CPW band-stop filter using unloaded and loadedEBG structures

M.F. Karim, A.-Q. Liu, A. Alphones, X.J. Zhang and A.B. Yu

Abstract: Two different structures of unloaded and loaded electromagnetic bandgaps (EBG) areproposed. The models of the unloaded and loaded unit structures are derived by equivalent circuitapproach and full-wave electromagnetic simulation is used for extracting the values of the lumpedelements in the circuit. A band-stop filter (BSF) has been designed with flat response at a selectedfrequency by cascading the unit EBG structures. The EBG filter is fabricated on high resistivitysilicon substrate employing a microelectromechanical systems (MEMS) surface micromachiningprocess. The measurement results for the loaded EBG reveals a 20dB stop-band with a bandwidthof 13.2GHz. The lower and the higher pass-band insertion losses are less than 2dB and 4.5dB,respectively. EBG band-stop filters fabricated by the MEMS process have immense potential to beintegrated with CMOS devices owing to compactness and low cost.

1 Introduction

Periodic structures have recently attracted much attention inthe microwave and millimetre-wave community owing totheir filtering properties or inhibition of signal propagationin certain directions. These structures have been usuallyreferred to as electromagnetic bandgaps (EBGs) or electro-magnetic crystals. EBG structures can be embedded in thedielectric substrate or etched in the metal layer. An exampleis EBG structures with air holes embedded in the substrateto use as substrate for antennas. This will help to suppressthe surface waves and result in a better radiation patternand higher antenna efficiency [1]. EBG structures realisedon metal layers are useful for constructing filters includingband-stop filters, low-pass filter and band-pass filter [2, 3],phase shifters [4], and antennas [5]. Examples of EBGstructures patterned on metal layers are the microstriptransmission line with EBG etched holes on the groundplane [6] or on the signal line [7, 8]. Other examples are thecoplanar waveguide (CPW) with EBG etched holes on theground plane [9], signal line [10] and on the ground andsignal lines respectively [11]. CPWs offer greater designflexibility and require a single metal level compared tomicrostrip structures. The CPW structures are moreattractive because of the ease of characterisation andfabrication, not forgetting promising applications in micro-wave integrated circuits (MICs), monolithic microwaveintegrated circuits (MMICs), and microelectromechanicalsystems (MEMS) devices.

CPW EBG structures with rectangular aperture patternsetched on the ground plane and located adjacent to the gap

between the signal and ground have been reported [12]. Thedimensions of the rectangular aperture are the onlyparameters that can be adjusted. CPW defected groundstructures (DGS) have been employed for bandpass filterapplications [13]. Fortunately, there exists another CPWEBG structures that offer greater design flexibility, wherebya pattern consisting of slots and square apertures are etchedon the ground plane [9]. Initial studies in EBG structuresfabricated on the silicon substrate have been investigated[14]. Most researchers have focussed mainly on thepossibility of obtaining a stop-band in EBG structuresbut little has been reported on the modelling and theperformance of the band-stop filters.

In this paper, a new CPW EBG structure based onunloaded and loaded design is investigated. The frequencycharacteristics of the EBG will be demonstrated byemploying different circuit parameters. An equivalentcircuit model for unloaded and loaded unit structures willbe derived based on circuit analysis theory. The dispersiondiagram will be obtained for the both structures bycombining the commercial software and the Floquet’stheorem in order to analyse the electromagnetic wavebehaviour within a unit cell. The band-stop filter will bedesigned by cascading the unit EBG structures. TheMEMSsurface micromachining process will be used to fabricate thefilter. Finally, the discussion on the measurement results interms of flat responses for the stop-band and pass-band ofthe filter will be analysed.

2 Unloaded EBG structure

An unloaded lattice shaped unit EBG structure, as shown inFig. 1, is proposed. The substrate is high resistivity silicon(r¼ 4000O � cm) with dielectric permittivity of er¼ 11.9 andthickness of H¼ 200mm. The EBG structure is designed for50O in CPW configuration, with a signal line width ofW¼ 108mm and the gap width, G¼ 60mm.

