The authors of this paper would like to thank Dr. Mahesh P. Abegaonkar, Centre of Applied Research in


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Electronics (CARE), IIT- Delhi, New Delhi, India, for fabrication and measurement support.

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