Miniaturized loop resonator filter using capacitively loaded transmission lines

MINIATURIZED LOOP RESONATOR FILTER USING CAPACITIVELY LOADED TRANSMISSION LINES MAK HON YEONG DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2005 MINIATURIZED LOOP RESONATOR FILTER USING CAPACITIVELY LOADED TRANSMISSION LINES MAK HON YEONG B.Eng (Hons.), NUS A THESIS SUBMITTED FOR THE DEGREE OF MASTERS IN ENGINEERING DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2005 Table Of Contents List of Tables iii List of Figures iv Abstract vii Acknowledgements viii Chapter : Introduction 1.1 Introduction 1.2 Objectives 1.3 Scope of Work 1.4 Organization 1.5 Publications Arising from the Present Work Chapter : Microstrip Resonators and Slow Wave Structures 2.1 Introduction 2.2 Microstrip Transmission Line 2.3 Microstrip Resonator 2.4 Ring Resonator 2.4.1 Ring Equivalent Circuit and Input Impedance 11 2.4.2 Modes, Perturbations, and Coupling Methods of Ring Resonators 12 2.4.3 Applications Using Ring Resonators 14 2.5 Slow Wave Structures 15 2.5.1 Lossless Transmission Line 15 2.5.2 Capacitive Loaded Transmission Lines (CTL) 16 Chapter : Closed Loop Resonator Miniaturisation 19 3.1 Introduction 19 3.2 Novel Closed Loop Resonator 20 3.3 Resonator Synthesis Procedure 25 3.3.1 3.4 Example 31 Summary 38 Chapter : Filter Synthesis Using Arbitrary Resonator Structures 39 4.1 Band Pass Filters 39 4.2 Coupled Resonator Filter 40 4.2.1 General Coupling Matrix for Coupled Resonator Filters 40 4.2.2 General formulation for Extracting Coupling Coefficient K 43 4.2.3 Formulation for Extracting External Quality Factor Qe 45 i 4.3 Procedure for Coupled Resonator Filter Design 47 Chapter : Loaded Q and Coupling Coefficient of Resonators 48 5.1 Introduction 48 5.2 Loaded Q of Resonators 48 5.2.1 Coupled Line Coupling 48 5.2.2 Tapped Line Coupling 51 5.2.3 Other Explored Feed Structures 55 5.3 Coupling Coefficient K of Resonators 58 5.4 Summary 60 Chapter : Miniaturized Closed Loop Resonator Filter 61 6.1 Introduction 61 6.2 Chebyshev Filter of 0.01dB Ripple, N=3, BW=10% Using Square Closed Loop resonator 61 6.3 Chebyshev Filter of 0.01dB ripple, BW=10% Using Resonator dbl_d66 64 6.4 Fabricated and Measured Results 66 6.5 Summary 68 Chapter 7.1 Chapter : Conclusion 69 Suggestion for Future Works 70 : Appendix 71 References 80 ii List of Tables Table 1: QL of tapped line coupled structures for dbl_d66 54 Table 2: Coupling coefficient measurement results 59 Table 3: Normalized K and Q values for Chebyshev filter 0.01dB ripple 10% bandwidth 61 iii List of Figures Figure 1: Microstrip Line Figure 2: Ring resonator Figure 3: ring resonator with one feed line 10 Figure 4: Equivalent circuit of ring resonator 11 Figure 5: Maximum field points for different resonant modes 13 Figure 6: Ring resonator with slit 13 Figure 7: Lossless transmission line circuit 15 Figure 8: Capacitively loaded transmission line 16 Figure 9: Square closed loop f0=1.42 GHz 22 Figure 10: Double_stub f0=1.19 GHz 22 Figure 11: dbl_w35 f0=1.14 GHz 22 Figure 12: dbl_d66 f0=1.08 GHz 22 Figure 13: Square closed loop at f0=1.08 GHz 22 Figure 14: Resonator frequency response 23 Figure 15: Compare resonance frequency 23 Figure 16: Single stub unit cell 28 Figure 17: Double stub unit cell 28 Figure 18: Cascaded unit cells for a single side 28 Figure 19: Circuit model of miniaturized resonator in ADS 29 Figure 20: Resonator Synthesis Procedure 30 Figure 21: Double stub EM model 32 Figure 22: Double stub ADS circuit model 33 Figure 23: Double stub response 33 Figure 24: Single stub EM model 34 Figure 25: Single stub circuit model 34 Figure 26: Single stub response 35 Figure 27: Synthesized resonator 36 Figure 28: Synthesized resonator dB|S21| 37 Figure 29: Equivalent circuit for n-coupled resonators (a) loop equation formulation, (b) network representation 40 Figure 30: Singly loaded resonator S11 46 Figure 31: Parallel coupled line feed 49 Figure 32: Coupled line with interdigital stubs feed 49 iv Figure 33: Coupled