INVESTIGATION AND DESIGN OF NOVEL SPATIAL POWER COMBINER HUI SO CHI (B.Eng (Hons.), University of Queensland, Australia) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2005 Acknowledgement I would like to express my sincere gratitude to both of my supervisors, Professor Leong Mook Seng and Associate Professor Ooi Ban Leong from the Department of Electrical and Computer Engineering, for their invaluable guidance, suggestions and constructive criticisms during the course of doing the project In particularly, I am indebt to Professor Ooi’s special arrangement on the purchase of MMIC amplifiers At the same time, I would like to show my appreciation to Mdm Lee Siew Choo for her effort on the tedious manual wire-bonding work which may cause more wrinkles around her eyes Besides, I must thank those staffs in Microwave Laboratory who have helped me in one way or another during the prototype fabrication and measurement stages I would also like to thank Mdm Guo Lin for her valuable discussion on miniaturizing the Vivaldi antenna Last but not least, I want to thank my dearest wife Without her consistent support and encouragement, this thesis cannot be completed in time i Abstract The aim of this project is to create a novel spatial power combiner The design utilizes the broadband coupling technique and the EBG ground-slot structure to enhance the bandwidth and the out-of-band stability This power combiner is a multilayer circuit-feed spatial power combiner, which consists of the active power divider block and the transmitting block The ground-slot coupling technique is first proposed to enhance the gain bandwidth of the active power divider block Two broadband ground slots are investigated and implemented in Design This prototype is able to achieve -3 dB gain bandwidth of 40% at GHz centre frequency To improve the bandwidth of Design 1, the critical Tee-junction coupler is modified by inserting a 100 Ω resistor The resistance value is derived by using the “even- and odd-mode” analysis This simple but effective modification leads to Design and the final -3 dB gain bandwidth is increased from 40% to 52% Moreover, the output return loss of Design is also better than Design while there is no significant trade-off in gain performance To have a greater performance, the direct signal coupling technique and the EBG structure are proposed and implemented to design the active power divider block Moreover a miniaturized Vivaldi antenna is designed for the 2-elements antenna array in the transmitting block The complete prototype is a multilayer circuit-feed spatial power combiner It can achieve 5.8 GHz impedance bandwidth at the design centre frequency of GHz Moreover there is no oscillation or instability phenomenon across the entire testing frequency from 0.5 GHz to 20.5 GHz Therefore the proposals are valid and the objectives of the project have been achieved ii Table of Contents Page Acknowledgment i Abstract ii Table of Contents iii List of Figures vi List of Symbols ix Chapter 1: Introduction 1.1 Background of Spatial Power Combiner 1.2 Objectives of Project 1.3 Scope of Thesis 1.4 Original Contributions and Publications 1.4.1 List of Publications Chapter 2: Preliminary Theories and Concepts 2.1 Planar Transmission Lines Theory 2.1.1 Strip-line 2.1.2 Microstrip Line 10 2.1.3 Coplanar Strip-line (or CPW) 11 2.2 Ground-Slot Coupling 12 2.3 Multilayer Line Coupling 13 2.4 Antenna Concepts and Array 14 2.4.1 Basic Antenna Concepts 14 2.4.2 Antenna Array 18 2.4.3 Vivaldi Antenna 20 2.5 EBG Ground 21 2.6 Stability of Amplifier Design 22 iii Chapter 3: Design of Broadband Multilayer Active Hybrid-Ring 24 3.1 Investigation of Broadband Power Dividing Technique 24 3.2 Design 25 3.2.1 Analysis of Five-Ports Hybrid Ring Power Divider 25 3.2.2 Modified Hybrid-Ring Power Divider 28 3.2.3 Ground-Slot Coupling Technique 30 3.2.4 Amplifier Stage and Matching 35 3.2.5 DC Biasing and Overall Structure 38 3.3 3.4 3.5 Design 41 3.3.1 Bandwidth Improvement from Design 41 3.3.2 Broadband Multilayer Power Divider 45 Fabrication and Testing of Prototypes 46 3.4.1 Results of Design 48 3.4.2 Results of Design 50 Discussions 53 Chapter 4: Investigation and Design of Novel Spatial Power Combiner 56 4.1 Investigation 56 4.2 Design of Components 58 4.2.1 Overview of SPC Prototype 58 4.2.2 Vivaldi Antenna and Array 59 4.2.2.1 Analysis of Simple Vivaldi Antenna 59 4.2.2.2 Vivaldi