A New miniaturized Planar Electromagnetic Bandgap (EBG) Structure with Dual Slits

S. Mohamad Dawood Farzan *, Amarnath Sankar **C-DOT Alcatel Lucent Research Centre

Chennai, TN, India{mohamad.d.farzan *, amarnath.sankar **}@carc.co.in

Abstract enhances the inductance of the branches that connect theThis paper proposes a new planar EBG structure with dual patches.

slits included into the large metal patches and small The EBG structures are analogous to band stop filtersconnecting branches that connect adjacent EBG cells in the constructed with the lumped circuit elements. The centerpower/ground (P/G) plane pair. The paper then lunges into the frequency of the bandgap in plane pair constructed withstudy of a tuning method, to achieve isolation at the frequency Electromagnetic bandgap structure etched on either of theof interest by scaling the extents of either of the slits in the P/G planes is controlled by the inductance of the branches (Lproposed Dual Slits EBG (DS-EBG) structure without scaling and the gap capacitance between the edges of the twothe entire geometry. Further this paper investigates the neighboring lattices (Cg), is given by,bandwidth optimization method without affecting the center F - 1 1frequency of interest and the size of the metal patches. This 2ff L )paper supports the proposal with simulation results and 2 b gmeasurements made on the proposed structure with unit cell The unit cells ofAI-EBG structure and EBG structure withsize of 5.4 mm x 5.4 mm, the center frequency was tuned from long slits included into the metal patches is shown in the Fig.1.7 to over 6 GHz. The bandgap was widened to 2 GHz 1. The cutoff frequency of planar EBG structure is controlledagainst the original bandgap of 1 GHz without changing the by the inductance of the branches and the capacitance of thesize of the DS-EBG structure for a given center frequency of patch and an attempt to miniaturize the structure by scaling2.4 GHz, by using the bandwidth optimization method the structure of the unit cells and shift the center frequency ofproposed. the resulting structure to the higher frequencies as the patch

capacitance is decreased. The stop bands are moved towardsthe left of the spectrum by either increasing the inductance of

Introduction the branch or the capacitance of the patch. Increasing theThe recent electronic packages hold very high frequency capacitance of the patches for a given permittivity, demands

digital and analog chips. The wide band electromagnetic noise increase in size of the patch. The common method tooriginating from the switching digital circuits are the major miniaturize the EBG structure is to use High-K dielectricsources of disturbance in the P/G plane pairs of such densely material, by increasing the capacitance of the metal patchpacked electronic system. Many literatures have been [5][6].discussing the origination of simultaneous switching noise The miniaturization technique proposed by the inventors(SSN) or ground bounce noise (GBN) and suppression/ of the AI-EBG structure increased the branch inductance byisolation methods including terminating boards at the edges, introducing two narrow slits in to the patches [5]. The otherplacing multiple discrete decoupling capacitors, embedded method to miniaturize the EBG structure interposed highcapacitors and splitting the planes. At a frequency greater than permeability magnetic metal sheet between the parallel planes1 GHz embedded capacitances were found to be ineffective in [7]. These methods increased the inductance of the connectingsuppressing the resonant peaks in the impedance profile of the branch by enhancing the inductance of the connecting branchplane pairs. Splitting either the power or ground planes are by either including slits or by interposing high permeabilityeffective in isolating the region of interest from the propagated magnetic metal sheets between the planes and achievedEM noise till a frequency of 1 - 1.5 GHz, but only narrow miniaturization as they moved the stop bands for a given unitband isolation is achieved at higher frequencies. High cell dimensions by several GHz.impedance surfaces (HIS) proposed in [1] to mitigate thepropagation of surface waves were later modified in [2] to fit Branchin to the plane pairs in the PCBs to mitigate switching noise atfrequencies >1GHz and were named Embedded EBG (EEBG)structures. The planar EBG structures introduced in [3][4]were compact enough than the EEBG structures as they were Sieasily integrated into thin boards to suppress EM noise in theGHz range, but the unit cells were at least 15 mm wide for astop band with a lower corner or on-set frequency of 2 GHz. (a) (b)The structure proposed in this paper is derived from theAlternating Impedance EBG (AI-EBG) structure with long Figure 1: Unit cells of (a) AT-EBG structure and (b) AT-EBGslits added to the either sides of the square patches that structure with narrow slits in the metal patch.enhances the branch inductances. In the proposed structure, In the structure proposed in this paper, a second level ofone more level of slit is added to the metal patch which slit in the metal patch is introduced and the inductance of the

