A New Transformation of Bandpass Filter to BandstopFilter Using Multilayer-Technique and U-Defected Ground

Structure DGSAhmed BoLJTEJDAR, Anatoliy BATMANOV', Jan MACHAci, EdmundBuRTE', Abass OMAR], IEEE, Fellow

'Chair of Microwave and Communication Engineering University of Magdeburg, Germany2Faculty of Electrical Engineering, Czech Technical University, Prague, Czech Republic

Ahmed.Boutejdargovgu.de

Abstract. A novel compact microstrip bandpass/Bandstop filters, especially narrow-band bandpass filters which play anfilters realized by combining new Multilayer Method with important role modem communication and electronic systemselectrically coupled microstrip U-shaped DGS resonators, is [1]-[2]. In order to obtain compact three-pole microstripproposed The proposed filters have low insertion loss and bandpass filters with two transmission zeros, low rejectionare compact in size due to the slow-wave effect. They also band performance, low insertion loss in passband and highhave sharp transition regions due to the presence of two- selectivity, several structures were proposed such as the filterstransmission zeros on both sides of their passband. The using end-coupled slow-wave resonators, slow-wave open-measured center frequency, bandwidth and insertion loss are loop resonator filters and slow-wave open stub-tapped3.4 GHz, 4O0'o, and 0.5 dB respectively. The simulated and resonator filters. Another techniques involve employing crossmeasured results show good agreement and validate the coupling and Quasi-elliptic function filters which are able toproposed approach. place the transmission zeros near the cutoff frequencies so

that higher selectivity with less resonators can be obtained.

Keywords U-Resonator50Q.-Microstrip line

DGS, Microstrip slots, Suspended layer, BPF, BSF. Substrate<,, ~~~~~~~Substrate1. Introduction IUha G

Metallic GroundRecently, defected ground structures (DGS) and plane

electromagnetic band gap (EBG) structures have receivedincreased attention because of their use in mobilecommunication systems which require compact high Fig. 1. Three-dimensional view of the U-Head-DGS cell.performance filters and couplers. One of the very successful In order to improve the selectivity and efficiency of spectrumapproaches to achieve siginifcant size reduction is to use DGS . .components which also has the capability of suppressing utilisatn, erswit two or mrestorasmission zerosthavharmonics. DGS elements can be used in various kinds of bed aed . Usn wo 4resnto with a stub-components such as lowpass filters and bandpass filters as trappe inveerr [3 lwa proposedstopachie ahsinglewell as RF phase shifters. DGS, which evolved from EBG are tansmssonzero at lo origh to in i ch trealized by etching certain patter in the metallic ground attached open-ended stub iS attributed to zeroi mpedance atplane of a microstrip line which perturbs the curent thseparated X/2lineresonator). Another two transmission zerosdistribution causing an increases in the effective inductance s . . A t tand capacitance of the line. Thus, a DGS elements is in [4], were realized using open stub lines and DGS-circles. In

equivalent to an LC circuit. Planar bandpass filters [1] have [3], a slow-wave resonator filter using two coupled hairpinbeen extensively studied and exploited as key circuit blocks microstrip folded Lines was presented. The bandpass filter,wit oprtn fucin of inbn trnmiso an out-of-1 designed at fundamental resonant frequency of the resonator,

ban reecio. To mee th reureet in moe .irls show low insertion loss in the passband.In this paper, a new type of compact microstrip bandpasscommunication, much effort has been made in the past years. '

to develop a variety of compact bandpass filter with sharp and fitr,wt'lwwv fetraie sn -Gdee reeto ousd the pasbn by genrain resonators and Multilayer Method has been developed.

tranmisionzersorattnuaionpols. Rcen adanc in Transmission zeros can be implemented on both sides of thehigh-empeaturesupecondcting (HTS circits nd passband. The proposed filter iS fabricated and measured. The

.. . , . .,, ~~measurement results were in good agreement of themicrowave monolithic integratedl circuits (MMIC) has siuaonrul.additionally stimulated the development Of various planar