Antenna Array 62 4.2.2.3 Simulation of S11 Parameter 63 EBG Ground and Active EBG-coupler 64 4.2.3.1 EBG Ground 65 4.2.3.2 Active EBG-coupler 67 Broadband Matching Network 72 4.2.4.1 Output Matching Network 72 4.2.4.2 Input Matching Network 74 DC-Bias Conditions and Circuits 78 4.2.3 4.2.4 4.2.5 iv 4.3 Overall Performance and Prototype Fabrication 81 4.4 Testing and Measurement 85 4.5 Discussions 91 Chapter 5: Conclusion 95 5.1 Conclusion 95 5.2 Proposal for Future Works 98 References 99 Appendices 102 A Supplementary Measurement Data B Author’s Publications C Data Sheets C1: FHC40LG Transistor C2: DAL-118-DALTIA Amplifier v List of Figures Fig 1.1: A four-finger HBT unit cell from Knowledge-On Company Fig 1.2: A compact parallel-connection power transistor unit made of 4x4 HBT Fig 1.3: Three basic topologies for power-combining techniques (a) Circuit Feed / Circuit Combine (b) Circuit Feed / Spatial Combine (c) Spatial Feed / Spatial Combine Fig 1.4: Block diagram of the multilayer power combiner prototype Fig 2.1: Planar Transmission lines (a) Strip-line (b) Microstrip line (c) Slot-line (d) Coplanar strip-line (CPW) Fig 2.2: Ideal coplanar waveguide structure for characteristic impedance calculation Fig 2.3: Cross-section of a simple ground-slot coupling between microstrip lines Fig 2.4: Cross-section of a simple multilayer line coupling Fig 2.5: Antenna radiation pattern in polar and rectangular format Fig 2.6: One-dimensional antenna array with N-elements Fig 2.7: Radiation patterns for different element number of isotropic-antenna array Fig 2.8: Illustration of Vivaldi Antenna with transition Fig 2.9: Proposal to reduce high cross-polarization in Vivaldi antenna Fig 2.10: Examples of EBG Structure for perforation on ground plane Fig 2.11: Simplified block diagram of an amplifier Fig 3.1: Four-ports and five-ports hybrid ring Fig 3.2: Configuration of five-ports hybrid-ring impedance transformation Fig 3.3: Dimensions of hybrid-ring power divider: (a) Original; (b) Modified Fig 3.4: Cross-section view of breaking path at Z0 branch Fig 3.5: 3-D structure of bowtie-slot coupler in Design Fig 3.6: Simple model for bowtie-ground-slot coupling Fig 3.7: Blow-up view of Tee-junction coupler with trapezium-like slot Fig 3.8: Dimensions of Tee-junction coupler vi Fig 3.9: Simulation results of bowtie and trapezium-like couplers (a) Input return loss, S11 (b) Insertion loss, S21 Fig 3.10: Schematic diagram of amplifier block in Design Fig 3.11: Dimensions of matching networks for Design Fig 3.12: Gain simulation of amplifier for Design Fig 3.13: DC biasing circuit for transistors in Design Fig 3.14: Overall topology structure of Design1 (RF circuit only) Fig 3.15: Stability analysis on Design Fig 3.16: Topology of modified Tee-junction coupler in Design Fig 3.17: Bisection of the circuit with even-mode and odd-mode excitation Fig 3.18: Analysis of Wilkinson power divider to find Zin and S11 Fig 3.19: Stability analysis on Design Fig 3.20: Block diagram of power measurement for Design & Fig 3.21: Photos of prototypes dismantled for illustration (a) Design with ground-slots indicated (b) Design with 100Ω resistor indicated Fig 3.22: Simulated and measured results of gain and isolation for Design Fig 3.23: Simulated and measured results of return losses for Design Fig 3.24: Measurement of P1dB at GHz centre frequency for Design Fig 3.25: Comparison of gain measurement for Design and Design Fig 3.26: Measurement of power and gain at different frequencies for Design Fig 3.27: Measured results of return losses for Design and Design Fig 4.1: Multilayer topology of power combiner prototype Fig 4.2: Miniaturized Vivaldi antenna for antenna array Fig 4.3: Rate of curvature for each exponential edge Fig 4.4: Final dimensions of miniaturized Vivaldi antenna Fig 4.5: Vivaldi antenna array for power combiner prototype Fig 4.6: Concept of E-field skew correction in Vivaldi antenna array Fig 4.7: Simulated S11 of the Vivaldi antenna set and the Vivaldi antenna array Fig 4.8: Dimensions of EBG hole structure for EBG ground Fig 4.9: Simulation results of microstrip line on proposed EBG ground Fig 4.10: Schematic diagram of active EBG-coupler vii Fig 4.11: Dimensions of EBG coupling structure Fig 4.12: Simulation results of S-parameters of passive EBG coupler Fig 4.13: Active-EBG coupling and connection topology Fig 4.14: Return loss and gain simulation of active