line QL 49 Figure 34: Coupled line with X interdigital stubs (gap=15 mils) 50 Figure 35: QL vs no of stubs 50 Figure 36: Tapped line coupling for dbl_d66 51 Figure 37: Tapped line coupling for Square closed loop resonator 51 Figure 38: Tapped line with series inductor 51 Figure 39: Effect of series inductor on QL 52 Figure 40: Multiple tap feed structures 53 Figure 41: QL of tapped line coupled structures 54 Figure 42: Single tap with vertical shifted tap positions 55 Figure 43: Single tap Q-loaded vs Offset 56 Figure 44: Multiple tap with vertical shifted feed positions 56 Figure 45: Multiple tap Q-loaded vs Offset 57 Figure 46: Coupling measurement of resonator dbl_d66 58 Figure 47: Coupling measurement of Square closed loop resonators 58 Figure 48: Coupling Coefficient K vs Gap 59 Figure 49: Layout of the 3rd order Chebyshev filter using Square closed loop resonator 62 Figure 50: Circuit simulation of 3rd order Chebyshev filter using Square closed loop resonator with series inductors placed at each port to increase QL 63 Figure 51: Simulated response of the 3rd order filter using Square closed loop resonator 63 Figure 52: Layout of the 3rd Order Chebyshev filter using dbl_d66 65 Figure 53: Simulated response (IE3D) of the 3rd order filter using dbl_d66 65 Figure 54: Fabricated 3rd order filter using dbl_d66 66 Figure 55: VNA measurement from 50 MHz to 2400 MHz 66 Figure 56: VNA measurement 50 MHz to 1600 MHz 67 Figure 57: Compare simulated vs measured result 67 v List of Symbols ε0 permittivity µ0 permeability λ wavelength CTL capacitively loaded transmission line TL transmission line up phase velocity vi Abstract This thesis details the design and investigation of a miniaturized microstrip closed loop resonator using slow wave structures in the form of capacitively loaded microstrip lines The primary objective is to achieve resonator and hence filter miniaturization with a secondary objective of achieving improving resonator coupling to aid filter synthesis A novel miniaturized closed loop resonator structure that achieves both miniaturization and improved coupling has been developed The miniaturized resonator is demonstrated to achieve a 37% reduction in area when compared against a square closed loop resonator of equivalent resonant frequency Also developed are feed structures to provide improved control of external QL To aid resonator design, a methodology to synthesize the new structure based on frequency requirements is provided A 3rd order Chebyshev filter using the new resonator structure has been fabricated In comparison to a filter synthesized using a closed loop resonator of similar size, the new structure achieves a 22% lower resonant frequency and also an additional area reduction of 6% which is possible due to space savings provided by the structure vii Acknowledgements I would like to thank Professor Leong Mook Seng, Associate Professor Ooi Ban Leong and Doctor Chew Siou Teck for their invaluable advice and guidance to this project I would also like to thank the staff from the Radio Frequency Laboratory at DSO for providing support for fabrication processes Lastly, I would like to thank Mr Ng Tiong Huat and friends at the Microwave Laboratory of NUS for their company and friendship They have made my academic experience a fulfilling and enriching one viii Figure 56: VNA measurement 50 MHz to 1600 MHz m2 m1 m3 -10 dB(S(4,3)) dB(S(2,1)) -20 -30 m1 freq=1.080GHz dB(S(2,1))=-1.505 -40 -50 m2 freq=1.005GHz dB(S(4,3))=-4.683 -60 -70 m3 freq=1.155GHz dB(S(4,3))=-4.526 -80 0.2 0.4 0.6 0.8 1.0 freq, GHz 1.2 1.4 1.6 Figure 57: Compare simulated vs measured result Measured Response: F0 = 1.08 GHz ( |S21| = -1.12 dB) F-3dB = 1.01 GHz ( |S21| = -4.15 dB ) F+3dB = 1.15 GHz ( |S21| = -4.12 dB ) BW = 13% 67 1.8 Measured Result Simulated Result 6.5 Summary In this chapter filter synthesis using the miniaturized resonator has been successfully demonstrated The measured result of the fabricated filter corresponds closely to the simulated