In the unloaded unit EBG structure, a square slot isetched in the ground plane with a side length, a, and this isreferred to as an unloaded structure. The square etched slotis connected to the gap by a narrow transverse slot withlength of ws and width of ds. The centre cut-off frequency,

M.F. Karim, A.-Q. Liu, A. Alphones and A.B. Yu are with the School ofElectrical & Electronic Engineering, Nanyang Technological University,Nanyang Avenue, Singapore 639798

X.J. Zhang is with Intel Corporation, Yan’an Road, Shanghai 200336,People’s Republic of China

E-mail: eaqliu@ntu.edu.sg

r IEE, 2005

IEE Proceedings online no. 20050096

doi:10.1049/ip-map:20050096

Paper first received 14th April and in revised form 10th August 2005

434 IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 6, December 2005

which is the resonant frequency, depends on the transverseslot and the square etched hole of the ground plane. Toinvestigate the influences of these parameters on thefrequency characteristics, two separate designs havebeen constructed. The frequency characteristic for theproposed unit EBG structure is studied and analysed using(i) Momentum (an EM simulation tool by Agilent) and(ii) an equivalent circuit model.

2.1 Influence of the unloaded squareaperture sizeThree different sets of dimensions for the unloaded unitEBG structure are evaluated. The influence of the squareaperture size, a, on its frequency characteristic is investi-gated. The dimensions ds¼ 60mm and ws¼ 200mm are keptconstant, for all the three different square aperture size,a¼ 350mm, 500mm and 600mm. From the simulationresults shown in Fig. 2, the resonant frequency of theEBG cell decreases as the etched area of the square aper-ture increases. This frequency characteristic can also be

explained using the parallel resonant circuit. As the size ofthe square aperture increases, the inductance also increases,whereas ds and ws are responsible for the capacitance. Sinceds and ws are the same for all the three cases, the capacitancedoes not vary much compared to the inductance. This inturn reduces the resonance frequency of the equivalentparallel circuit.

2.2 Influences of the transverse slot widthThe influence of the transverse slot width ds is investigatedin this Section. The square aperture size, a, is kept constantat 500mm� 500mm, and the length of the transverse slot,ws, is fixed at 200mm. The width of the transverse slot isallowed to vary and set at 10mm, 60mm and 200mmrespectively. The simulation results are obtained as shownin Fig. 2.

From Fig. 2, it can be observed that the resonantfrequency shifts from 25.48GHz to 37.98GHz when ds

changes from 10mm to 200mm. Since the size of theunloaded square aperture is fixed for all the three cases, theinductance variation is smaller. Thus, as ds increases, theparallel capacitance decreases and the resonance frequencyof the equivalent parallel circuit increases.

2.3 Modelling of the unloaded EBGAn equivalent parallel LC circuit can be used to model theunloaded unit EBG structure as shown in Fig. 3. It consistsof a series inductor and a parallel capacitor. From apractical point of view, the unloaded unit EBG structurecan serve as a replacement for the parallel LC resonantcircuit in many applications. To apply the unloaded EBGcell to a practical circuit, it is necessary to extract theequivalent circuit parameters, which can be obtained fromthe simulation result of the unloaded unit EBG structure.The lumped capacitance, C, is mainly contributed by thetransverse slot on the ground, while the inductance, L, isrelated to the magnetic flux passing through the apertureson the ground.

The equivalent impedance equation of the single resonantmodel may be expressed as

Z ¼ joC þ 1

joL

� ��1ð1Þ

The resonant frequency of the parallel circuit is defined as

o0 ¼1ffiffiffiffiffiffiLCp ð2Þ

The 3dB cut-off angular frequency, oc, can be determinedby

S21j j ¼2Z0

2Z0 þ Z

�������� ¼ 1ffiffiffi

2p ð3Þ

Substituting (1) into (3), the capacitance is obtained as

C ¼ oC

2Z0 o20 � o2

C

� � ð4Þ

0 10 20 30 40 50 60

−10

−20

−30

−40

0

5

S21S11

mag

nitu

de, d

B

frequency, GHz

ds = 10 µm

ds = 60 µm

ds = 200 µm

a = 600 µm

a = 500 µm

a = 350 µm

Fig. 2 Simulated S-parameters for the unloaded unit EBGstructure, illustrating the effect of variations in square aperture size,a, and the transverse slot width, ds