EBG-coupler Fig 4.15: Block diagram of output matching network Fig 4.16: Dimensions of output matching network Fig 4.17: Quarter-wave transformer example Fig 4.18: Block diagram of power combiner with pi-network Fig 4.19: Balance type of pi-network structure Fig 4.20: Input matching network on EBG ground (a) Topology (b) Dimensions Fig 4.21: Overall performance of power combiner with/without EBG ground Fig 4.22: S-parameters of MMIC amplifier chip: measured vs supplier’s data Fig 4.23: Testing of MMIC amplifier on substrate board condition (a) Equipment setup (b) Testing under wafer probe Fig 4.24: Schematic diagram of DC-Bias circuit for power combiner Fig 4.25: S11 simulation results of power combiner prototype in two different cases Fig 4.26: Assembly of power combiner prototype Fig 4.27: Testing set-up for S11 measurement of power combiner prototype Fig 4.28: Input return loss of prototype: simulated vs measured data Fig 4.29: Testing and equipment set-up for radiation pattern measurement of power combiner prototype (a) Prototype mounted on turn-table in anechoic chamber (b) Antenna analyzer system Fig 4.30: Maximum system gain measurement of power combiner prototype with and without active power divider block Fig 4.31: Relative radiation power measurement of power combiner prototype at GHz design centre frequency Fig 4.32: Relative radiation power measurement of power combiner prototype at GHz and GHz Fig 4.33: Comparison of E-field skew correction in [23] and prototype viii List of Symbols Z0 Characteristic impedance εr Relative dielectric constant εeff Effective dielectric constant S11 Input return loss S-parameter S21 Gain / Forward transmission S-parameter S12 Isolation / Reverse transmission S-parameter S22 Output return loss S-parameter Φ(θ,φ) Radiation intensity as a function of space co-ordinates ΘE Half-power bandwidth in E-plane ΘH Half-power bandwidth in H-plane Γ Reflection coefficient BW Bandwidth CPW Coplanar Waveguide DUT Device Under Test EBG Electromagnetic Bandgap EM Electromagnetic HBT Hetero-junction Bipolar Transistor HPBW Half Power Bandwidth LTCC Low Temperature Co-fired Ceramic MMIC Monolithic Microwave Integrated Circuit RF Radio frequency RFIC Radio Frequency Integrated Circuit SLC Single Layer Chip SMD Surface Mount Device SPC Spatial Power Combining TEM Transverse Electro-Magnetic VSWR Voltage Standing Wave Ratio ix For the maximum system gain evaluation, two sets of measurement are obtained and plotted in Fig 4.30 The first set is the “w/amp” condition measurement in which the power combiner prototype with the active power divider block and the Vivaldi antenna array is tested as a complete unit The second set is the “wo/amp” condition measurement in which a similar Vivaldi antenna array without any active power divider block is tested as a “comparator” With and without the active power divider block, the maximum system gain is differed by about 5.5 dB 15 Max Gain (dB) w/amp wo/amp 10 5 freq (GHz) Fig 4.30 Maximum system gain measurement of power combiner prototype with and without active power divider block When taking the far-field relative radiation power patterns, the prototype is rotated from -88° to +88° with respect to the principle axis of its antenna array Data are collected for both E-plane and H-plane with the co-polarization and the crosspolarization condition Before plotting in the rectangular format for illustrations, the data are computed with respect to the peak data value of the co-polarization radiation 88 power in E-plane For brevity, there are three frequency plots enclosed in this section while the rests are attached in Appendix A These three frequency plots are measurement at GHz, GHz and GHz given in Fig 4.31 and Fig 4.32 6GHz Relative Radiation Power (dB) -10 -20 -30 -40 -90 -60 E-co -30 Angle (deg) E-cross 30 H-co 60 90 H-cross Fig 4.31 Relative radiation power measurement of power combiner prototype at GHz design centre frequency 89 3GHz Relative Radiation Power (dB) -10 -20 -30 -40 -90 -60 E-co -30 Angle (deg) E-cross 30 H-co 60 90 H-cross 8GHz Relative Radiation Power (dB) -10 -20 -30 -40 -90 -60 E-co -30 Angle (deg) E-cross 30 H-co 60 90 H-cross Fig 4.32 Relative radiation power measurement of the power combiner prototype at GHz and GHz 90 4.5 Discussions In this chapter, a novel spatial power combiner is investigated and designed to achieve “broad bandwidth” and “out-of-band