A wide stopband low-pass filter design based on multi-period taper-etched EBG structure

A

Wide-stopband Low-pass Filter Design Based

on

Multi-period

Taper-etched EBG Structure

Yin-Chi Chen, An-Shyi Liu, and Ruey-Beei

Wu

Department

of Electrical Engineering and Graduate Institute of Communication Engineering National

Taiwan University Taipei, Taiwan 106, R.O.C.

Abstract - Electromagnetic bandgap (EBG) microstrip

structures have been proposed in theapplication ofantennaand microwave filter design. The paper demonstrates a design procedurefor the EBG structure to achieve a wide stopband low-pass filter, by combining tapered etched holes and multi-period EBG structure. The measurement result shows the rejection band is from 5.2 GHz to 25 GHz, which meets the simulation result.

Index Terms Electromagnetic bandgap, multi-period EBG

D D 'D ' D ' D ' D 'D D D

D:DD :D : Di:

(a)

:01 :02 :03: 0D4 : 5: 0D4 : 2:02 1

I. INTRODUCTION

(b)

A periodic structure which may exhibit the rejection of a certain frequency band is called photonic bandgap (PBG) or

electromagnetic bandgap (EBG)structure. The applications of EBG structure in microstrip technology investigated in literatures are concentrated on antenna and microwave filter design [1]-[7]. The main mechanism of EBG integrated

antennaisto suppress surfacewavewhile the EBG integrated filteris to rejecta frequencyband. The aim of this work is to

designanEBGstructure as awidestopband low-pass filter.

In conventional microwave filter design, the undesired

frequency suppression has been usually implemented using

shorted-circuited stub or surface-mounted capacitor. The shorted-circuited stub is one-halfguided wavelength long at

the frequency to be rejected and occupies some circuit area.

Onthe other hand, the surface-mounted capacitoris suitable for low-frequency applications and its parasitic effect becomes severe as the frequency increases. It is difficult to

estimate theparasitic effect owing to fabrication discrepancy

in the design procedure. Therefore, another resolution which is a simple pattern ofperiodic holes or structures etched in

ground plane beneath the strip conductor, has beenproposed

forlow-pass filter design[2]-[7].

Two-dimensional (2-D) periodic patterns etched in ground plane formicrostrip lines were first proposed [2]-[3]. Owing

to the highly confined field around the strip conductor, the

structure is then reduced to one-dimensional (1-D) ones in

some literature [4]-[7]. Although atheoretic studyto analyze

therejection frequencyband of 1-DEBGmicrostripstructure

has been proposed [4], it considers only the design that the ratio of radius of etched circular holestoseparationbetween

This work was supported in partby the National Science Council, Taiwan, ROC,under Grant NSC 93-2752- E-002-003-PAE.

D1:2: D3: D D3 D2 D1

Fig. 1 Schematic representations of differentEBGstructures.

(a) single-period, (b) multi-period, and (c) multi-period with

tapered etched holes.

centers ofadjacent holes are kept the same. Such a design does not provide optimal bandwidth ofrejection band for a

given number of etched holes. Other designs for wider rejection bandcanbeimplemented, suchasby cascaded EBG

microstrip circuit [3], taper structure in ground plane [6], or

multi-period EBGstructure

[6]-[7].

The structure proposed in this paper is based on a combination of tapered etched holes and multi-period EBG structure. With suitable choice of the radius ratio and the

separation of adjacent holes, a novel wide rejection band, which is from 5.2 GHzto 25 GHz, canbe obtained. The first section describes how the EBG structure was designed. It is followedby asectiondescribingthe simulation and measured

results,and then is the conclusion.

II.DESIGNANDANALYSISOFEBG STRUCTURES

SeveraltypicalEBGstructureswith etched circular holes in

ground plane ofa50Q microstriplineare shown inFig. 1. In the design procedure, single-period structures shown inFig. l(a) were first analyzed by Ansoft Ensemble 8.0 [8] with

separations D as a parameter. The central frequency, depth, and bandwidth of the forbidden band for the 1-D EBG structure can be determined by the separation of adjacent

holes in the ground plane (D), the hole radius (R), and the number of holes

(N)

[4].

For a single-period EBG structure, the central frequency of the first stopband can be roughly determined by the periodD

using the formula [1]:

c 1

f

=~-e

- (1)

where c is thespeedoflightinfree space, and

Ag

=2 Dis the

Bragg condition. Recently, a more accurate expression for the Bragg condition has been proposed [5], which takes into account the change in the effective dielectric constant of the microstrip line

eff

produced by the pattern etched in the ground plane. For a microstrip line with etched ground plane, the effective dielectric constant

6eff

canbe estimated as that of the line withunperturbed ground plane, i.e.,

8rC+1 cr 1 1

8eff

2 + 2 + (2)

where d and W are the thickness of the substrate and the width of theline, respectively.

The structure considered here has parameters -r= 3.38 (Rogers R04003), d=20mils, and W=1.2 mm, corresponding

to a50Q conventional microstripline. Tobegin with, the unit lattice is chosen to be a circle with uniform radius for investigating the bandgap characteristics. The bandwidth of

bandgap can be estimated by full-wave analysis with the

separation of aduacent holes, D, as a parameter. In order to

achieveatnon-overlapping band gap,one canthen determine

a setofseparationsbetween adjacent circles,

Di's.

