DESIGN OF BROADBAND VECTOR-SUM PHASE SHIFTERS AND A PHASED ARRAY DEMONSTRATOR WINSON LIM Bachelor of Engineering (Hons), NUS A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2013 DECLARATION I hereby declare that the thesis is my original work and it has been written by me in its entirety I have duly acknowledged all the sources of information which have been used in the thesis This thesis has also not been submitted for any degree in any university previously _ Winson Lim 7th September 2013 ii Acknowledgements I thank Dr Koen Mouthaan and Dr Tang Xinyi for their guidance in my research and the MMIC lab for providing the facilities to carry out my research I also thank my family for their kind understanding to allow me to pursue my research studies iii Table of Contents Summary vi List of Tables vii List of Figures viii Chapter - Introduction 1.1 Motivation 1.2 Thesis objectives 1.3 Thesis organization 1.4 List of publications Chapter - Phase Shifters 2.1 Introduction 2.2 Reflection type phase shifters 2.3 Switched network phase shifters 2.4 Loaded-line phase shifters 11 2.5 Vector sum phase shifter 13 2.5 Discussions 14 Chapter - Vector sum phase shifter 16 3.1 Introduction 16 3.2 Literature review of vector sum phase shifter 17 3.3 Fundamental design of an I/Q network vector sum phase shifter 20 3.4 Proposed design and implementation of wideband VSPS at VHF band 21 3.1.1 Experimental setup 26 3.1.2 Experimental results 28 3.5 Conclusions and recommendations 30 Chapter - Vector sum phase shifter (L–Band design) 32 4.1 Introduction 32 4.2 Proposed design and considerations at L-band 33 4.3 Modification of topology using simulation results 34 4.3.1 Experimental setup 37 iv 4.3.2 Experimental results 41 4.4 Conclusions and recommendations 44 Chapter - Phased array antenna system demonstrator 46 5.1 Introduction 46 5.2 Design and implementation of wideband phased array demonstrator 47 5.3 Experimental setup of the phased array demonstrator 48 5.4 Experimental results of the phased array demonstrator 52 5.5 Conclusions and recommendations 54 Chapter - Conclusions and Future Works 56 6.1 Conclusions 56 6.2 Recommendations 59 References 60 Appendix A: (VHF band) Datasheet information for COTS components used 63 Appendix B: (L band) Datasheet information for COTS components used 65 Appendix C: Datasheet information for patch antenna 67 Appendix D: Further readings 69 v Summary Phased array antenna systems provide significant advantages and are used in modern radar and wireless communication systems Phase shifters are one of the key components in electronically steered phased array antennas There are a few phase shifter topologies but the vector-sum phase shifter (VSPS) topology can generally provide a 360º higher resolution performance Vector-sum phase shifters (VSPS) are usually narrowband and the phase and amplitude error increase as the bandwidth widens Designs have been explored to increase the bandwidth of VSPS with low Root-Mean-Square (RMS) phase and amplitude error using commercial-off-the-shelf components on printed circuit board (PCB) In addition, other S-parameters like return losses, gain variation over frequency were also analyzed and improved In this thesis, a wideband VHF VSPS has been designed for a bandwidth ratio of up to 20:1 with desirable RMS phase error for a 3-bit phase shifter The realized example demonstrates a measured input return loss larger than 15 dB, amplitude imbalance less than dB and RMS phase error less than 1.5º from 10 to 100 MHz For the wider frequency range of 10 to 200 MHz, the measured input return loss is larger than 10 dB, amplitude imbalance less than 2.5 dB and RMS phase error less than 5º Broadband L-band VSPS is then also designed to cover the entire L-band and also improve on the S-parameters Measured input and output return losses are larger than 18 dB and 25 dB respectively from to GHz The gain variation is about dB, amplitude imbalance less than 1.5 dB and the RMS phase error less than 2.5º Lastly, a 4-element phased array demonstrator using the L-band VSPS has been built and has successfully demonstrated beam steering capability at 1.8 GHz vi List of Tables Table I Performance comparison of phase shifters using discrete components on PCB 31 Table II Performance comparison of phase shifters using discrete components on PCB 45 vii List of Figures Figure 1-1: Basic scanning array architectures (a) Linear passive array with phase shifters for every element (b) An active array with TRMs at every element (taken from [1]) .2 Figure 1-2: Active array systems architecture Figure 2-1: Block diagram of a typical RTPS Figure 2-2: Block diagram of a SNPS using difference in electrical lengths Figure 2-3: Block diagram of a SNPS using two networks 10 Figure 2-4: Third