stability” targets This power combiner consists of two main blocks: the transmitting block and the active power divider block The transmitting block is made of two miniaturized Vivaldi antennas designed in Section 4.2.2 The active power divider block is the integration of the active EBGcoupler from Section 4.2.3.2, the broadband matching networks from Section 4.2.4 and the DC-bias circuit from Section 4.2.5 Although the broadband power divider has been created in Chapter 3, its bandwidth performance is limited to 52% To make a greater improvement, the “direct coupling” technique is derived from the multilayer line coupling technique in Section 2.3 This coupling technique is then implemented on the design of the active EBG-coupler to achieve the first novelty in the power combiner design For the out-of-band stability in a practical design, the proposed EBG ground slot structure in Section 4.2.3.1 is applied to create the spurious-free characteristic in the power combiner After that, a spatial power combiner prototype is built according to Section 4.3 Then the prototype is tested and verified in Section 4.4 with respect to the simulation results During the small-signal measurement of the prototype, the S11 data is obtained by using the HP8510C network analyzer which is calibrated from 0.5 GHz to 20.5 GHz In Fig 4.28, the measured data follows the trend of the simulated curve but has a slight shift to the higher frequency side The reason is most likely due to the thickness deviation of the 0.127 mm substrate board which is soft and flimsy The measured S11 curve also shows that the input return loss is below -10 dB level from 2.5 GHz to 8.3 GHz Although this -10dB return-loss bandwidth is lower than the simulation, the input characteristic of the power combiner is significantly broad Moreover there is no 91 measured data exceeding above dB across the entire testing frequency range Therefore, based on the stability definition in Section 2.6, this power combiner is stable as well The far-field antenna measurement is conducted in the anechoic chamber with equipments calibrated from GHz to 12 GHz The maximum system gain of the prototype is taken by the antenna analyzer system from GHz to GHz, and is presented at GHz step in Fig 4.30 The curve is peak at GHz while the -3dB gain bandwidth is approximately 5.5 GHz During the design of the input matching network in Section 4.2.4.2, it has been stated that the antenna array has higher gain at the higher frequency and so the amplifier block is purposely designed in favour of lower frequency range to achieve a wider operating frequency As a result, the overall maximum system gain will drop after 8.5 GHz when it is compared to the system without the active power divider block The far-field radiation patterns of the prototype are measured like a normal antenna in the anechoic chamber with calibrated equipments For each testing frequency, the relative radiation power is recorded from -88° to 88° in E-plane and Hplane with the co-polarization and the cross-polarization conditions All curves are processed with respect to the peak value data of the co-polarization in E-plane before consolidating together in each radiation plot Three radiation plots at GHz, GHz and GHz are shown in Section 4.4 while the rest are attached in Appendix A In general, the radiation patterns of this prototype behave like a single Vivaldi antenna As the frequency increases, the HPBW reduces similar to the results in [22] Additionally, the cross-polarization level is lower in H-plane than E-plane Furthermore the radiation patterns vary significantly from frequency to frequency Since the main beam is not well defined, the directivity or the antenna efficiency 92 cannot be computed accurately by using equations 2.15 and 2.17 This phenomenon is partly due to the drawback of miniaturizing the Vivaldi antenna design in Section 4.2.2 More research works can be done in future to get a balance between the antenna size and the main beam pattern To review the proposed idea of the cross-polarization reduction, data from the E-plane cross-polarization at the principle axis are consolidated and compared with [23] in Fig 4.33 below The cross-polarization data of the prototype are not as good as the reference in most frequency points because the Vivaldi antennas are not absolutely parallel in the array However it is obvious that the proposal