Proceedings of 2010 IEEE Asia-Pacific Conference on Applied Electromagnetics (APACE 2010)
Compact Two Pole Bandpass Filter Using Symmetrical Composite Right/Left Handed Transmission Line with Vias Viveka Nand Mishra1, Raghvendra Kumar Chaudhary2, Kumar Vaibhav Srivastava3, Animesh Biswas4 Department of Electrical Engineering Indian Institute of Technology Kanpur, INDIA. 1
[email protected],
[email protected],
[email protected],
[email protected].
Abstract- A compact two-pole bandpass filter based on the composite right/left-handed (CRLH) metamaterial transmission line structure is proposed in this paper. In detail, subwavelength resonators are realized through the zeroth order resonance (ZOR), and inverter structures are proposed to control the coupling between neighbouring ZOR resonators. The proposed technique is validated by the EM predictions, the proof of metamaterial properties with the ZOR field distributions and extracted constitutive parameters, and measurements. Size independent resonance property of the ZOR combined with homogeneity condition of the LH TL unit cell is utilized in the size reduction of bandpass filter. It is found that the suggested method enables the remarkable size reduction from the conventional filters such as the parallel coupled type which is designed on the basis of the half-wavelength resonance. The proposed technique is also validated by the experimental results. Keywords - Bandpass Filter (BPF), Composite right-/left-handed (CRLH) transmission lines (TLs), Left-handed transmission line (LH TL) Metamaterial, Zeroth Order Resonators (ZORs).
I. INTRODUCTION RF bandpass filters play an important role in mobile and satellite communication. Technology nowadays is more directed towards miniaturisation and energy efficient systems [1, 2]. Techniques have been reported to provide for passive component size reduction, especially, the bandpass filter cases which are briefly mentioned now. Left-handed (LH) materials realised using transmission line (TL) approach [3] generated great attention from the scientific and engineering community working in RF and microwave field. The planar implementation of LH TLs consists of series capacitor and shunt inductor with unavoidable parasitic series inductor and shunt capacitor forming a composite right/left handed (CRLH) structure [4]. Various novel LH devices have been developed based on this structure, because of its easier integration into printed circuits, advantages of size reduction, lower loss and wide bandwidth [4].
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In this paper, a detailed design procedure with motivation from previous work [5-8] is presented to synthesize compact bandpass filter that is based on the zeroth order resonance of LH TL. The frequency of the ZOR is independent of the number of the unit cell. This property is used to design a novel filter whose central frequency is independent of the physical length of the TL [5-8] hence bringing about size reduction, since these resonators consisting of unit cells do not rely on the half-wavelength resonance of transmission lines. In this paper, we have proposed a design of compact bandpass filter based on the metamaterial CRLH ZOR with improved insertion loss using general PCB etching facility. In the sections to come, the design process including the circuit simulation is presented. The filter is implemented, and the field distributions of the ZORs obtained by the 3D full-wave EM simulation and effective constitutive parameters are shown. Finally, the proposed technique is validated by the fabrication and measurement which is followed by technical discussions and conclusions. II. THE CRLH UNIT CELL A. Unit Cell Design Parameters
Figure 1. Symmetrical CRLH unit cell design.
