INTERNATIONAL JOURNAL OF ELECTRONICS AND COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)

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1 INTERNATIONAL JOURNAL OF ELECTRONICS AND COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET) International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN (Print) ISSN (Online) Volume 4, Issue 7 (2013), pp IAEME: Journal Impact Factor (2013): (Calculated by GISI) IJECET I A E M E Experimental Investigations of Microstrip Distributed MEMS Transmission Line Phase Shifter V Janardhana 1, S Pamidighantam 2, N Chattoraj 1, J S Roy 3, K S Reddy 4, R G Kulkarni 5, Kamaljit Rangra 6 1 Faculty/Electronics & Communication, Birla Institute of Technology, Mesra, Ranchi, India 2 Birla Institute of Technology (BIT-P), Hyderabad, India 3 School of Electronics Engineering, KIIT University, Bhubaneswar, Orissa, India 4 Department of Informatics, University of Oslo, Oslo 5 HMC-Lab, Bharat Electronics Limited, Bangalore, India 6 Central Electronics Engineering Research Institute, Pilani, India 1 janardhanav@yahoo.com, nila_chwdhry@yahoo.com, 2 sayanu@hotmail.com, 3 drjsroy@rediffmail.com, 4 srinivar@ifi.uio.no, 5 kulkarnirg@bel.co.in, 6 kjrangra@gmail.com ABSTRACT: In this work RF MEMS phase shifter on Microstrip DMTL is attempted for the first time. Microstrip conductor is lifted from the ground plane to form a parallel plate bridge capacitance between the Microstrip conductor & ground. The measured and simulated results of the structure are presented in a table below: Frequency (GHz) Model Length (µm) Phase Shift ( φ ) Loss (db) Simulated Measured Simulated Measured KEYWORDS: Microstrip, Phase shifter, DMTL I. INTRODUCTION Voltage & current on the air filled Microstrip line (Fig. 1) are uniquely defined as the dominant mode is TEM. With the inductance L & capacitance per unit length C of the transmission line, the characteristic impedance and phase velocity can be written as [1, 4, 11]: Z o V = L LC 1 = = C C cc 1 1 = ε o µ LC = (4.1) o (4.2) B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India Page 222

2 International Journal of Electronics and Communication Engineering & Technology (IJECET), Fig. 1: Electromagnetic Field of Air-Filled Microstrip-TEM TEM mode Equation (4.1) indicates that the analysis of the characteristic impedance can be reduced to the determination of the capacitance per unit length C the capacitance per unit length of an ideal air-filled parallel plate transmission mission line of effective width weff & height h reads C =εo weff h (4.3) The effective width weff can be calculated from w eff = w + 2h w ln 2πe h 2h (4.4) Microstrip Inductance per unit length L can be written as [1,2,3,4]: L = µo II. h (4.5) w eff THEORY & DESIGN The structure of MDMTL phase shifter (Fig. 2) 2) consists of CPW input and output feed lines with 50 Ω impedance. The DMTL line is lifted above the ground that forms a parallel plate bridge capacitance with ground plane. The equivalent circuit of MDMTL is shown in Fig. 4. Fig. 2: 3D view of Microstrip Distributed MEMS Transmission Line Phase Shifter MDMTL International Conference on Communication Systems (ICCS-2013) (ICCS B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India October 18-20, 2013 Page 223

3 International Journal of Electronics and Communication Engineering & Technology (IJECET), Fig. 3: CST - Electromagnetic magnetic MDMTL Simulation Model DC bias line is under the MDMTL line (Fig. 3) 3) to pull it down through electrostatic actuation, with bridge and the DC bias lines acting as two electrodes. The MDMTL Bridge in an initial position has a capacitance Cup, and subsequently Cdn, with the ground plane, when it is pulled down through DC electrostatic ectrostatic actuation. The change in the capacitance alters the phase velocity of the electromagnetic field leading to a phase shift. Fig. 4: Equivalent Equivalent Lumped Element Model of MDMTL Substituting (4.3) & (4.4) in (4.1), the characteristic impedance of the air-filled air microstrip transmission line is calculated from: Z o (air ) = µo εo w 0.5 w + ln 2πe h π 2h ( w > 2h) 1 (4.6) Hammerstad and Jensen have derived an accurate formula for characteristic impedance Z o of a Microstrip transmission line and are given below [5,6,9,11,12]: w h η 2h Z o ( Ham) = o ln F π h w w 2 (4.7) h w F1 = 6 + (2π 6) exp w h Where ηo = 120π is the wave impedance in free space. International Conference on Communication Systems (ICCS-2013) (ICCS B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India October 18-20, 2013 Page 224