978-1-4244-2138-1/08/$25.OO ©)2008 IEEE

2. Characteristics of the DGS 3. Design of the proposed Compact BPF

The proposed U-DGS in Fig. 1-2 is etched in the ground In order to improve the performance of the previous bandpassplane and consists of two capacitive arms, which are filter shown in Fig. 3, a multilayer structure is used. The newconnected to a rectangular slot, with the same dimension as structure is similar to the original filter but the central DGSshown in Fig. 2. The etched slot (DGS) is equivalent to a resonator is moved to the bottom layer as shown in Fig. 3.capacitance. The metal area between the two arms on the top This was found to improve the performance and reduce thelayer corresponds to an inductance. The conventional circuit overall size of the filter. The new bandpass filter has the sameparameters can be extracted from an electromagnetic bandwith (400 o) and the same center frequency (fO:3.4GHz)simulation by matching to a one pole Butterworth band stop but with improved passband characteristics. In order to obtainfilter response, as in [5]. The U-Slot in the ground plane the coupling matrix of the new topology, the specifications ofexcited by the 50Q line acts as a parallel resonant circuit [5]. the filter are defined and then the desired parameters areIt can be modeled by a parallel LC circuit as shown in Fig. 2. extracted by using an optimization-based scheme [6-7]. TheThe values ofL and C can be computed using: coupling coefficient and quality factor curves are then used

25 to realize the obtained coupling coefficients. In our case theC 2 pF & C= nH (1) third order filter is required to designed to have a bandwidth

f ( )JCp BW = 1300 MHz, return loss RL = 20 dB, and centerfrequencyfo = 3.4 GHz. The obtained coupling matrix from

The values of the cut-off frequency f, and resonance the optimization scheme isfrequency fp can be found from the transmissioncharacteristics of the U-slot. The simulation results of the U- M 0 0.721 (2)slot show one-pole low-pass filter characteristics. It is clear L0.721 0that employing the slot in the metallic ground plane increasesthe effective permittivity, leading to an increase of the and the external quality factors are qin= qout= 1.24. To realizeeffective inductance of the microstrip line. From the Figs. 2. the normalized coupling matrix and quality factors, we usewe can see the dependence of the resonance frequency (cutoff the required fractional bandwidth FBW = BW/fo. The actualfrequency) on the dimensions of the U-shape on the top and (denormalized) coupling matrix becomes and Ql = Q2 = 3.1bottom layers. The etched arm has a significant effect on the where m = FBW x M , and Q = q/ FBW. The m-couplingresonance frequency. Actually, it is well known that an coefficients will be inserted in the The unknown distance s isattenuation pole can be generated by a combination of 2mm. The proposed DGS-Bandpass filter wasinductive and capacitive elements. This explains thefrequency characteristic of the proposed U-DGS-element. Fig. 0mF 0.288 (3)2 shows that if b is kept constant while varying d, it is easy to o.288 ocontrol the positions of the (cutoff) and attenuation poles.This means that the length of gap (arm) controls the effectiveseries inductance of a microstrip. The microstrip 50Q-Microstrip line