The microstrip layout of symmetric LH TL unit cell is shown in Fig.1. The design parameters of the symmetrical CRLH unit cell with vias on RT Duroid 6010.2 substrate (εr = 10.2,
tanδ = 0.0023, Substrate height = 1.27mm and copper trace = 0.35um) as setup in high frequency structure simulator (HFSS) is as given in Table 1. Table 1: Design parameters of Unit Cell with vias, with RT Duroid 6010.2 as substrate [ r = 10.2, tan = 0.0023, Substrate height = 1.27mm and copper trace = 0.35um] Unit cell period : p = 6.8 mm
S = p-lc = 1.2 mm
Finger length : lc = 4 mm
stub length : ls = 3.2 mm
finger width : wc = 0.3 mm
stub width : ws = 0.8 mm
IDC spacing :gap = 0.2 mm
via radius : r = 0.24 mm
B. Unit Cell Equivalent Lumped Parameters The S-parameters are obtained using full wave EM simulations in HFSS with reference planes at the input and output terminals of the unit cell. Using the extracted Sparameters for unit cell and network analysis, lumped parameters are found for the equivalent circuit of unit cell. The equivalent lumped model for unit cell as in Fig. 2. when used as a resonator is set up in ADS planar circuit simulator and the circuit is as shown in Fig. 3. The lumped parameters of equivalent circuit of unit cell is given in Table 2. C. Unit Cell Design when used as Resonator: Equivalent Lumped Circuit
Port P1 Num=1
Term Term1 MGAP Num=1 Gap1 Z=50 Ohm Subst="MSub1" W=0.625 mm S=S1 mm
L L2 L=LL nH R=
C C2 C=CR pF
L L1 L=LR nH R=
C C1 C=CL pF
L L3 L=LL nH R=
C C3 C=CR pF
MGAP Gap2 Subst="MSub1" W=0.625 mm S=S1 mm
Term Term2 Num=2 Z=50 Ohm
Port P2 Num=2
Figure 3. Symmetrical CRLH open circuited unit cell lumped element equivalent circuit.
D. Two pole filter using CRLH equivalent Lumped Circuit A two pole filter using the unit cell equivalent circuit as resonators and capacitive gap as inverters is set up in ADS circuit simulator. The set up values are given in Table 3 and circuit is as shown in Fig. 3. Table 3: Two pole lumped equivalent circuit of filter design in ADS circuit simulator with RT Duroid 6010.2 as substrate [εr=10.2, tanδ = 0.0023, Substrate height = 1.27mm and copper trace = 0.35μm] CR = 0.2pF
LR = 0.21nH
CL = 5 pF
L L = 5.2 nH
C1 = 0.2pF
C2 = 5 pF
Gap S1 = 0.2 mm
Inter-resonator gap S2 = 1.2 mm
The S-parameter response for the filter circuit is as shown in Fig. 4. The two poles are clearly visible from the return loss of the lumped equivalent circuit. Further it verifies the correctness of the unit cell model in the narrow-band of operation.
Figure 4. Two pole BP filter: Lumped equivalent circuit Response. Fig.2. Symmetrical CRLH open circuited unit cell lumped element equivalent circuit.
Table 2: Lumped parameters of equivalent circuit of Unit Cell = ܴܥ0.2pF
ܮR = 0.21nH
CL = 5 pF
LL = 5.2 nH
fse = 4.91GHz
fsh = 4.93 GHz
III. UNIT CELL SIMULATION IN HFSS Further the unit cell is simulated in HFSS for confidence as ADS circuit simulator does not solve for fields. A. Unit Cell Eigen Mode solution in HFSS The setup of symmetrical CRLH unit cell structure in HFSS is as shown in Fig. 4. The boundary for the vacuum box in HFSS was set PEC.
Table 4: Eigen Mode Solution for Symmetrical CRLH Unit Cell with PTH vias Model in HFSS. Eigen Mode Mode1 Mode2 Mode3
Frequency (GHz) 2.51556+ j 0.00541072 4.75327+ j 0.00969507 5.90778+ j 0.0158036
Q 232.461 245.139 186.913
Figure 5. Symmetrical CRLH Unit Cell : Eigen Mode Setup in HFSS with RT Duroid 6010.2 as substrate [εr = 10.2, tanδ = 0.0023, Substrate height = 1.27mm and copper trace = 0.35um]
B. Unit Cell Excitation The configuration for Symmetrical CRLH Unit Cell excitation as shown in Fig. 8. was setup in HFSS for two port S-parameter simulations after inference from Eigen mode field solution in HFSS so as to maximize Q-loaded.
Figure 8. Excitation of symmetrical CRLH Unit Cell with vias as resonator as resonator for minimum insertion loss with Rogers Duroid 6010.2 as substrate [εr = 10.2, tanδ=0.0023, Substrate height = 1.27mm and copper trace = 0.35um] XY Plot 1
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HFSSDesign1
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Eigen mode solution of the resonator clearly shows that mode 2 is excited in the resonator at 4.75 GHz with an unloaded Q of 245 as shown in Table 4.
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Figure 6. Electric Field lines in the symmetrical CRLH resonator with vias as plotted in HFSS.