Z0 Z0

L

C

Fig. 3 Equivalent parallel resonant circuit for the unloaded unitEBG structure

WG

wsds

a

G

square etchedhole

transverse slot

Fig. 1 Unloaded unit EBG structure

IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 6, December 2005 435

The inductance can be determined by

L ¼ 1�o2

0C ð5Þ

a, ws and ds (see Fig. 1) are chosen at a¼ 350mm,ws¼ 200mm and ds¼ 60mm to provide an example to theparameter extraction procedure. The equivalent circuitparameters are extracted and are presented in Table 1.The resonant frequency of the proposed unit EBG isaffected by the variation of the unloaded square aperturedimensions. Based on the tabulated results (see Table 1) andthe simulated results (see Fig. 2), it can be deduced thatby varying from a¼ 350mm to 600mm, the change incapacitances is merely 15%. This is different from the caseof inductance variation, where as much as 178% has beenobserved. The inductance variation rate is nearly 10 timesgreater than that of the capacitance. These observationsverified that the dimension of the transverse slot isresponsible for the parallel capacitor and the size ofthe square aperture relates directly to the series inductor.The series inductance is significantly affected by the sizeof the square aperture.

For the second design in which only transverse slotwidth, ds, is also allowed to vary, the resonant frequency ofthe proposed circuit changes slightly. Table 1 presents theextracted equivalent circuit parameters. The simulationresult is shown in Fig. 2. From the case of ds¼ 10mm to thecase of ds¼ 200mm, the inductance varied by 5%, whereasthe capacitance changes by almost 57%. The capacitancevariation rate is nearly 10 times greater than that of theinductance. This again verifies that the dimension ofthe transverse slot controls the capacitance and the sizeof the square aperture affects the inductance in theequivalent circuit. Since the width of the transverse slot,ds, is varied, the parallel capacitance should also changeaccordingly.

2.4 Propagation characteristics of unloadedEBGThe propagation characteristic of the unloaded EBG isanalysed by Floquet’s theorem. By simply considering theunit structure, the EM simulation result of the methodproposed by [15] can be used to get the dispersion diagram

of the proposed unloaded EBG structure. The propagation

factor in this case is e�g�, where � ¼ 2020 mm is the periodof the EBG structure, and g ¼ aþ jb is the complexpropagation constant in the direction of propagation andcan be calculated by (6)

g ¼ 1

�cosh�1

1þ S11ð Þ 1� S22ð Þ þ S12S21

h�þ Z01

Z02

� 1� S11ð Þ 1þ S22ð Þ þ S12S21

i4S21

0BBBBBB@

1CCCCCCAð6Þ

where Z01 and Z02 are the impedances of port 1 and 2, inthis study 50O.

The normalised phase constant b/k0 and the attenuationconstant a/k0 of a single EBG cell are illustrated in Fig. 4.When b/k0 is close to the Bragg condition (b� ¼ p), thesignal is highly attenuating. The stop band is very wide andwithin which the propagation is prohibited. The simulationresult of the higher frequency band (i.e. larger than 43GHz)is unfavourable when compared with those of the lowerfrequency band (i.e. less than 18GHz).

3 Loaded EBG structures

In this Section, a loaded unit EBG structure is designed. Aring slot is etched in the ground plane with side length, aand ring width, rs. The ring slot is connected to the gap bya narrow transverse slot with length, ws, and width, ds. Inaddition, a square etched hole with side length, b, is etched inthe centre of the ring slot as shown in Fig. 5. As compared tothe EBG structure without centre etched hole (i.e. unloaded),the proposed EBG structure includes a ring slot and a squareetched hole and hence, is named as loaded EBG structure.

When the side length of the square etched hole andthe ring slot side length are equal (i.e. at b¼ a), the squareetched hole and the ring slot will merge to form a squareaperture. When that happens, the unit cell becomes anunloaded EBG structure as mentioned in the previousSection. In this Section, the effect of the dimensions of thesquare etched hole (i.e. side length, b) on the frequencycharacteristic, the equivalent circuit model and the disper-sion diagram of the loaded EBG are examined.

Table 1: Extracted equivalent circuit parameters for theunloaded unit EBG structure, with variation in squareaperture size, a, and variation in the transverse slotwidth, ds

ds¼60mm, ws¼200mm:

Square aperture size, a, mm 350 500 600

Resonant frequency, GHz 38.79 30.6 27.13

3dB cut-off frequency, GHz 29.12 21.92 18.78

Inductance, nH 0.229 0.342 0.434

Capacitance, pF 0.073 0.079 0.079

ws¼200mm, a¼500mm:

Transverse slot width ds, mm 10 60 200

Resonant frequency, GHz 25.48 30.6 37.98

3dB cut-off frequency, GHz 19.38 21.92 24.56

Inductance, nH 0.339 0.342 0.357

Capacitance, pF 0.115 0.079 0.049

10 15 20 25 30 35 40 45 500

1

2

3

4

5

6

7

8

0

1

2

3

4

5

6

7

8

frequency, GHz

Brillouin zone

�/K0� /K0

�/K

0

�/K

0

Fig. 4 Simulated dispersion diagram of unloaded EBG structurewith ds¼ 60mm and a¼ 500mm

436 IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 6, December 2005

3.1 Effects of the square etchedhole-loaded EBG structureIn this part of our study, four different sets of dimensionsare chosen. The square aperture size, a, is 500mm� 500mm,and the dimension of transverse slot is maintained atds¼ 60mm and ws¼ 220mm for all the four cases. A centreetched square hole is added in the area of unloaded squareaperture (see Fig. 5), while the width of the ring slot, rs ischosen to be 30mm for all the four cases, the length of thesquare etched hole, b is allowed to vary.

Based on the simulation results as shown in Fig. 6, all thethree cases with b¼ 0mm, 200mm and 440mm are noted toexhibit similar behaviour except for the case of b¼ 380mm.The resonant frequencies for all the four cases are almostidentical, but the frequency response begins to drift after theresonant frequency. The loaded structures in the squareaperture area is the main contributor to such a variation.It is widely known that low insertion loss and high returnloss are expected in the pass-band of the band-stop filter.An optimum width of the square etched hole (i.e. atb¼ 380mm) can clearly be deduced in Fig. 6 where the

return loss is the highest and the insertion loss is the lowestamong all the four cases. The existence of a loaded structurein the square aperture perturbs the whole circuit, whichimplies that the transverse slot is no longer the only factorcontributing to the equivalent capacitor. Likewise, thesquare aperture can no longer act as the main determiningfactor to the equivalent inductor. Therefore, the additionalstructure in the square aperture area can significantlyimprove the pass-band insertion loss as well as the returnloss. Single resonant equivalent circuit is no longer effectiveto model such loaded structure.

3.2 Modelling of the loaded EBG structureIn the previous Section, the single resonant parallel LCcircuit model is employed to model the unloaded unit EBGstructure. In the case of the loaded EBG cell, such a modelwould not be accurate. When the single resonant circuitmodel is used as shown in Fig. 3, the simulation results forthe insertion and return losses cannot match with the EMsimulation results especially when the frequency becomesgreater than the resonant frequency. Hence, the loadedEBG cell has to be modelled using a different circuit. Bycascading two parallel resonant circuits, the loaded unitEBG structure may be represented in Fig. 7.

The equivalent impedance of the cascaded parallelresonant circuits can be expressed as

Z ¼ joC1 þ1

joL1

� ��1þ joC2 þ

1

joL2

� ��1ð7Þ

where the subscripts ‘1’ and ‘2’ denote the two differentresonant circuits.

The reflection coefficient, S11 is defined as

S11j j ¼Z

2Z0 þ Z

��������

¼

1

oC1 � 1=oL1þ 1

oC2 � 1=oL2

��������ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

4Z20 þ

1

oC1 � 1=oL1þ 1

oC2 � 1=oL2

� �2s ð8Þ

The maximum value of 7S117 can be obtained bydifferentiating it with respect to the angular frequency, o,and the two roots obtained are

o1 ¼ L1C1ð Þ�12; o2 ¼ L2C2ð Þ�

12 ð9Þ

The insertion loss is given by

S21j j ¼2Z0

2Z0 þ Z

��������

¼ 2Z0ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi4Z2

0 þ1

oC1 � 1=oL1þ 1

oC2 � 1=oL2

� �2s

ð10Þ

Z0 Z0

L1

C1

L2

C2

Fig. 7 Equivalent circuit for a unit cell of the cascaded parallelresonant EBG structure

WG

ws ds

rs

a b

G

ring slot

square etchedhole

transverse slot

Fig. 5 Loaded unit EBG structure

0 10 20 30 40 50

−5

−20

−30

−25

−15

−10

0

mag

nitu

de, d

B

frequency, GHz

S11S21

b = 0 µm

b = 200 µm

b = 380 µm

b = 440 µm

Fig. 6 Simulated S-parameter for the loaded unit EBG structure,with variation in the centre slot within the ring

IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 6, December 2005 437

By differentiating S21, the first notch angular frequency, on

can be obtained as

on ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiL1 þ L2

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiL1

�o2

2 þ L2

�o2

1

q ð11Þ

The 3dB cut-off angular frequency, oi, can be determinedas

S21j jjo¼oc

¼ 2Z0ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi4Z2

0 þ1

ocC1 � 1=ocL1þ 1

ocC2 � 1=ocL2

� �2s ¼ 1ffiffiffi

2p

ð12ÞFrom (9), (11) and (12), the equivalent circuit parameterscan be extracted, whereby

L1 ¼2Z0 o2

n � o21

� �o2

1 � o2c

� �o2

2 � o2c

� �oco2

1 o22 � o2

1

� �o2

n � o2c

� � ð13Þ

and

L2 ¼2Z0 o2

2 � o2n

� �o2

1 � o2c

� �o2

2 � o2c

� �oco2

2 o22 � o2

1

� �o2

n � o2c

� � ð14Þ

The capacitance values can be obtained as

C1 ¼ 1�o2

1L1; C2 ¼ 1�o2

2L2 ð15ÞAll the four parameters L1, L2, C1 and C2 can be calculatedusing (13), (14) and (15), respectively. To illustrate theparameter extraction procedure, the values of a, rs, ws, ds

and b are 500, 30, 220, 60 and 380mm respectively. Theparameters for both the single resonant circuit model andthe cascaded circuit model are extracted and summarised inTable 2.

In order to determine the suitability of the two models fortheir possible application in the loaded EBG structure, boththe simulation results of the single resonant and cascadedmodels are shown in Fig. 8. The cascaded circuit simulationresults obviously provided good agreement with the EMsimulation results at frequency, which is below the notchfrequency (i.e. at 60.76 GHz). The simulation result for thesingle resonant circuit model drifts from the EM simulationresult after 40GHz. Thus, for the loaded EBG structure, thecascaded circuit model is more reasonable than the single

resonant circuit model, and is valid up to the first notchfrequency. The extracted equivalent circuit parameters thatare simulated in Fig. 8 and the parameters for the single andthe cascaded resonant model are listed in Table 2.

3.3 Propagation characteristics of theloaded EBGThe dispersion diagram of the loaded EBG structure whichis useful in understanding the guided and leaky wavecharacteristics is shown in Fig. 9. The shaded region from18GHz to 42GHz represents that the electromagnetic waveis prohibited in the periodic structures. There exist aspurious peak in the phase constant curve obtained throughEM simulation and can be overcome by increasing themesh resolution. The band gap region of the loaded EBG isless compared to the unloaded EBG.

4 Bandstop filter measurement results anddiscussion

The surface micromachining process is used for fabricatingthe band-stop filters. The process begins by growing a0.5mm-thick SiO2 layer on a 200mm-thick high resistivitysilicon substrate that served as a buffer layer. A 1.5mmaluminum layer is evaporated on the buffer layer, to define

Table 2: Extracted equivalent circuit parameters for theloaded EBG unit structure

ds¼60mm, ws¼220mm, a¼ 500mm, rs¼ 30mm

Square etched hole b, mm 0 200 380 440

First resonant frequency, GHz 29.24 29.24 29.24 29.19

First 3dB cut-off frequency, GHz 22.42 22.42 23.20 22.34

Second resonant frequency, GHz 81.77 81.42 77.26 77.77

First notch frequency, GHz 66.67 66.67 60.76 65.13

Single resonant model:

L, nH 0.285 0.285 0.243 0.286

C, pF 0.104 0.104 0.123 0.103

Cascaded model:

L1, nH 0.282 0.282 0.243 0.285

C1, pF 0.105 0.105 0.122 0.104

L2, nH 0.023 0.022 0.028 0.021

C2, pF 0.168 0.172 0.152 0.195

0 10 20 30 40 50 60 70 80 90 100

−10

−20

−30

−40

−50

−60

−70

−80

0

S21

S11

mag

nitu

de, d

B

frequency, GHz

S21

single resonant circuit

cascaded circuit

EM simulation

S11single resonant circuitcascaded circuitEM simulation

Fig. 8 Comparison on parameter extraction and EM simulationresults for the loaded unit EBG structure

10 15 20 25 30 35 40 45 500

1

2

3

4

5

6

7

8

0

1

2

3

4

5

6

7

8

frequency, GHz

Brillouin zone

�/K

0

�/K0

�/K

0

� /K0

Fig. 9 Simulated dispersion diagram of a loaded unit EBGstructure

438 IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 6, December 2005

the structure. Photoresist coating and device patterning areperformed next. Finally, aluminum wet etching is per-formed to remove the aluminum thin film and to form theEBG structures. The process needs only one mask tofabricate the filters.