results as shown in Figure 54 Comparing the size and performance of a filter created using the simple closed loop resonator (Figure 53) against one created using the miniaturized resonator dbl_d66 (Figure 51), the following is observed: • Filter using dbl_d66 occupies a slightly smaller area of 2.058 in2 whereas the filter using the simple loop resonator occupies a slightly larger area of 2.096 in2 This equates to approximately 6% reduction in area • Filter using dbl_d66 has a lower resonant frequency of 1.08 GHz as compared to 1.4 GHz This equates to a 22% reduction in resonant frequency 68 Chapter : Conclusion In this thesis, the objective of closed loop resonator miniaturization using capacitively loaded transmission lines (CTL) has been successfully achieved The miniaturized resonator has been demonstrated to achieve 37% reduction in area over the square closed loop resonator of equivalent resonant frequency A method to synthesize miniaturized closed loop resonators of the desired resonant frequency has been developed The miniaturized resonators offer improved coupling performance and control through the interdigital capacitor like structures that are formed when two resonators are placed adjacent to each other Planar feed structures to control the external QL of the miniaturized resonators have also been developed The structures designed enable fine tuning of QL without the use of discrete lumped inductors Filter synthesis has successfully been performed on the newly developed structure A 3rd order Chebyshev filter of 0.01dB ripple and 10% bandwidth has been fabricated and measured with simulation results corresponding closely to the measured results In comparison to a filter synthesized using a closed loop resonator of similar size, the new structure achieves a 22% lower resonant frequency and also an area additional area reduction of 6% which is achieved due to the interdigital structure 69 7.1 Suggestion for Future Works In this thesis, the application of slow wave structures, in the form of capacitive open circuited microstrip line stubs to the closed loop resonator, has successfully enabled resonator miniaturization to be achieved For future studies, other forms of capacitive loading structures could be explored This could be in form of lumped capacitors or other microstrip structures such as radial stubs 70 Chapter : Appendix Parallel Coupled Line Feed Feed gap (mil) 10 15 20 25 30 F1 (GHz) 1.092 1.099 1.101 1.102 1.1026 1.103 F2 (GHz) 1.108 1.113 1.114 1.114 1.114 1.114 F0 (GHz) 1.100 1.106 1.107 1.108 1.108 1.108 71 Qloaded 137.50 158.00 170.31 184.67 194.39 201.42 Coupled Line with Interdigital stub feed Reson sep (mil) F1 (GHz) F2 (GHz) F0 (GHz) Q-loaded 1.073 1.11 1.090 58.92 10 1.078 1.11 1.095 68.44 15 1.081 1.11 1.095 75.52 20 1.084 1.108 1.095 91.25 25 1.086 1.107 1.095 104.29 30 1.087 1.107 1.095 109.50 72 Coupled Line with X interdigital stubs (gap=15 mils) stubs stubs stubs stubs # of stubs F1 (GHz) F2 (GHz) F0 (GHz) Q-loaded 1.093 1.107 1.1 157.14 1.085 1.107 1.095 99.55 1.082 1.11 1.095 78.21 1.081 1.11 1.095 75.52 10 1.081 1.11 1.095 75.52 73 Tapped line coupled structures Show S-parameter Plot beside each structure Center stub stubs stubs + pts stubs stubs 74 stubs + 4pts stubs + pts stubs + pts stubs + 6pt stubs 75 stubs + 8pts stubs + 8pts Feed F+90 (GHz) F-90 (GHz) F0 (GHz) Q-loaded Center Stub 1.02 1.21 1.12 5.89 stubs 0.995 1.18 1.09 5.89 stubs 0.985 1.155 1.075 6.32 stubs 0.985 1.145 1.07 6.69 stubs 0.995 1.14 1.07 7.38 3stubs + 2pt 0.99 1.17 1.08 6.00 3stubs + 4pt 0.975 1.147 1.065 6.19 5stubs + 4pt 0.975 1.14 1.06 6.42 5stubs + 6pt 0.97 1.125 1.05 6.77 7stubs + 6pt 0.975 1.125 1.05 7.00 7stubs + 8pt 0.98 1.12 1.05 7.50 9stubs + 8pt 0.98 1.115 1.05 7.78 76 Coupling Measurement of Resonator dbl_d66 833.5 gap 833.5 77 Reson gap (mil) F1 (GHz) F2 (GHz) F0 (GHz) k 1.04 1.15 1.094 0.10 1.04 1.14 1.089 0.09 10 1.05 1.14 1.094 0.08 15 1.055 1.135 1.094 0.073 20 1.06 1.135 1.097 0.068 25 1.065 1.13 1.097 0.059 30 