order high-pass/low-pass phase shifter 10 Figure 2-5: Loaded line phase shifter [31] 11 Figure 2-6: Bandwidth versus the phase shifts for different phase errors and return losses of the lumped element loaded Class III phase shifters [31] 12 Figure 2-7: Block diagram of a typical VSPS 13 Figure 3-1: Block diagram of VSPS with base vectors generation network 17 Figure 3-2: Vector sum phase shifter using polyphase filter network (taken from [48]) .18 Figure 3-3: VSPS realizing 0º and 90º base vectors using APN (taken from [16]) 19 Figure 3-4: Block diagram of a basic I/Q network vector sum phase shifter 20 Figure 3-5: Proposed design of wideband VSPS 21 Figure 3-6: Phase of simulated setup in ADS using ideal blocks 22 Figure 3-7: AD835 Analog multiplier connected as wideband VGA [Appendix A] .23 Figure 3-8: Amplitudes and polarities of bit phase states 24 Figure 3-9: Simulated bit phase shifts .25 Figure 3-10: Simulated RMS phase error .25 Figure 3-11: Photo of the fabricated phase shifter (8.4cm x 7.8cm) 26 Figure 3-12: Photo of the VSPS casing of cm height 27 Figure 3-13: Casing resonances using Sonnet: | | 27 Figure 3-14: Measured input return loss 28 Figure 3-15: Measured phase shifts of bit VSPS 29 Figure 3-16: Measured insertion loss of fabricated phase shifter for phase states 29 Figure 3-17: Simulated and measured RMS phase error 30 viii Figure 4-1: Proposed design of VSPS using analog multiplier circuits 32 Figure 4-2: Analog multiplier circuit (Topology 1) 33 Figure 4-3: Simulated S-parameters of VSPS (Topology 1) 34 Figure 4-4: Simulated RMS phase error of topologies and 35 Figure 4-5: Modified voltage multiplier circuit (Topology 2) 35 Figure 4-6: VSPS using analog multiplier circuit topology and attenuator 36 Figure 4-7: Simulated S-parameters (Topology 2) 36 Figure 4-8: Photo of the modular VSPS using ADL5391 evaluation boards 37 Figure 4-9: Circuit of the fabricated VSPS .38 Figure 4-10: Fabricated PCB substrate for the VSPS (front side) 39 Figure 4-11: Fabricated PCB substrate for the VSPS (back side) 39 Figure 4-12: Photo of the fabricated phase shifter (6.4 cm x 4.3 cm) 40 Figure 4-13: Metallic casing of height cm and lowest box resonance at 4250 MHz 41 Figure 4-14: Measured |S11| and |S22| 42 Figure 4-15: Measured insertion loss of fabricated VSPS for all phase states 42 Figure 4-16: Measured phase shifts of bit VSPS 43 Figure 4-17: Simulated and measured RMS phase error 43 Figure 5-1: Beam steering of main-lobe towards the target of interest 46 Figure 5-2: Block diagram of phased array demonstrator and the 360º VSPS .48 Figure 5-3: Photo of the phased array demonstrator .49 Figure 5-4: Photo of the DAC AD5390 control of analog voltages for beam steering .50 Figure 5-5: Radiation pattern and specifications of ZDAD17002500-8 [Appendix C] .50 Figure 5-6: Interior structure of the patch antenna .51 Figure 5-7: Photo of the array of patch antennas 51 Figure 5-8: Experimental Setup for measurements 52 Figure 5-9: Radiation beam pattern at progressive 0° phase shifts 53 Figure 5-10: Radiation beam pattern at progressive 45° phase shifts .53 Figure 5-11: Radiation beam pattern at progressive 90° phase shifts .54 ix Chapter - Introduction 1.1 Motivation Beam scanning capabilities of phased array antennas provide significant system advantages and thus are used by the military and industry in various modern radars and wireless communication systems for space, airborne, surface and ground-based applications There are generally two types of phased arrays: passive and active phased arrays In passive phased arrays, antenna elements have a central transmitter/receiver (T/R) and a power distribution network There is generally no element amplitude control [1] In active phased arrays, each of the antenna elements or sub-arrays has its own T/R module (TRM) and is able to provide complete flexibility in amplitude and phase control for both transmit and receive Another advantage of the active array is that the system sensitivity is increased because the system noise figure is set and the RF power is generated at the aperture A second advantage of an active array is that the feed networks need not be optimized for lowest loss; thereby allowing design flexibility and the ability to minimize size (volume) and weight Of course, these performance improvements come with increased array complexity and cost Thus comparing with passive arrays, active arrays can provide added system capability and reliability but are generally more complex and expensive As the technology of integrated circuits matures, it is now possible to integrate the whole RF TRM into a single monolithic microwave integrated circuit (MMIC) which reduces the cost of the active phased arrays [2], [3] With the advent of technologies such as CMOS