Increase of Characteristic Impedance of MicrostripLine Using a simple Slot in Metallic Ground Plane

A. Arbabi , A. Boutejdar, M. Mahmoudi* and A. Omar

Chair of Microwave and Communication Engineering University of Magdeburg, GermanyE-mail: Ahmed.Boutejdarge-technik.uni-magdeburg.de

*School of Electrical and Computer Engineering, University of Tehran, Iran

Abstract - In this paper, a novel compact wideband high-rejection LPF using defected ground structure(DGS) ispresented. The proposed LPF consists of etched slots in groundmetallic plane as defected ground structure(DGS) and of Hi-Lostructure, which corresponding to capacitance and Inductance onthe top layer. The effect of the DGS slot on the characteristics ofthe investigated filter is examined. In this work we have proposeda simple method for increasing microstrip line characteristicimpedance that is used extensively in microwave filter design. Inorder to prove the efficiency of the method, a comparison ismade between the new DGS LPF and conventional filters, whichshows that the proposed filter with simple etched cells is enoughto obtain better performance by suppressing ripples and a verylarge stopband. Measured results are found to be in goodagreement with the simulation results.

I. INTRODUCTION

EMERGING applications such as wireless communicationschallenge microstrip filters with ever more stringentrequirements-higher performance, smaller size, lighter weight,and lower cost. Also, a wide stop-band, sharp cutoff response,and compact size are important factors in the design of thefilters. Recently, novel microstrip filters using defected groundstructures (DGS) are reported [1], [2], [3]. All of thesestructures have slots in ground plane which are perpendicularto wave propagation direction, disturbing current distributionin ground plane and have resonant behavior. In this paper wehave introduced another type of slot that is parallel to wavepropagation direction. This new structure, like a microstripline, is a planar transmission line that supports quasi-TEMmode. The advantage of this new structure over microstrip isit's higher characteristic impedance that can be used forimplementing quasi-lumped inductors. These elements areused extensively in low-pass microstrip filter design. Forexample we can use this new transmission line to implementinductors in the high-low (Hi-Lo) filter design method. Havinga higher characteristic impedance, this structure leads to betterapproximation of lumped inductors that yields better stopbandresponse for low-pass filters [4]. Smaller length that is neededfor implementation of a given inductor results in a less filterarea. As another choice, we can use this structure to increasemaximum power handling of filters without changing their

responses. Narrow lines, which are used to implementinductors, limit this factor. However, by using a wider strip atthe top we can solve this problem. In the next section we haveintroduced the new structure and provided it with a simpleanalysis. Although this method is applicable to all structuresthat have quasi-lumped inductors, for simplicity, in Section IIIwe have designed and simulated an 3-pole elliptic filter and itsconventional counterpart. Two designs have been comparedand the comparison shows that this new structure hassuppressed spurious response of this filter and has enlargedstop-band bandwidth. In addition to this improvement ofresponse, the area of the new filter is 3000 smaller than that ofits conventional counterpart. As we will see, the new structureis a quasi-TEM transmission line and we can use classic filterdesign formulas

wA kL

SW

Fig. 1. Proposed structure.

of [3]. The only parameters that are needed for design arecharacteristic impedance and propagation constant of the linethat can be obtained from a separate full-wave EM simulationof the new structure (as we have calculated them in proceedingsections).

II. ANALYSIS OF THE PROPOSED STRUCTURE

Fields in the microstrip extend within two media (air aboveand dielectric below) so that the structure is inhomogeneous.Due to this inhomogeneous nature, the microstrip does notsupport a pure TEM wave. When the longitudinal componentsof the fields for the dominant mode of a microstrip line remainvery much smaller than the transverse components, they maybe neglected. In this case, the dominant mode then behaveslike a TEM mode, and the TEM transmission line theory isapplicable for the microstrip line as well. In quasi-TEM

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approximation a homogeneous dielectric material with aneffective dielectric permittivity F, replaces the inhomogeneousdielectric-air media of microstrip. Transmission characteristicsof microstrips are described by two parameters, namely, theeffective dielectric constant 6r, and characteristic impedanceZC, which may then be obtained by quasi-static analysis [4]-[5]. In quasi-static analysis, the fundamental mode of wavepropagation in a microstrip is assumed to be pure TEM. Theabove two parameters of microstrips are then determined fromthe values of two capacitances as follows [4]:

Ca

Ze 1 (2)cCC

In which Cd is the capacitance per unit length in the presenceof dielectric substrate, Ca is the capacitance per unit lengthwith the dielectric substrate replaced by air, and c is thevelocity of electromagnetic waves in free space (c = 3.0 x