4 go (µm) (h) weff (µm) Zo (Ω) Zo(air) (Ω) Zo(Ham) (Ω) L (nh) C (pf) Table 1: Microstrip Impedance Calculation from Various Formulae From (4.4), (4.1) & (4.6), (4.7), (4.5) & (4.3) - weff, Zo, Zo (air), Zo (Ham) L, & C, are calculated respectively for various heights go (h). Fig. 5: Simulated S- Parameter and Smith Chart for MDMTL Model With go (h)= 1.3 µm At S11 peak the transmission line acts as a quarter wave transformer and hence Zl = Zo at this point. 1± Γin Zl = 50 (4.8) 1mΓin From Fig. 5, S11 peak is 28 db, Γ = and hence load impedance Zl = 53 Ω from (4.8), it can be observed from the Table 1 that for go = 1.3 µm the calculated value for Zo(Ham) is 42 Ω, hence it is underestimating the Zl, as the simulate value is 53 Ω. From Fig. 5, The value of impedance Zo(Ham) needs to be multiplied by B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India Page 225

5 go (µm) Zo (Ham) (Ω) Table 2: Corrected Microstrip Impedance Zo (Ham) from Table 1 is multiplied by 1.26 times to arrive at Zo (Ham) values at Table 2. Phase Shift φ: The phase shift is found from [8]: φ = Z o πf (4.9) c Zlup Zldn A. MDMTL Single Bridge-3 Cascaded Units The microstrip bridge consists of center conductors with width w = 15µm. The length of the bridge is 3 x1202 µm. The total length of the structure L = 5257 µm. Fig. 6: CST Model of Single Beam MDMTL three units cascaded Fig. 7: Simulated & Measured S-Parameter of Single Beam MDMTL Three units cascaded Model at go = 3.5 µm B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India Page 226

6 S11 peak 12.8 db & 10.0 db of measured & modeled structure respectively, the load impedance from (4.8) is 71 Ω & 84 Ω respectively. The S11 Null of measured from Fig. 7, is closer compared to the simulated S11 Nulls the reason being the height of the transmission line go from the ground plane in the fabricated structure is less & non uniform compared to go in the modeled structure. Fig. 8: Simulated Phase shift of Single Beam MDMTL Three units cascaded at go = 3.5 µm & 2.3 µm Fig. 9: Optical Microscope Picture of Fabricated Three Cascaded Single Beam MDMTL phase shifter Fabrication: Surface micromachining technology process is used, unlike bulk micromachining where a substrate is selectively etched to produce structures, surface micromachining is based on the deposition and etching of different structure layers. Corning glass substrate is used. The detailed technology used for fabricating the structure is discussed in [13] Measurement: Wafer level measurements are carried out using Cascade Microtech probe station along with Agilent PNA 8362 B Network analyzer. 0 0 Frequency-(GHz) Loss -(db) Microstrip 3 Cascaded Single Beam 2.3 Simulated Microstrip 3 Cascaded Single Beam 2.3 Calculated Microstrip 3 Cascaded Single Beam 34V Measured Fig. 10: Line loss for length L: Calculated from (4.10), Simulated & Measured loss for Single Beam MDMTL Three Cascaded Units at go = 2.3 µm. International Conference on Communication Systems (ICCS-2013) B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India October 18-20, 2013 Page 227