In order to show the validity of the equivalent circuitand the extracted parameters for the proposed unitEBG structure (both unloaded and loaded), band-stopfilters are designed by employing the proposed EBG cellstructure.

4.1 Unloaded EBG band-stop filterThree unloaded EBG cells are cascaded to form a band-stop filter, which is fabricated and measured. These cells areperiodically placed apart at a distance of 2020mm, which isequal to the half guided wavelength of the CPW at 30 GHz.The values of a, ws, and ds for designing the filter arepresented in Table 1. The measurement results are shown inFig. 10. From the measurement results, one can notice thatthe 20dB stop-band is from 24.4GHz to 40.6GHz. Thestop-band return loss is less than 2dB. For the lowerpass-band, the return loss is greater than 10dB but for thehigher pass-band the return loss reaches to 6dB. The lowerpass-band insertion loss is less than 2dB and the higherpass-band insertion loss is below 5dB.

4.2 Loaded EBG band-stop filterThree of the proposed loaded EBG cells are cascaded toform a band-stop filter. The SEM micrograph of theproposed three-cell EBG band-stop filter is shown inFig. 11. The values of a, rs, ws, ds, and b, for the loadedEBG are presented in Table 2. The measured results of thefabricated EBG band-stop filter together with its simulateddata from both the EM software and the equivalent circuitare shown in Fig. 12. Both the simulation results and theequivalent circuit are in good agreement with the measuredresults, which show the validity of the proposed equivalentcircuit. The measured results show a relatively wide 20dBstop band, from 24.8GHz to 38GHz, with a return lossbelow 2dB. For the pass-band, the return loss is higher than10dB except for those at the higher frequencies. The lowerpass-band insertion loss is less than 2dB and the higherpass-band insertion loss is less than 4.5dB. The 2dBinsertion loss exhibited in the pass-band of the filter atfrequencies below 5GHz is a result of material loss,conductor/metallic loss and MEMS fabrication error.

A comparison on the performances of the loaded and theunloaded EBG band-stop filters shows that except for the20dB stop-band width, it is obvious that the performancesof the unloaded EBG band-stop filter are more inferiorcompared to the loaded EBG band-stop filter in all aspects.In particular, the higher pass-band performances of theloaded EBG filter are better than those of the unloadedcase. The reason for this improvement lies in the equivalentinductance of the loaded unit EBG structure, which ismuch lower than that of the unloaded unit EBG structure.As for the equivalent capacitance, the loaded cell yieldshigher capacitance than that of the unloaded cell. It shouldbe noted that both the pass-band insertion loss and thestop-band return loss characteristics are very flat andcontain less than 0.2 ripples for both filters, which is in facta concern for band-stop filters realised using other EBGstructures.

5 Conclusions

A new band-stop filter using unloaded and loaded structurehas been investigated. The frequency characteristics of theEBG unit cell are studied by employing different circuitparameters. An equivalent circuit model has been derivedfor the unloaded and loaded EBG cell. The extractedparameters show that the bandgap effect of the EBG cells

0 10 20 30 40 50

−10

−20

−30

−40

−50

−60

0

S11S21

mag

nitu

de, d

B

frequency, GHz

measurement

EM simulation

circuit simulation

Fig. 10 Measurement results for the fabricated unloaded EBGband-stop filter with the simulation results (EM and circuitsimulations) for comparison

CPW

ring slotsquareetched hole

15 kV × 25 1 mm 45 46 SE I

Fig. 11 SEM micrograph of the loaded EBG band-stop filter

0 10 20 30 40 50

−10

−20

−30

−40

−50

−60

−70

−80

0

mag

nitu

de, d

B

frequency, GHz

S21S11

measurement

EM simulation

circuit simulation

Fig. 12 Comparison of the measurement and simulation results ofthe loaded EBG band-stop filter

IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 6, December 2005 439

can be interpreted accurately based on the circuit analysistheory. The dispersion diagram obtained was useful fordescribing and understanding the propagation character-istics in the EBG unit cell. The complex characteristicimpedance estimated was helpful for analysing the effect ofimpedance change in the EBG structure. A band-stop filteris designed by cascading the unloaded and loaded EBGcells. MEMS surface micromachining process is employedto fabricate the EBG structures and is compatible with theCMOS process, which can be integrated easily with CMOSdevices for wireless front-end systems. The measured resultof the loaded structures shows enhanced pass-bandperformance owing to higher equivalent capacitance andlower equivalent inductance, which are helpful to increasethe insertion loss and decrease the return loss at higherfrequencies. In addition, the proposed structure can alsoyield flat return loss performance in the stop-band. The20dB stop-band is as wide as 13.2GHz. The lowerpass-band insertion loss is less than 2dB with 0.2dB ripple,and the higher pass-band insertion loss is less than 4.5dB.

6 References

1 Gonzalo, R., De Maagt, P., and Sorolla, M.: ‘Enhanced patch-antenna performance by suppressing surface waves using photonicbandgap substrates’, IEEE Trans. Microw. Theory Tech., 1999, 47,pp. 2131–2138

2 Karmakar, N.C.: ‘Improved performance of photonic bandgapmicrostripline structures using chebyshev distribution’, Microw. Opt.Technol. Lett., 2002, 33, (1), pp. 1–5

3 Radisic, V., Qian, Y., Coccioli, R., and Itoh, T.: ‘A novel 2-Dphotonic bandgap structures for microstrip lines’, IEEE Microw. Guid.Wave Lett., 1998, 8, pp. 66–71

4 Elamaran, B., Chio, I.M., Chen, L.Y., and Chiao, J.C.: ‘A beam-steerer using reconfigurable PBG ground plane’. 2000 IEEE MTT-SInt. Microwave Symp. Dig., 2000, pp. 835–838

5 Chang, I., and Lee, B.: ‘Design of defected ground structures forharmonic control of active microstrip antenna’. 2002 IEEE AP-S Int.Symp. Dig., 2002, pp. 852–855

6 Karmakar, N.C., and Mollah, M.N.: ‘Investigations into nonuniformphotonic-bandgap microstripline low-pass filters’, IEEE Trans.Microw. Theory Tech., 2003, 51, pp. 564–572

7 Zhang, G., Yuan, N., Fu, Y., and Zhu, C.: ‘A novel PBG structure formicrostrip lines’. Proc. Int. Conf. on Microwave and Millimeter WaveTechnology, 2002, pp. 1040–1042

8 Xue, Q., Shum, K.M., and Chan, C.H.: ‘Novel 1-D microstrip PBGcells’, IEEE Microw. Guid. Wave Lett., 2000, 10, pp. 403–405

9 Fu, Y.Q., Zhang, G., and Yuan, N.: ‘A novel PBG coplanarwaveguide’, IEEE Microw. Wirel. Compon. Lett., 2001, 11,pp. 447–449

10 Xue, Q., Mo, T.T., Shum, K.M., and Chan, C.H.: ‘Characteristics of acompact CPW resonant cell’, Microw. Opt. Technol. Lett., 2003, 37,(6), pp. 408–410

11 Zhu, L.: ‘Slow-wave and bandgap transmission characteristics offinite-periodic coplanar waveguide’. Asia-Pacific Microwave Conf.Dig., 2002, pp. 1153–1156

12 Yun, T.Y., and Chang, K.: ‘Uniplanar one-dimensional photonic-bandgap structures and resonators’, IEEE Trans. Microw. TheoryTech., 2001, 49, pp. 549–553

13 Her, M.L., Wang, Y.Z., Chang, C.M., and Lin, K.Y.: ‘Coplanarwaveguide (CPW) defected ground structure (DGS) for bandpass filterapplications’, Microw. Opt. Technol. Lett., 2004, 42, pp. 331–334

14 Zhang, X.J., Liu, A.Q., Karim, M.F., Yu, A.B., and Shen, Z.X.:‘MEMS-based photonic bandgap (PBG) band-stop filter’. IEEEMTT-S Int. Microw. Symp. Dig., 2004, pp. 1463–1466

15 Mao, S.G., and Chen, M.Y.: ‘Propagation characteristics of finite-width conductor-backed coplanar waveguides with periodic electro-magnetic bandgap cells’, IEEE Trans. Microw. Theory Tech., 2002, 50,pp. 2624–2628

440 IEE Proc.-Microw. Antennas Propag., Vol. 152, No. 6, December 2005