and BiCMOS, reliable low-cost commercial-off-the-shelf (COTS) components, automated assembly of microwave components, and low-cost high-speed high-throughput spacing for phased array antenna elements can be reduced to for example λ/4, the beam steering angle achievable for array VSPS progressive phase shifts of 45º will be 30º However, further analysis can be done to optimize the spacing for wider beam steering angles with lower grating lobe levels One recommendation for future work on the phase array demonstrator is to perform testing at more frequencies to further analyze the behavior of the phased array demonstrator and its beam pattern The phased array demonstrator can be modified to use a different radiating element such as the wideband Vivaldi antenna Based on the design of a Vivaldi antenna, it allows further compactness of the array arrangement which can then increase the maximum resultant beam steering angle possible Lastly, the array of radiating elements can also be varied in terms of the number, orientation and spacing to further study the corresponding beam-forming pattern and behavior 55 Chapter - Conclusions and Future Works 6.1 Conclusions Phase shifters are the most essential elements in active electronic beam-steering systems such as phased-array antennas To further increase the functionality and capability of the phased array antennas, broadband performance is highly desirable However, broadband phase shifters are more difficult to design In addition, phase-shifter critical design parameters for phase shifters include RF insertion loss, phase error, amplitude variation with phase shift, switching times, power-handling capability, and the power required to shift phase Unfortunately, no single type of phase shifter can satisfy the requirements for all these parameters In chapter 2, various phase shifter topologies have been reviewed including the reflection type phase shifter, the switched network phase shifter, the loaded-line phase shifter and the vector sum phase shifters The loaded-line phase shifter has the narrowest bandwidth There is high design flexibility in the switched network phase shifters as various types of high-pass and low-pass networks can be used in the design However to increase the bandwidth, higher order networks are required This increases the number of discrete components which increases the circuit size and intrinsic variations due to tolerances which requires delicate tuning The reflection type phase shifter can achieve a large bandwidth only if both the coupler and the terminations have wide bandwidths The vector sum phase shifter can lead to very compact circuits and provides continuous phase shifts over wide operation bandwidths However, the power consumption and the linearity should also be considered as VGAs and DACs are required for the weighting of the base vectors 56 In chapter 3, a literature review and a discussion of the fundamentals of a typical active vector sum phase shifter is provided The vector sum phase shifter is composed of a base vectors generation network, a power combiner, and control circuits such as VGAs and DACs which set the different amplitude weightings of the base vectors in the power combiner for the necessary phase shifts The commonly used phase difference between the base vectors can be around 90º or around 120º The main advantage of investigating the use of more complex base vector generation networks like the poly-phase filter networks and the all-pass networks is to maximize the potential for more wideband and accurate base vectors phase and amplitude splitting Here, the quadrature hybrid coupler is used for the base vectors generation network although it has a narrower phase shift bandwidth of around an octave or less This is because the quadrature hybrid coupler is less complex and implicitly allows for a similar bandwidth for the input return loss of the vector sum phase shifter With the advent of better quality and low cost analog multipliers, quadrature hybrids and combiners, a wideband phase shifter with a 20:1 bandwidth using the VSPS topology with COTS components on PCB is then presented The design leverages on the improved performance of commercially available COTS components The realized example has a measured input return loss larger than 15 dB, amplitude imbalance less than dB, and RMS phase error less than 1.5º from 10 to 100 MHz For the wider frequency range of 10 to 200 MHz, the measured input return loss is larger than 10 dB, amplitude imbalance less than 2.5 dB and RMS phase error less than 5º Table I has made a performance comparison of previous phase shifters using discrete components on PCB with the presented work This work has shown a wide bandwidth with low RMS phase error and good amplitude balance