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Figure 9. S-parameter response of symmetrical CRLH unit cells with vias as resonators in HFSS on RT Duroid 6010.2 as substrate [εr = 10.2, tanδ = 0.0023, Substrate height = 1.27mm and copper trace = 0.35um]
The minimum insertion loss (S21) achieved under the design constraint of 0.2 mm resolution was -1.3dB at 4.72 GHz as shown in Fig.9. IV. BANDPASS FILTER DESIGN
Figure 7. Magnetic Field pattern around the resonator as plotted in HFSS.
The optimized excitation gap for the 2-pole BP filter shown in Fig. 10. was g1 = 0.2 mm and inter-resonator capacitive spacing g2 = 0.4 mm near the via ends. The via diameter was 0.24 mm and was symmetrically placed with its centre 0.4 mm from three sides of the stub.
Fig. 6. clearly shows that maximum E-field exists in between the inter-digital capacitors. This inference was very useful in deciding the way the resonator has to be excited for getting maximum Q-unloaded, hence minimum insertion loss. The magnetic field around the vias as seen in Fig. 7. clearly depicts that current flow through vias to the ground. It can be concluded that when these symmetrical CRLH resonators are placed in configuration as shown in Fig. 8. there has to be mixed coupling between them hence mode 2 gets coupled.
Figure 10. BPF Layout in HFSS with RT Duroid 6010.2 as substrate [εr = 10.2, tanδ = 0.0023, Substrate height = 1.27 mm and copper trace = 0.35 um]
With initial inter-resonator spacing of 1 mm and excitation gap of 0.2 mm at the ends in the configuration depicted in Fig. 10. and Fig. 11., the bandpass filter response was optimized using optimetrics utility in HFSS for maximum S21 between 4.6 GHz - 4.8 GHz.
Figure 11. Two pole Filter (top view) using symmetrical CRLH unit cells with vias as resonators in HFSS on RT Duroid 6010.2 as substrate [εr = 10.2, tanδ = 0.0023, Substrate height = 1.27mm and copper trace = 0.35um]
The optimized value for g1 and g2 were 0.2 mm and 0.4 mm respectively. The Wide Band frequency response is as shown in Fig. 12 and Fig. 13. S Parameters for CRLH with PTH Vias
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Effect of variation in via diameter on S-parameter was also studied. The via radius was kept 0.12mm, 0.24mm and 0.36mm. The HFSS simulations show a shift in pass-band and centre frequency of the BPF as shown in Fig.14.
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V. RESULTS
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Curve Info
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4.4600
-1.3793
m2
4.4950
-11.6988
dB(S(1,1)) Setup1 : Sw eep1 dB(S(1,2)) Setup1 : Sw eep1
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Figure 14. Variation of S21 response with change in Via diameter.
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Initial results with hand-soldered vias were not very encouraging as far as repeatability of results was considered, hence plated-through -hole (PTH) was used to create Vias. A major advantage of using PTH instead of hand soldered vias is that they help in mass production of filters with reproducible response. Another advantage is that PTH vias being a machine process result in high degree of precision where as hand soldering of vias is time consuming, laborious and requires good soldering skills.
Figure 12. Two pole Filter optimized frequency response (Narrowband) using symmetrical CRLH unit cells with vias as resonators in HFSS on RT Duroid 6010.2 as substrate [εr = 10.2, tanδ = 0.0023, Substrate height = 1.27mm and copper trace = 0.35um]
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Compact 2_Pole BPF with symmetrical CRLH TLs with Vias: WB Response
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Figure 15. Measured and simulated Return loss comparison for BPF using CRLH resonators with PTH vias.
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4.50 Frequency [GHz]
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Figure 13. Two pole Filter optimized frequency response (wideband out of band spurious response) using symmetrical CRLH unit cells with vias as resonators in HFSS on RT Duroid 6010.2 as substrate [εr = 10.2, tanδ = 0.0023, Substrate height = 1.27mm and copper trace = 0.35um]
Fig. 15. and Fig. 16. clearly show that measured S11 plot and S21 plot matches very much with the simulated S11 & S21 plot in HFSS. The fabricated BPF parameters are given in Table 5. There is marginal shift in pass-band and centre frequency.