978-1-4244-2318-7/08/$25.00 ©2008 IEEE SPI 2008

connecting branch is increased. The center frequency of thebandgap for a given dimension was moved to much lowerfrequency by adding two levels of slit into the metal patch,which is presented in the following sections of this paper.

Design and Analysis of EBG structure with Dual SlitsThe unit cell of the new miniaturized EBG structure with

dual slits is shown in the Fig. 2. The size of the unit cell of theDS-EBG structure measured 5.4 mm x 5.4 mm. The unit cellswere simulated to extract the transmission coefficients withports placed as shown in the Fig. 2. MoM based fullwave field (a) (b)solver, Sonnet em lite was used for the simulations presented 0in this paper. Each grid shown in the Fig. 2 measured 0.2 mm -10x 0.2 mm size and the thickness of the dielectric layer with -20dielectric constant of 4.5 and loss tangent of 0.02 measured -300.4 mm. The Fig. 2 shows the structure with slits of maximum ,40length. S parameters were simulated for the unit cells by -50varying the length of both the slits and the detailed simulation E -60 Nom-Slitresults are presented in the next section of the paper. -70 Max-Slit

-80

-90

w w w w w w w w w/~~~~~~~~~~~~~~~~~ X- N Cs N 10 CD CD N1Port I Port 2 Frequency(Hz)

(C)Figure 3: Test boards constructed by cascading 5x5 DS-EBGstructures with (a) maximum slit length and (b) nominal slitlength; (c) Transmission coefficients (S21) measured between theports in the test boards shown in (a) and (b).

It was clearly seen from the measurement that an attemptto decrease the center frequency by increasing the slit length

(a) resulted in poor bandwidth. A detailed analysis on theFigure 2: (a) Unit cell of the new ERG structure with Dual Slits bandwidth Vs center frequency trade off in tuning the slit(DS-EBG). length of the DS-EBG structure was carried out by simulating

Two layered test boards with top metal layer patterned the unit cells and 1-D cascaded cells. The results are presentedwith 5 x 5 unit cells of the new EBG structure with dual slits in the next sections of this paperof (a) maximum slit length and (b) nominal slit length were Tuning the center Frequencyfabricated. The dimension of the EBG cells in the test board A detailed analysis was done on the unit cells of the DS-was similar to the one shown in the Fig. 2 but for the width of EBG structure shown in Fig. 4, in tuning the center frequencythe slits and the metal branches, which measured 0.25 mm. of the bandgap. The dimensions of the structure were similarThe metal layers were made of 35 micron copper and the to the geometry shown in Fig. 2. The center frequency of thedielectric layer was made using standard FR4 with a thickness unit cells can be tuned by changing the length of either innerof 0.4 mm. The dimension of the boards was 26.5 mm x 26.5 or outer slits. The Fig. 4 shows DS-EBG structure withmm x 0.4 mm. The S21 measured from the test boards portion of the slits shaded in light gray. The shaded regionbetween the ports 1 and 2 connected as shown in the Fig. 3(a) shows the extent to which the slits length was varied for theand 3(b) are plotted in Fig. 3(c). The EBG structure with analysis. The number inside the cells of the lightly shadedmaximum slit length had a -30 dB bandgap of 2 GHz with region shows the cell in the slit that was metallized or de-peak isolation at 3 GHz. While the board constructed with metallized, in order. The metallization levels in the inner slitstructures having nominal slit length had a -30 dB bandgap of was limited to 7 as shown in the Fig. 4, as the properties of the2.4 GHz with peak isolation near 4.5 GHz. When the slits of structure were approaching that of the AJ-EBG structure thenminimal length were formed on the patches, the bandgap of after.the structure was very close to that of the AJ-EBG structure. Analysis was carried out by simulating the transmissionMeasurements were not carried out on the AJ-EBG structure coefficients between the ports 1 and 2, starting with no metaland DS-EBG with minimal slit length of similar size due to deposited in both inner and outer slits. In other words the firstthe frequency limits of the Network Analyzer used. set of the simulation was carried out on the structure, with