12 U-Resonator

1010 < \3-jX ~~~~~c trate

0

~~ 4 i~~~ Metallicb > '' - - < X F Ground

~~~~ ~~~~~planeo2 U- DGS

0 1 2 3 4 5 6 Fig. 3. Three-dimensional view of the new U-Bandpass filter.0 1 2 3 4 5 6

Lenght (d) of the arm in mm simulated on a Rogers R04003 substrate experimental curveFig.2. ComparisonofresonancefrequencyofDGS(-) and Mircostrip [6] in order to get the optimal distance between the DGS

resonator ). ~~~~~~~~~~resonators.with relative dielectric constant £r of 3.38 and aline on the top of the substrate in Fig. 2 has a width of thickness h of 0.813mm. Simulation is performed usingw=1.9mm to obtain a 50Q2 characteristic impedance of the Microwave Office and CST Microwave Studio.M. All themicrostrip line. The substrate dielectric constant is 3.38 and dimensions ofthe U-slot are g=lmm,b=3mm and d=ll mm.its height is h=0.813mm. The dimensions shown in Fig. 3 are The simulation results ofthe bandpass filter are shown in Fig.g =lmm, b =3mm and d =1 1mm. The DGS cell is simulated 5. Although the filter consists of three resonators, only twousing Microwave Office. Simulation results are depicted in poles exist in the passband. This is because one of theFig. 2, which shows the characteristics of a one-pole LPF. resonators is in the bottom layer.

4 Design and Fabrication of the BPF 50Q-Microstrip Ue aline _ U-Resonator

The simulation results showed that the designed filter has agood sharpness factor, symmetrical response and smallerlosses in the pass band as shown in Fig. 5. In order to verify Metallicthe simulation results, the filter was fabricated and measured Groundusing an HP8722D network analyzer. The measurement planeresults are shown in Fig. 5. together with the simulationresults. A very good agreement between the simulated andmeasured results is observed. In the passband, the measuredinsertion and return loss were less than 0.7dB and 20dB, Fig. 6. Three-dimensional view of the cascaded U-bandstop filter.respectively. The results shows significantly improved performance because the losses is not negligible in pass bandperformance over the filters previously presented in [5-6]. and the stop band is not large enough. In order to improveThe BPF was simulated and fabricated on a substrate with a these characteristics and in the same time to reduce the size ofrelative dielectric constant £r of 3.38 and a thickness h of the filter, we have used another reduction method, which will0.813mm. Simulation is performed using CST Microwave be shown in later chapter.Studio and Mocrowave Office. Fig. 4 shows photographs ofthe fabricated BPF filter. The total area is 20 x 15Mmm2.

-10

~~~~~~-15

-2

-06-35

pg1 3 i ! 1X1 * l | 5i1 E _-40Si11

Fig. 4. Photograph of the fabricated U-DGS BPF. s21P 0 2 4 6 8 10

Frequency[GHz]

-10 --- Fig. 7. Simulated S-parameters ofthe proposed cascaded BSF.

-20 -L

m-30 6. Design of the Multi-Layer BSFuo -3011

-40 In order to improve the performance of the Previouslystructure and further reduce its size, we moved one of the

-50 - microstrip resonators to the upper layer, thus the new compact- Measurement PSF will be consist of two connected microstrip U-resonators~EM-Simulation-601 2 3 4 5 6 7 8 9 on the Top layer and one DGS-U-slot on the ground plane.

Frequency[GHz] This proposed geometrical idea is based on the use of severalFig. 5. Measured and Simulated S-parameters of the proposed BPF. layers on top of each other. The new structure is similar to

three cascaded structure but the central resonator is moved to

5. Design of the Cascaded BSF theBottom layer. The simulation results of the resultingcom-50Q-Microstrip line

The Fig. 6. shows that the transformation of band-pass filter /Rotband stop filter is feasible by connecting the three Microstrip U-ResonatorU-elements together. The 3 U-resonators are similar and itpresents a 3 pole cascaded band stop filter. The Fig. 6. showsthe cascaded structure, which is designed on a R04003S.srtsubstrate with a relative dielectric constant £r of 3.38 and a 111\ _thickness h of 0.8 13mm. in this case, the ground plane is full- /copper with a Thick of 35p~m and it is separated from top /_layer through the substrate. Fig. 7. shows the simulated ; __ Metallicparameters ofthe cascaded band stop filter. The filters'answer G7 Iroundshows that the structure has an insertion loss of 2 dB from DC ~_ planeto 4.3GHz and the return loss is - 6dB over the both pass- U- DGS \

bands The esigof te casadedstrucure desn' givegood Fig. 8. Three-dimensional view of the proposed U-DGS-bandstop filter.

-pact structure are much better, compared to the conventional the etched arm is equivalent to a series inductance. So, theBSF structure. As shown in Fig. 9. and Fig. 8. We designed DGS unit is equivalent to a resonant circuit, which is shownand simulated this filter using AWR. This new modification in Fig. 2. the etched arm length. The slot head dimensionsof the structure does not have a significant influence on the were kept constant and the length of the arms was varied. Theperformance of the filter as compared to the previous ones. simulation results are shown in Fig. 10. and Fig. 11. theWe can use the top and Bottom layers without sacrificing the attenuation pole location moves up to a higher frequency,good response of the filter. The cutoff and resonance while the length decreases.frequencies did not change from their previous positions, butthe advantage is that, the new BSF is 35% more compact thanthe conventional [3], as Fig. 8 shows. 10 /