remains effective to reduce the cross-polarization Ref [23] Prototype dB -10 -20 -30 -40 freq (GHz) Fig 4.33 Comparison of E-field skew correction in [23] and prototype In summary, the novel spatial power combiner prototype has been investigated and designed to operate from GHz to GHz The direct coupling technique and the EBG concept are proposed to achieve the ultra-broadband and the out-of-band 93 stability requirement From the testing results, the prototype shows more than 90% bandwidth at the GHz design centre frequency Moreover there is no oscillation or instability phenomenon across the entire testing range from 0.5 GHz to 20.5 GHz Therefore it can be concluded that the proposal is valid and the objectives of the project have been achieved 94 Chapter Conclusion 5.1 Conclusion In this project, a novel spatial power combiner is investigated and designed to achieve the broad bandwidth and the out-of-band stability targets The design centre frequency of the power combiner is set at GHz while the target bandwidth is from GHz to GHz Moreover, it must be stable from 0.5 GHz to 20.5 GHz The design consists of two main blocks: the transmitting block and the active power divider block At the end, a prototype of this spatial power combiner is built and tested to verify the objectives of this project In Chapter 3, a broadband multilayer active hybrid ring has been investigated and designed to fulfill the requirement of the active power divider block To reduce the radiation interference and the overall PCB size, the active circuit of this power divider must be isolated from the feed lines of antennas Based on the design specifications, the first prototype “Design 1” is created It has a three-layer structure (Fig 3.14) made of two substrate boards stacked together The ground-slot coupling technique is proposed to carry out the vertical signal transition The key components and their designs are stated from Sections 3.2.2 to 3.2.4 From the small-signal measurement results in Fig 3.22, the -3dB gain bandwidth has about 40%, which is close to 44% of the Tee-junction coupler’s bandwidth limit To improve the performance of Design 1, this critical Tee-junction coupler is extracted for further study Since this coupler is a 3-port reciprocal device where all the ports cannot be matched simultaneously, its performance is improved in Section 3.3.1 by using the ‘Wilkinson power divider’ concept Based on the even- and oddmode analysis, a 100 Ω resistor is derived and inserted between the output ports of the coupler This modification leads to Design which is modified from Design and 95 tested similarly Based on the measured results in Fig 3.25, Design has achieved 52% of -3dB gain bandwidth as compared to 40% of Design 1.This improvement is mainly due to the fact that the added resistor helps to absorb the reflected power from the load terminations in the lower circuit of the Tee-junction coupler Besides the bandwidth improvement, Design has also shown a better return-loss performance according to Fig 3.27 The only drawback of this modification is that the lumpcomponent resistor causes a slight drop in the gain level It is because the resistor used has the parasitic effects and the lead connections Further investigation on the coupling technique is necessary if the bandwidth requirement increases to 100% In Chapter 4, a novel spatial power combiner has been investigated and designed to achieve “broad bandwidth” and “out-of-band stability” targets for this project This power combiner consists of the transmitting block and the active power divider block The transmitting block is made of two miniaturized Vivaldi antennas designed in Section 4.2.2 The active power divider block is the integration of the active EBG-coupler from Section 4.2.3.2, the broadband matching networks from Section 4.2.4 and the DC-bias circuit from Section 4.2.5 Although the broadband power divider has been created in Chapter 3, its bandwidth performance is limited to 52% To make a greater improvement, the “direct coupling” technique is derived from the multilayer line coupling technique in Section 2.3 This coupling technique is then implemented on the design of the active EBG-coupler For the out-of-band stability in a practical design, the proposed EBG ground slot structure in Section 4.2.3 is applied to create the spurious-free characteristic in the power combiner After that, the spatial