1.07 1.13 1.100 0.055 35 1.075 1.135 1.105 0.054 40 1.077 1.125 1.101 0.044 45 1.08 1.125 1.102 0.041 50 1.085 1.12 1.102 0.032 78 Coupling Measurement of Square Closed Loop Resonator 833.5 gap 833.5 Reson sep (mil) F1 (GHz) F2 (GHz) F0 (GHz) k 1.335 1.465 1.398 0.09 10 1.365 1.455 1.409 0.06 15 1.385 1.445 1.415 0.042 20 1.395 1.44 1.417 0.032 25 1.4 1.43 1.415 0.021 30 1.41 1.425 1.417 0.011 35 1.42 - 1.42 coupling too weak 40 1.42 - 1.42 coupling too weak 45 1.42 - 1.42 coupling too weak 50 1.42 - 1.42 coupling too weak 79 References [1] J.S Hong and M.J Lancaster, "Capacitively loaded microstrip loop resonator" Electron Lett, vol 30, no 18, pp1494-1495, Sept 1994 [2] A.gorur, C Karpuz and M alkan, "Characteristics of periodically loaded CPW structures" IEEE Microwave Guided Wave Lett., vol.8, pp.278-280, Aug 1998 [3] K.W Eccleston and H.M Ong, "Compact Planar Microstripline Branch-Line and Rat-race Couplers" IEEE Trans MTT Vol 51, No 10, pp 2119 – 2124, Oct 2003 [4] R.E Collin, Foundations for Microwave Engineering New York: Mc-GrawHill, 1992, ch [5] J.S Hong and M.J Lancaster, Microwave filters for RF/Microwave applications, Wiley, 2001 [6] D.M Pozar, Microwave Engineering, 2nd ed New York: Wiley,1998 [7] W.R Randall, HF filter design and computer simulation, Noble Publishing Associates, 1994 [8] A.B Williams and F.J Taylor, Electronic Filter Design Handbook, 2nd ed, McGraw-Hill, 1988 [9] Kai Chang, Microwave Ring Circuits and Antennas, Wiley-Interscience, 1996 [10] Bal S.Virdee, Christos Grassopoulosf', “Folded Microstrip Resonator” Microwave Symposium Digest, 2003 IEEE MTT-S International , Volume: , 8-13 June 2003 [11] [12] C.C Yu, Kai Chang, “Transmission-Line Analysis of a Capacitively Coupled Microstrip-Ring Resonator” IEEE Trans MTT, Volume: 45, No 11, pp 20182024, Nov 1997 R.J Cameron, “General Coupling Matrix Synthesis Methods for Chebyshev Filtering Functions” IEEE Trans MTT, Volume: 47, No pp 1559–1564, Sep 2000 [13] I.J Bahl and P.Bhartia, Microwave Solid State Circuit Design, Wiley, 1988 [14] G Mattaei, L Young, and E.M.T Jones, Microwave Filters, Impedance matching networks, and coupling structures, Artech House, 1980 [15] K.C Huang, D.Hyland, A.Jenkins “A miniaturized interdigital microstrip bandpass filter” IEEE Trans on Applied Superconductivity, Vol.9, no.2, pp.3889-3892 ,June 1999 [16] D.Ahn, J.S Lim, I.S Kim “Design of a 2-pole bandpass filter using closed loop resonator and coupled lines” IEEE MTT-S digest, pp 1643-1646, 1996 80 [17] K.Y Kang, S.K Hang “HTS pole bandpass filter with closed loop resonators and coupled lines using YBCO thin films for PCS” Asia Pac Microwave Conference, pp 105-108, 1997 [18] D.Ahn, J.S Lim, I.S Kim “The design of closed loop type BPF with two attenuation poles at the upper side of passband” Asia Pac Microwave Conference, pp 821-824, 1997 [19] L.Zhu and K.Wu “A general purpose circuit model of interdigital capacitor for accurate design of low-loss microstrip circuit” IEEE MTT-S digest, pp 17551758, 1998 [20] G.D Alley “ Interdigital Capacitors and their application to lumped-element microwave integrated circuits” IEEE Trans MTT, Vol: 18, No 4, pp.1028-1033, Dec 1970 [21] J.S.Hong, M.J Lancaster “Microstrip slow-wave open-loop resonator filters” IEEE MTT-S digest, pp 713-716 ,1997 [22] ADS2002, Agilent [23] Zeland Software Inc., IE3D simulator 81 [...]... closed loop resonators A common closed loop resonator type is the ring resonator This consists of a transmission line formed in a circular closed loop The basic ring resonator circuit consists of feed lines, coupling gaps, and the resonator (Figure 2) Power is coupled into and out of the resonator through feed lines and coupling gaps Figure 2: Ring resonator The ring resonator is a full wavelength resonator. .. loop resonators to achieve miniaturisation A miniaturised closed loop resonator using slow wave structure is developed here Also developed is a methodology for resonator synthesis The third portion explores coupled resonator