whole light gray shaded region de-metallized, which resultedin a bandgap with the lowest center frequency of 1.7 GHz. Inthe first set of, several iterations of simulations were carriedout by metallizing each levels in the inner slit. Thus, with

maximum slit length in the outer slit the center frequency was 6tuned between the ranges, 1.7 GHz to 2.2 GHz by scaling the 5.5 0"InnerSlitlevelOinner slit alone. N 5 linnerSlitieveli

SThen next sets of simulations were done after metallizing o4.5 - InnerSlitlevel2

outer slit by one more step. In the final run, center frequency 4j3.5 InnerSlitlevel3of the structure with minimum slit length in the outer slit was ' 3 InnerSlitlevel5tuned from 2.3 to 5.7 GHz by scaling inner slit alone. The Fig. 2 2.54(b) shows the transmission co-efficient extracted from the et2simulation done on the unit DS-EBG cell shown in Fig. 4(a) 1.5 =with outer slit completely de-metallized and inner slit 2metallized from levels 1 through 7. The tunability of the DS- 0 1 2 3 4 5

EBG structure can be seen from the Fig. 4(c) that shows the Outer Slit Metallization Levelscenter frequency tuned by metallizating inner and outer slits. (d)The bandwidth of the structure at -10 dB isolation level, with Figure 4: (a) Unit cell of DS-EBG structure showing the cells inrespect to the change in the slit length is shown in Fig. 4(d). the slits that are metallized / de-metallized to study the tunability

of the DS-EBG structure. (b) S21 extracted from the simulationresults of the DS-EBG cell with outer slit being completely de-metallized. (c) Center frequency of the structures with slits beingmetallized. (d) -10 dB bandwidth of the structures with slits beingmetallized.

It was noted that the center frequency was greatlyl l l 01 dependent on the metal area of the unit cells. The removal of

metal to deepen the outer slit and deposition of same area ofmetal into the inner slit resulted in no or little change in thecenter frequency. But the bandwidth and the magnitude of theisolation at the center frequency were greatly changed. Thisparticular property is studied In detail in the next section ofthis paper.

Slit length Vs bandwidth optimization for a given center(a) frequency

0 It was seen from the previous results that the center|1/ / X i frequency oftwo structures were almost same ifthe slits ofthe

-10 / N / structures were metallized to same levels irrespective of inner|\\\// /5 \\ it 1 or outer slit it was. A model shown in Fig. 5(a) was analyzed

to study this property in detail. The model was constructed byV20 llii / / V cascading two DS-EBG cells shown in the Fig. 2. The regions.E lliI \ I v in the slit shown in light gray were metallized or de-metallized-30 in the iterations of the simulation. For example in the first run

lInnerSlit-Level I all the cells shown in light gray in the inner slit were de--40 -InnerSlit-Level3 metallized and the outer slit was completely metallized. In the

lnnerSlit-Level 7 next run, cells numbered in the order were removed from the-50 outer slit and were deposited in to the inner slit. This way

-R 6 g,iO.N,,. inner slit was metallized and outer slit was de-metallized byFrequency (GHz) one level. With every count in iteration, it was noted that the

(b) bandwidth and the magnitude of the isolation were enhancedwithout much change in the center frequency and the size of