0~~~~~~~~~~~~~~~~~~~~~~~1

C~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~"

-10X d=1om;

-40 -

Ci) -30 &m

-06

-50 811~~~~~~~~~~~~~~~~-5 ~=m

tj~ ~ ~ 2 Frequency[GHz]

-60 2 4 6 8 l 0 Fig. 11. Simulated 21-parameters for different length, d.Frequency[GHz]

Fig. 9. Simulated results of the new compact BSF.8. Conclusion

7. Tuning of The Filter Characteristics A new U-bandpass filter with multiple transmission zeros forsharp transition band has been presented in this paper. In

In order to investigate the frequency characteristics of the order to realise a compact, symmetrical structure and toetched slot, we simulated the DGS unit section using simplify the implementation, DGS-method and couplingMicrowave Office. The variation of the dimensions of the method were used. The measured insertion loss and returnetched gap shifts the cutoff frequency and the attenuation pole loss are less than 0.7dB and 20dB in the passband of 1.3GHzlocation in the frequency domain. As is well known, a at center frequency 3.2GHz, respectively. The proposed Filterresonance frequency can be generated by a combination of was designed, simulated, fabricated and measured. Goodinductive and capacitive elements [3]. The etched gap area, agreement between simulated and measured results has beenwhich is placed under the microstrip line, corresponds to achieved.capacitance and the metal area, which is between the botharms is equivalent to a series inductance. So, the DGS unit is Referencesequivalent to a resonant circuit, which is shown in Fig. 2. Theparameters of this DGS equivalent circuit have been found [1] Awida, Mohamed; Boutejdar, Ahmed; Safwat, Amr; El- Hennawy,using curve-fitting. The found results were: C =0.33pF and L Hadia; Omar, Abbas Multi-bandpass filters using multi-armed open2.33nH. Next we investigated the effect of 0.33pF of loop resonators with direct feed In: 2007 IEEE MTT-S International

Microwave Symposium , Honolulu, Hawaii, June 03 - 08, 2007. -

o <zrX7[Piscataway, NY]: IEEE Operations Center, S. 913-916[2] A. Abdel -Rahman, A. K. Verma, A. Boutejdar and A. S. Omar,

,,Compact stub type microstrip bandpass filter using defected ground-10-Y -plane," IEEE Microwave and Wireless Components Letters, vol. 14,

-1 5 --{ -\ ---pp. 136-138, 2004. (MTT- Journal).[3] Boutejdar, Achmed; Elsherbini, A.; Omar, Abbas Sayed

-20 -t --"Improvement of bassband and sharpness factor of parallel coupled-25 -X ;1- microstrip bandpass filters using discontinuities correction"

Mediterranea Microwave Symposium 2007. Budapest, pp. 121-124-3o0 i XX_ l[4] S. Amari, "Synthesis of Cross-Coupled Resonator Filters Using an

-35 _ t / _ l~~~~~~~~~~~Analytical Gradiant-Based Optimization Technique", IEEE Trans.a / \1 11 -~~~~~~~~~~~~Microwave Theory Tech., vol. 48, No. 9, pp. 1559-1564, Sep. 2000.

-40 X _ |-~~~~~~~~~~~[5] C. Kim, J. S. Park, A. Dal, and J. Kim, "A novel 1-D periodic-45 ; :~~~~~~~~~~~~~~defected ground structure for planar circuits," IEEE Microwave

^ { ~~~~~~~~~~~~~~~~GuidedWave Lett., vol. 10, pp. 131-133, Apr. 2000.-50( ) 1 23 4 s 6 7 t ~~~~~~[6]J. S. Hong and M. J. Lancaster, Microstrip Filters for RF/Microwave

Frequency[GHz] Applications. New York: Wiley, 2001.[7] D. AHN, J. S. PARK, C. S. KIM, Y. QIAN, AND T. ITOH, "A design of

Fig. 10. Simulated 511-parameters for different length, d. the low-pass filter using the novel microstrip defected groundstructure," IEEE Trans. Microwave Theory Tech., vol. 49, no. 1, pp.86-93, Jan. 2001.