108m/ s ). As can be seen from (2) Zc can be increased bydecreasing two capacitances Ca and Cd. This work wasconventionally done by decreasing microstrip line width (W),which is limited by two problems. The first one is theminimum size that can be fabricated in a specialmanufacturing process and the second is the limit, which isimposed by power handling of structure.

impedance and effective dielectric permittivity, respectively.For decreasing the two capacitors (i.e. Ca and Cd) we mustdecrease electrical field. It is well known that electrical field inmicrostrip is mainly in the region bellow the strip. The mainidea behind this work is to decrease electrical field in thisregion. Using slot in ground plane, increasing the distancebetween ground and strip in slotted microstrip, electrical fieldbecomes weaker and the proposed structure shows smaller Cdand Ca and according to (2) a more characteristic impedance.We have calculated this characteristic impedance for GaAssubstrate with thickness of t = 0.813mm and "r = 3.38 usingSonnet, a full-wave EM simulator from Sonnet Software, Inc.,for different slot width (W) in term of slot width (SW). Theresponse is plotted in Fig. 2. As it was predictable,characteristic impedance increased when slot width becamewider. This increasing behavior is saturated to the value ofcharacteristic impedance of a strip without ground.

III. FILTER DESIGN USING NEW STRUCTURE

Although the proposed structure can be used in all circuits thatneed a high characteristic impedance line, in this section wewill design a simple low-pass filter using this structure todemonstrate its efficiency. A 3-pole, elliptic, (Hi-Lo) LPFwith 3dB cutoff frequency of 1 GHz has been chosen for thispurpose, which its lumped elements implementation is shownin Fig. 3(a). Response of this circuit for S11 and S21 has beencalculated using Advanced Design System (ADS), a circuit

PORT IND INDP= 1 ID= Li ID= L2Z= 50 Ohm L= 5.7 nH L= 5.64 nH

PORTP= 2Z= 50 Ohm

INDID= L3L= 2.3 nH

CAPID= ClC= 3.73 pF

(a)

4

Fig. 2. Characteristic impedance for different line width (W) in term of slotwidth (SW).

0°

-10

Proposed structure has been shown in Fig. 1. This structure isa microstrip line with a slot in its ground. Conventional slotsin microstrip structures are perpendicular to direction ofpropagation of wave in microstrip, disturbing currentdistribution in ground plane significantly and have resonantbehavior [2]. But the new structure's slot is parallel todirection of wave propagation and have no resonant response.In fact, the structure is a transmission line that doesn't supportpure TEM mode and the dominant mode of propagation isquasi-TEM. So we can use quasi-TEM analysis andconsequently (1) and (2) for evaluating characteristic

m -20

CO)

n)--30

-40

Si-50 )-

0

S21

~EM-sumulationI ----~~~~Equivalent circuit

1 2 3 4 5Frequency[GHz]

6U)

Fig. 3. (a) Equivalent circuit of 3-pole elliptic LPF. (b) Its S-parameters.

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simulator from Agilent Technology, and depicted in Fig. 3(b).For microwave frequencies, this filter may be implemented asFig. 4(a). In this implementation WI = 0.2mm and W, =

10.2mm have been used for to implement inductors andcapacitors respectively. The substrate for this circuit is GaAswith thickness of t = 0.813mm and Fr = 3.38. Characteristicimpedance and propagation constant of microstrip lines havebeen calculated using TX-Line (a software for computingplanar line parameters from Applied Wave Research, Inc.).The calculate the line dimensions, these two formulas havebeen used:

/3/ LR 0

Zh// =CZL

Ro

(3)

(4)

where Ro = 50Q is the filter Impedance and L and C arenormalized element values of the low-pass prototype [5]-[6].A filter using proposed structure is designed in the same way.The only difference is in calculating the characteristicimpedance and propagation constant, which in this case isaccomplished by full-wave EM simulation using Sonnet.

The two filters are illustrated in Fig. 4. Fig. 5 shows their S-parameter responses which are also obtained by Sonnet's full-wave EM simulation. From Fig. 5 It can be seen that usingproposed structure for inductors has suppressed spuriousresponse in stop-band. Also the second filter has 3000 less areain comparison with the first one.

0

-10 - ,

EF~~~~t-20 F

C/)

-t

-50

-600 1 2 3 4

Frequency[GHz]

-with slot-without slot

5 6

0

-10

50Q-line

Microstrip,-,capacitance

-20

cE' -30-

1 2 3 4Frequency[GHz]

Slot (DGS)

\- Ground plane

Fig. 4. (a) Conventional filter. (b) New DGS filter.