filter synthesis using the miniaturized closed loop resonator The new resonator structure is characterized for its design curves such as external Q and coupling coefficient K Using. .. To develop compact closed loop resonators by using capacitively loaded transmission lines (CTL) o To develop resonators capable of providing improved coupling performance o To synthesize filters by using the newly developed resonators 1.3 Scope of Work The scope of this project can be divided into 3 main portions The first portion explores the various types of Microstrip resonators The second portion... Modified Miniaturized Loop Resonator Filter 4 Chapter 2 : Microstrip Resonators and Slow Wave Structures 2.1 Introduction Microwave resonators are used in a variety of applications, including filters, oscillators, frequency meters and tuned amplifiers [6] At microwave frequencies, distributed elements are commonly used to achieve resonance Resonators can be made using microstrip transmission lines, ... coupling which occurs if adjacent lines are located too close to each other In the case of the capacitively loaded microstrip loop resonator, miniaturization is achieved by using open stubs placed at regular intervals inside the loop The stubs provide capacitive loading and creates a slow wave effect 19 3.2 Novel Closed Loop Resonator This thesis proposes a novel closed loop resonator structure that achieves... square closed loop and miniaturized closed loop resonators which will be used to demonstrate resonator miniaturization and the effect of varying CTL parameters To determine resonator characteristics, the resonators are weakly coupled to ports using 50Ω feed lines that are separated from the resonators by a 10 mil gap This keeps loading to a minimal 20 A brief description of each of the featured resonators... coupling by using slow wave structures in the form of capacitively loaded transmission lines CTL is applied on the closed loop resonator by placing stubs at regular intervals around the circumference Unlike the previously explored structures, the new resonator structure uses both inward and outward pointing open stubs as shown in Figure 10 - Figure 12 By using double stubs instead of single stubs, the loaded. .. overview of filter synthesis using arbitrary resonator structures and explains coupled resonator filter synthesis Chapter 6 characterizes the external Q and coupling coefficient K of the newly developed resonator structure Feed structures for coupled line and tapped line coupling are developed for the new resonator structure Chapter 7 performs filter synthesis using the newly developed resonator structure... freq=1.420GHz m6=-29.167 type of resonator Figure 15: Compare resonance frequency 23 Resonator Miniaturization To demonstrate resonator miniaturization, two resonators with equivalent resonant frequency are compared • Miniaturized resonator shown in Figure 12 is compared against the • Square loop resonator shown in Figure 13 Comparing their sizes, the miniaturized resonator achieves 20% reduction in... be formed when an identical resonator is placed in close proximity The effect is an increase in coupling between resonators which aids filter synthesis This section demonstrates the effectiveness of resonator miniaturization using the new structure and formulates a method of synthesizing miniaturized closed loop resonators of a particular frequency For standardization, the resonators shown from this ... microstrip closed loop resonators The objectives can be listed as follows: o To develop compact closed loop resonators by using capacitively loaded transmission lines (CTL) o To develop resonators capable.. .MINIATURIZED LOOP RESONATOR FILTER USING CAPACITIVELY LOADED TRANSMISSION LINES MAK HON YEONG B.Eng (Hons.), NUS A THESIS SUBMITTED FOR... CTL capacitively loaded transmission line TL transmission line up phase velocity vi Abstract This thesis details the design and investigation of a miniaturized microstrip closed loop resonator using