The radius of etchedcircles, R, is chosen between the line width andone

half-wavelength. A small circle degrades the structure into a conventional microstrip and the band gap vanishes. A large

one acts as aslot patchwhen the radius reaches to one

half-wavelengthand increases radiation lossathigh frequency.

The second step is to determine the taper ratio ofadjacent

circles. For convenience, the ratio

Ri+IlRi

is set to be a constant so as to confine the radii of etched circles within the constraint mentioned in the above paragraph. Witha suitable selection of the taperratio, awide-band rejection bandcanbe obtained.

III.SIMULATIONANDMEASUREMENT

In orderto investigate the stopbandbehavior of theproposed

EBG structure, five single-period structures are analyzed, as

shown inFig. l(a). TheperiodD=5mm, 7mm, 9mm, 11.5mm, and 13mm is a parameter, while the etched holes radius is fixed at R=2mm.Simulation results of transmission (S21)are

shown inFig.2. The central frequencies of the first stopband

are 18.5GHz, 13.2GHz, 10.3GHz,

8.1GHz,

and 7.1GHz respectively, which shows that the central frequency of the

stopbandobtainedby using (1)isagoodestimation.

Also noticed in Fig.2 is the non-overlapping stopband. This indicates that a linear combination of these periods may

contribute to a wider stopband response. In addition, the ratio RID's are 0.4, 0.29, 0.22, 0.17, and 0.15 respectively. As shown in Fig.2, a larger RID results in a wider stopband bandwidth. It is in good agreement with the result shown in

[1].

0 -20 m 0az .) -40 -60 -80 -100 0 5 10 15 20 25 Frequency(GHz)

Fig. 2SimulatedS21parameter of single period 1-D EBG microstrip

structure.

TABLE I

DIMENSIONSFOR MULTIPERIOD

EBG STRUCTURE i

Di

(MM) R (mm)

RIo

/Ri 1 5 2 2 7 2 3 9 2

1.O

4 1t5 2

1tO

5 13 2

The five single-period EBG structures are cascaded in seriesto forma 1-Dmulti-periodEBGstructure, as shown in

Fig.l(b). Table I lists the separations

Di

ofadjacent circles. The simulated S parameters are shown in Fig.3. It is found

that the stopband of this multi-period EBG structure is

widenedowingto thecascading of the individual stopbands..

The stopbandobtained for themulti-period EBGisjustlike a

combination of the stopbandsof the fivesingle-periodEBG. The presence of apassband around 10 GHz in Fig.3 depicts

that the design is not good enough. A better approach for

performance upgrade is to adopt a tapered EBG microstrip

device [4], with the parameters listed in Table II. The simulated results in Fig.4 show that the passband around 10 GHz is successfully suppressed, and an impressive wide

stopband and high rejection level can be observed. Good agreement is also found between the measurements and the full-wave simulated data.

0 -20 5 15 IS21 -60-0 5 10 15 20 Frequency(GHz)

Fig. 3 Simulated S21 parameter of multi-period microstripstructure. m

'a

.) 1-D -_ - 1S111(Ensemble)

\4 - -- IS21 (Ensem ble)

. --- - IS111(Measurement)

IS211(Measurement) ;;

TABLE II

OPTIMAL DIMENSIONSFOR

MULTIPERIOD EBG STRUCTURE

I Dii(mm)

RI

(mm)}R

1+i/R

1 5 1.67 12 2 7 2400

1.2

3 9 240

12

4 11.5 2.88 1.2 5 13 3.46 2 IV. CONCLUSION

The behavior of EBG structures with etched circles in ground plane is studied. It is shown that their associated stopbands can be joined together to form a very wide stopband by cascading different periods. The multi-period

EBG structure constructed according to this principle still remains in 1-D, which is of immediate interest for the design

offilters.

25 It is also demonstrated that a wide stopband low pass

filter

can be achieved by employing the tapered etched circles in EBG ground plane. This novel device can be used to perform broadband filtering of undesired frequencies, overcoming the narrowbandtechniques commonlyusedtoday. Otherpossible applications, such asfilters, frequency-selective surfaces, and efficient antenna structures,requirefurther studies.

.I

'I

0 10 20

Frequency (GHz)

Fig. 4 Simulated and measured S21 parameter of optimal multi-period1-DEBGmicrostripstructure.

REFERENCES

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photonic bandgap structure for microstrip lines", IEEE MicrowaveGuided. Wave Lett., vol. 8, pp. 69-71, Feb. 1998. [3] I. Rumsey, M. Piket-May, and P.K Kelly, "Photonic bandgap

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Falcone, and M. Sorolla, "Analysisanddesignof1-D photonic bandgap microstrip structures using a fiber grating model", MicrowaveOptTechnol Lett., vol. 22, pp. 223-226, Aug.,1999. [5] M.A.G. Laso, T.Lopetegi, M.J.Erro, D. Benito, M.J. Garde,and

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[6] M. A. G.Laso, T. Lopetegi, M. J. Erro, D. Benito, M. J. Garde, and M. Sorolla, "Multiple-frequency-tuned photonic bandgap microstrip structures,"IEEEMicroave Guided Wave Lett., vol. 10,pp.220-222,Jun.2000.

[7]C.C.Chiau,X. Chen and C.Parin,"MultiperiodEBGstructurefor wide stopband circuits", IEE Proc.Microwave Antennas Propag., pp. 489-492,Vol.150,Dec.2003.

[8]AnsoftEnsemble,softwarepackage developed byAnsoft Co.

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