6 1 ~~~~~~~~~~~~thestructures.5.5 * Inner Slit level 0t

N 5 l Inner Slit level I From the Fig. 5(b) it was noted that the center frequency4.5 Inner Slit level 2 did not change much throughout the simulation. The -20 dB4 InnerSlitlevel3 and -30 dB bandwidth of the models are plotted in Fig. 5(b),

Q 3.5 Inner Slit level 4 _,pf Ashowing a steady increase in the bandwidth and the magnitudep 2.5Inner Slit level of the peak isolation. Better bandwidth and peak isolation2.5LI_ 2 were observed in the structure with inner slit of minimum

1.5 l length and the outer slit ofmaximum length. While for a samecenter frequency a poor performance was observed in the

0 1 2 3 4 5 structures with slits of minimum length in the outer level andOuter Slit Metallization Levels maximum length in the inner level.

(c)

[3] Jinwoo Choi, Vinu Govind, Madhavan Swaminathan, LixiWan, and Ravi Doraiswami, "Isolation in Mixed-SignalSystems Using a Novel Electromagnetic Bandgap (EBG)Structure", proceedings of Topical Meeting on ElectricalPerformance of Electronic Packaging, 2004

[4] Tzong-Lin Wu, Yen-Hui Lin, and Sin-Ting Chen, "ANovel Power Planes With Low Radiation and Broadband

§l 0ll Suppression of Ground Bounce Noise Using PhotonicBandgap Structures", Microwave Wireless Comp. Lett.,vol. 14, pp. 337-339, July 2004

[5] Yoshitaka Toyota, A. Ege Engin, Tae Hong Kim,(a) Madhavan Swaminathan and Swapan Bhattacharya, "Size

Reduction of Electromagnetic Bandgap (EBG) Structures2.5 0 with New Geometries and Materials", proceedings of2.25 -10 Electronic Components and Technology Conference, 200625 -20 [6] Baharak Mohajer-Iravani and Omar M. Ramahi,I 1.75 ~~~~~~~~~~~-30°D ^ ~~ CenterFrq"upeso

Q 1.25 C].t, F t40q_-20dBBandwidth "Suppression of EMI and Electromagnetic Noise in1.25- -4 -"" -2OdB Bandwidth

'A 1 j > >t -50 Packages Using Embedded Capacitance and MiniaturizedI>* u)---A-~~~~~~~~tj -30dB Bandwidth2 0.75 l X t -60 a X aP..kisOIation Electromagnetic andgap Structures With High-k0°-5 70 Dielectrics", IEEE Trans. Adv. Packag., vol. 30, pp-776-0 -90 788.

0 1 2 3 4 5 6 7 8 9 10 [7] Yoshitaka Toyota, Kengo lokibe, Ryuji Koga, Arif EgeMetallization / De-Metallization Levels Engin, Tae Hong Kim and Madhavan Swaminathan,

(b) "Miniaturization of Electromagnetic Bandgap (EBG)Structures with High-permeability Magnetic Metal Sheet",

Figure 5: (a) Model constructed by cascading two cells of DS- pruceswIth symposity 2007.EBG structure to study slit length Vs bandwidth (b) Plot showing proceedings of IEEE EMC symposium, 2007.the variations in the bandwidth and the magnitude of peakisolation with every step in metallization and de-metallization inthe inner and outer slits respectively.

ConclusionsThis paper presented a new miniaturized Electromagnetic

bandgap (EBG) structure with dual slits included into themetal patches of the EBG structure with large metal patchesand small connecting branches that connect adjacent metalpatches in the Power/Ground plane pair. Test boards werefabricated and the design was verified by measuring thetransmission coefficients. The center frequency of thebandgap was tuned by scaling the extents of the slits of theDS-EBG structure. An attempt to tune the structure above thelowest center frequency for a given geometry by scaling theinner slits alone to the maximum extent resulted in poorbandwidth. While a bandgap with same center frequency andbetter bandwidth was achieved by scaling both slits optimally.Unit cells were analyzed in detail to enhance the bandwidthfor the given center frequency and size by tuning both theslits.

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