It was shown by designing a planar two pole MTM-bandpass filter, that when left handed TL Unit Cells are used as resonators they help in terms of filter size reduction without compromising on insertion loss, which is very much needed for trans-receivers for satellite stations and MMIC Environment fabrication. The measurement results obtained from fabricated filter using PTH for vias are in agreement with HFSS simulation results and with respect to ADS circuit simulator results.
VI. CONCLUSION Figure 16. Measured and simulated Insertion loss comparison for BPF using CRLH resonators with PTH vias.
This is attributable to the fabrication limitation of the lab where drilling for PTH was done by hand using magnifying glass. Had it been done using the HFSS AutoCAD model on CNC machine the results could have been more matching. But still a good agreement in measured and simulated plots exist other than frequency shift. This clearly states that the filter design using symmetrical CRLH structure was a success and now the precision in PTH fabrication needs to be implemented using automated CNC PCB micro-machining.
The synthesis of microstrip bandpass filter using LH TLs is studied. The symmetric LH TL unit cell is characterized using lumped model, and the zeroth-order resonance of the open circuited symmetrical LH TL the unit cell is excited for maximum loaded Q, hence minimum insertion loss. The size independent resonance and shunt type resonance frequency of ZOR with admittance inverters is used for designing a compact two pole BPF, confirmed by both circuit and fullwave EM-simulation. At centre frequency fo = 4.5 GHz , half-wavelength is 12.48 mm on the same substrate for microstrip. The total size of 2 pole filter with normal half wave length resonators ≈ 24.96 mm. By implementing with ZORs, total size = 7.2 mm. There is reduction in size of 71% when compared with BPF realisation using half-wavelength resonators on the same substrate.
REFERENCES [1] Matthaei, G.L. , L. Young, and E.M.T. Jones, “Microwave Filters, [2]
Figure 17. Two- pole MTM BP filter with PTH vias after fabrication.
[3] [4]
The fabricated filter is as shown in Fig. 17. using PTH Vias. The size of the filter is 24mm×24mm×6mm. Table 5. The measured two pole BPF parameters using CRLH TL unit cells as resonator. Center Frequency f0=4.72 GHz
[5]
[6]
[7]
BW = 4.85 GHz- 4.61 GHz =0.24 GHz FBW = (.24/4.72)x100 =5.1 % Insertion loss = -1.6 dB In-band Return Loss = -14 dB Out-of-band rejection at ( 4.72±.3) GHz better than 18 dB
[8]
Impedence-Matching Networks and coupling structures”, Artech House,Norwood, MA, 1980. Richard J. Cameron , Raafat Mansour and Chandra M. Kudsia, "Microwave Filters for Communication Systems: Fundamentals, Design and Applications", Third Edition, John Wiley & Sons, 2007. A. Lai, T. Itoh, "Composite Right/Left-Handed Transmission Line Metamaterials", IEEE Microwave Magazine, pp.34-50, Sept. 2004. C. Caloz, T. Itoh, Electromagnetic Metamaterial, IEEE press, Wiley, Hoboken NJ, 2006 Yuanyuan Sun, Yewen Zhang, Fuqiang Liu, Li He, Hongqiang Li, Hong Chen , “A novel filter based on zeroth-order resonance by means of CRLH transmission line”, Microwave and Optical Technology Letters,Vol.49 Issue 5, Pg 1015 - 1018, Mar 2007. Allen, C. A., K. M. K. H. Leong, and T. Itoh, “Design of microstrip resonators using balanced and unbalanced composite right/left-handed transmission lines," IEEE Transactions on Microwave Theory and Techniques, Vol. 54, No. 7, 3104-3112,Jul. 2006. G. Naga Satish, K. V. Srivastava, A. Biswas and D. Kettle, "A Via-Free Left Handed Transmission Line with Radial Stubs," in Asia Pacific Microwave Conference-2009, Singapore in Dec 2009. (APMC-09). G. Naga Satish, K. V. Srivastava, A. Biswas and D. Kettle, "Band-pass Filter using Symmetrical Left-Handed Transmission Line Zeroth-Order Resonators" accepted for presentation in 5th German Microwave Conference (GeMiC) 2010 Berlin, Germany in March 2010.