power combiner prototype is built according to Section 4.3 Then this prototype is tested and verified in Section 4.4 against the simulation results 96 During the small-signal measurement of the prototype, the S11 data is obtained from 0.5 GHz to 20.5 GHz According to Fig 4.28, the measured result agrees with the simulated The reason of a slight frequency shift is most likely due to the thickness deviation of the 0.127 mm substrate board which is soft and flimsy The measured S11 curve also shows that the input return loss is below -10 dB level from 2.5 GHz to 8.3 GHz Although this “-10dB return-loss” bandwidth is slightly lower than the simulated, the input characteristic of the power combiner is significantly broad Moreover there is no measured data exceeding above 0dB across the entire testing frequency Therefore according to the stability definition in Section 2.6, this power combiner design is stable as well The far-field antenna measurement is conducted in the anechoic chamber with calibrated equipments The maximum system gain of the prototype is taken by the antenna analyzer system from GHz to GHz, and is presented at GHz step in Fig 4.30 The curve is peak at GHz and the “-3dB gain” bandwidth is approximately 5.5 GHz For the far-field radiation patterns at each frequency, the relative radiation powers of the prototype are presented separately in Fig 4.31, Fig 4.32 and Appendix A In general, the radiation patterns of this prototype behave like a single Vivaldi antenna As the frequency increases, the HPBW reduces similar to the results in [22] Additionally, the cross-polarization level is lower in H-plane than E-plane, and the radiation patterns vary significantly from frequency to frequency Since the main beam is not well defined, the directivity or the antenna efficiency cannot be computed by using equations 2.15 and 2.17 This phenomenon is partly due to the drawback of miniaturizing the Vivaldi antenna design in Section 4.2.2 More research works can be performed in future to get a balance between the antenna size and the main beam pattern 97 In Section 4.5, the proposed idea of the cross-polarization reduction is reviewed and compared to [23] in Fig 4.33 Although the cross-polarization data of the prototype are not as good as the reference in most frequency points, it is obvious that the proposal remains effective In conclusion, the novel spatial power combiner prototype has been investigated and designed to operate from GHz to GHz The direct coupling technique and the EBG concept are proposed to achieve the broadband and the out-ofband stability requirements Through the testing results, the prototype shows more than 90% bandwidth at the design centre frequency of GHz Moreover there is no oscillation or instability phenomenon across the entire testing frequency from 0.5 GHz to 20.5 GHz Therefore, the proposals are valid and the objectives of the project have been achieved 5.2 Proposal for Future Works During the course of this project, there are some restrictions on investigation and design works The first is the in-house fabrication process, which limits the direct coupling distance to 0.127 mm and the slot width to 0.2 mm If the coupler design can be implemented in LTCC technology where the substrate gap and the trace width can be much smaller, the performance of the active coupler and the EBG structure could be improved further On the other hand, the miniaturization of the Vivaldi antenna has reduced the consistency of radiation patterns across a wide range of frequency It needs more research to study their relationship and then balance between the radiation patterns and the size so that the main-beam patterns can be more prominent for combining power in the air 98 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Testing of Prototypes 46 3.4.1 Results of Design 48 3.4.2 Results of Design 50 Discussions 53 Chapter 4: Investigation and Design of Novel Spatial Power Combiner 56 4.1 Investigation 56 4.2 Design of. .. investigation, design and testing results of these prototypes lead to the creation of the novel spatial power combiner in Chapter Chapter reports the investigation and design of the novel spatial power combiner. .. feed-network in the spatial power combiner is replaced by a passive antenna array, this spatial power combiner will become a quasi-optical power combiner Therefore a good spatial power combiner design will