7 B. Ohmic Losses Within the conductors, losses result from the finite conductivity σ of the metal. The following approximate expression is found sufficient for loss in most situations [2,11,12]. R α = s c db/ m wz (4.10) o Where R s = C. SWR Loss ωµ 2σ is metal wall surface resistance, w is width of the conductor. The line losses increase when SWR is greater than 1:1, S11 > 27 db, Γ > 0.04 the net effect of standing waves on transmission line is to increase the average value of current and voltage compared to the matched line. An increase in current rises I 2 R ohmic losses in the conductor and increase in voltage increases E 2 /R losses in dielectric - Line loss increase with frequency, since the conductor resistance is related to skin effect and because of dielectric loss rise with frequency. The measured loss includes the loss due to SWR loss, ohmic losses and also includes the additional loss due to the difference in conductivity of the conductors (Bridge). The material used for fabrication of conductors may have less conductivity than the aluminum conductivity of 3.56 X 10 7 Siemens / met. The additional loading effects on the conductor due to the change in current distribution on the transmission line in the fabricated structure also add to the loss. This is not accounted for in the formulation of loaded line loss. These are the reasons for the measured loss to be more compared to simulated loss. Phase Shift -(Degrees) Frequency-(GHz) Microstrip Three Cascased Single Beam Simulated Microstrip Three Cascaded Single Beam Calculated Microstrip Three Cascaded Single Beam Measured 34V Fig. 11: φ - Phase Shift for length L µm, Calculated from (4.9), Simulated & Measured for Single Beam MDMTL Three Cascaded Units at go = 3.5 µm & 2.3 µm D. Phase Shift Fig. 11, The simulated phase shift values between two bridge positions are taken from Fig. 8. The calculated values of phase shift are from (4.9). Phase shift has not increased due to effective length increase of cascaded structure. This is due to the fact that the equivalent inductance per unit length of transmission line also changes B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India Page 228

8 along with equivalent capacitance per unit length of transmission line, when the bridge is moved down, even though at a lesser rate than the capacitance change. III. CONCLUSION The advantage of low loss in microstrip could not be fully taken in this design due to height go = 3.5 µm of the microstrip from ground & subsequent mismatch along the structure with input impedance Table 2 & resulting in standing waves that in turn increases the loss. The loss can be minimized with the optimum height go = 1.2 µm so that there is impedance match Table 2. Phase shift of 5 is achieved for height go = 3.5 µm & 2.3 µm, the loss can be minimized as discussed, however the phase shift will be in the same range as the line inductance also changes along with the line capacitance even though at a lesser rate. IV. ACKNOWLEDGMENTS The authors would like to thank Bharat Electronics, Bangalore, India, for their cooperation, by providing their facility for fabrication & measurements. We would also thank Prof. K C Gupta, university of Colorado for his invaluable guidance. CST - Microwave Studio software is used for simulation. REFERENCES [1] Ramesh Garg, "A Microstrip design guide ", IIT Kanpur, [2] R. K. Hoffmann, Handbook of Microwave Integrated Circuits ", Artech House, Inc., Norwood, MA, [3] Bharathi Bhat, Stripline-Like Transmission Lines for Microwave Integrated Circuits ", Centre for Applied Research in Electronics Indian Institute of Technology, New Delhi, Wiley Eastern Limited, [4] D.M.Pozar, Microwave Engineering ", Addison-Wesley Publishing Company, Reading, MA, [5] Dr. E.H.Fooks, Dr R.A. Zakarevicius, " Microwave Engineering using Microstrip Circuits", prentice Hall of Australia Pty Ltd., [6] Paul H. Young, Microstrip Design Laboratory ", Senior Member, IEEE, [7] K.C. Gupta, Ramesh Garg, Inder Bahl, Prakash Bhartia," Microstrip Lines and Slotlines", Second Edition, Artech House, Inc., [8] Nicolas Scott Barker, Distributed MEMS Transmission Lines ", A dissertation, University of Michigan, [9] T.C.Edwards & M.B.Steer, "Foundation of Inter connect & microstrip design", John Wiley & Sons, [10] Hector J. De Los Santos, RF MEMS Circuit Design for Wireless Communication ", The Artech House Micro electromechanical Systems (MEMS) Series, [11] Gunter Kompa, "Practical Microstrip design and applications ", ARTECH HOUSE, INC., [12] Kai Chang, Encyclopedia of RF and Microwave Engineering", John Wiley & Sons, Volume I, [13] Laha, R., B. Sriram, A. T. Kalghatki, K. Natarajan, and S. Pamidighantam, Design, development and characterization of surface micromachined passive components for radar applications," Proceedings of International Radar Symposium (IRSI), Bangalore, India, Dec B K Birla Institute of Engineering & Technology (BKBIET), Pilani, India Page 229