Fig. 5. S-parameter responses of conventional and new filter.

IV. EXPERIMENTAL RESULTS

In order to demonstrate the effectiveness of the proposed LPF,we compared the new elliptic defected ground structure LPFwith the conventional LPF without slot [7]-[8]. The two filtersare designed for the same cutoff frequency of 1.1GHz. Theywere simulated and fabricated on a substrate with a dielectricconstant of 3.38 and a thickness of 0.813mm. For the DGSlowpass filter, the maximum passband ripple was 1.3dB. Forthe proposed structure, a wide stopband with more than 15dBattenuation was obtained, up to 5 times of its cutoff frequency(nearly 5.5GHz) whereas with the Conventional filter [9], thestopband width with 15dB attenuation reaches only 2Ghz.The new DGS-lowpass filter was fabricated and its measuredresults are shown in Fig. 6and Fig. 7. The simulation resultsagree well with the measurement results.

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-40

-50 )0

-with slot-without slot

5 6

(a)

(b) 0 1 2 3

Fig. 6. Photography of the proposed LPF. (a) Top view. (b) Bottom view.

0 F---ML= -w

-10

O -20 --o h

06-30

-40

l 11

-500

MeasurementEM-simulationEquivalent circuit

designed and simulated. To verify the Performance, the newDGS filter has been fabricated, simulated and measured. Thecomparison between these two filters showed the advantagesof new DGS filter, which are a high rejection in the stopbandand a very low insertion loss in the passband. High harmonicsuppressions would make the introduced DGS meet therequirements ofmodem wireless communication systems.

ACKNOWLEDGEMENT

The author is grateful to Nlr. M. Mirshafiei and Nlr.Dempewolf for suggesting several improvements in thispaper's manuscript.

REFERENCES

[1] D. Ahn, J. S. Park, C. S. Kim, Y. Qian, and T. Itoh, "A design of thelow-pass filter using the novel microstrip defected ground structure,"IEEE Trans. Microwave Theory Tech., vol. 49, no. 1, pp. 86-93, Jan.2001.

[2] AbdelRahman, A. K. Verma, A. Boutejdar, and A. S. Omar, "Controlof bandstop response of hi-lo microstrip low-pass filter using slot inground plane," IEEE Trans Microwave Theory Tech., vol. 53, no. 3,pp.1008-1013, Mar. 2004.

[3] D. M. Pozar, Microwave Engineering, 3rd ed. New York:John Wiley & Sons, Inc., 2005.

[4] J. S. Hong and M. J. Lancaster, Microstrip Filters for RF/MicrowaveApplications. New York: John Wiley & Sons, Inc., 2001.

[5] A. Boutejdar, A.Ramadan, M. Makkey and A. S. Omar, "Design ofCompact Microstrip Lowpass filter Based on Novel U-Resonator UsingDefected Ground Structure and Compensated Microstrip Line" Proc.36th EuropeanMicrowave Conference 2006(EuMC), Manchester, UK,2006.

[6] Ahmed. Boutejdar, G. Nadim, S. Amari and A.S. Omar, ,,Control ofbandstop response of cascaded microstrip lowpass- bandstop filtersusing arrowhead slots inbackside metallic ground plane," IEEE AP-SSymp (Washington DC), 2005.

[7] J.-S. Lim, C.-S. Kim, Y.-T. Lee, D. Ahn and Nam, "Design of lowpassfilters using defected ground structure and compensated microstrip line,"Electronics Letters, vol. 38, no. 22, pp. 1357-1358, October 2002.

[8] Q. Xue, K. M. Shum and C. H. Chan, "Novel 1-D microstrip PBG cells,"IEEE microwave andWireless Component Letters, vol. 10, no. 10, pp.403-405, October 2000.

[9] Ahmed Boutejdar, Adel Elsherbini and A. S. Omar , "A CompactMicrostrip Multi-Layer Lowpass Filter Using Triangle Slots Etched inthe Ground Plane" Proc. 36th European Microwave Conference2006(EuMC), Manchester, UK, September 2006.

1 2 3 4 5 6Frequency[GHz] x 10

Fig. 7. Measured and simulated S-parameters of elliptic DGS LPF.

V. CONCLUSION

In this paper we proposed a simple method for increasingmicrostrip line characteristic impedance using only onerectangular DGS in metallic ground plane. In Section II asimple analysis of new structure as a transmission line wasprovided. In Section III, using proposed structure, a simple 3-pole elliptic LPF and its conventional counterpart were

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