Chapter presents the L-band vector sum phase shifter using low-cost COTS components The proposed design of the vector sum phase shifter using analog multiplier circuit topology was simulated As the insertion loss and the output return loss were not sufficient, modification of the design were made to improve the scattering parameters performances by introducing a buffer amplifier and an attenuator The modified design was 57 then fabricated and the measured input and output return losses are larger than 18 dB and 25 dB respectively from to GHz The gain variation is about dB, amplitude imbalance less than 1.5 dB and the RMS phase error less than 2.5º The vector sum phase shifter is then placed in a metallic casing with a RS232 connector to supply the phase shifter with the required DC supply voltages With the modified phase shifter performances and the metallic casing, this vector sum phase shifter can be used as a phase shifter block for a broadband phased array antenna system with digital or continuous analog phase control A performance comparison with other phase shifters using discrete components is also provided in Table II This work has shown good performance over the entire L–band with good return losses, low RMS phase error, and low amplitude imbalance Chapter demonstrates a typical phased array antenna system composed of many radiating elements each with a phase shifter Beams are formed by shifting the phase of the signal emitted from or received by each radiating element, to provide constructive/destructive interference to steer the beams in the desired direction In a phase steering system, the beam is positioned electronically by adjusting the differential phase between elements of an array in a predetermined manner A unique beam position corresponds to a progressive phase shift setting in the network feeding the array A low cost L–band 4-element phased array demonstrator is then proposed and implemented using the L–band vector sum phase shifter The phased array demonstrator with an array of four patch antennas has demonstrated beam steering capabilities at 1.8 GHz At VSPS progressive phase shifts of 0º, 45° and 90°, beam steering angles of 0º, 6.7° and 13.4° are achieved and the main reason for the small beam steering angles is largely due to the large centre-to-centre spacing between each patch antenna element 58 6.2 Recommendations Although the implemented vector sum phase shifters have adequate performance for integration into a phased array demonstrator, there are several recommendations for future research The phased array demonstrator has only demonstrated beam steering capability at 1.8 GHz More frequencies should be tested to further analyze the behavior of the phased array demonstrator and its beam pattern for other frequencies The phased array demonstrator can be modified to use a different radiating element, such as the wideband Vivaldi antenna This allows further compactness of the array arrangement which can then increase the maximum resultant beam steering angle The array of radiating elements can also be varied in terms of number, orientation, and spacing to further study the corresponding beam-forming pattern and behavior The VHF and L–band vector sum phase shifter can also be explored further using a more complex but wideband I/Q network to achieve ultra-wideband VSPS phase shifting performance of bandwidth ratio greater than 20:1, better return losses and a flatter gain 59 References [1] D Parker, and 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components used http://www.minicircuits.com/pdfs/QCN-19.pdf http://www.minicircuits.com/pdfs/SYPS-2-252+.pdf 65 http://www.analog.com/static/imported-files/data_sheets/ADL5391.pdf http://www.minicircuits.com/pdfs/TC1-1-13M+.pdf 66 Appendix C: Datasheet information for patch antenna http://www.zdacomm.com/images/PDF/ZDADI7002500-8.pdf 67 http://www.analog.com/static/imported-files/data_sheets/AD5390_5391_5392.pdf 68 Appendix D: Further readings [1] [2] [3] [4] [5] D W Kang, S Hong, “A 2-10 GHz digital CMOS phase shifter for ultra-wideband phased array system,” IEEE Radio Frequency Integrated Circuits Symposium, 2007 H Erkens, R Wunderlich, and S.Heinen, “A comparison of two RF vector-sum phase shifter concepts,” German Microwave Conference, 2009 Y Zheng and C Saavedra, “Full 360º vector-sum phase shifter for microwave system applications,” IEEE Trans on Circuits and Systems-I: Regular papers, vol 57, no 4, Apr 2010 K J Kohl and G M Rebeiz, “A 6–18 GHz 5-bit active phase shifter,” IEEE MTT-S Intl Microwave Symp Digest, pp 792–795, 2010 H Nosaka, M Nagatani, S Yamanaka, K Sano, K Murata and T Enoki, “Voltage-controlled phase shifter in a surface mount package for 40- and 100-Gb/s optical transceivers,” Compound Semiconductor Integrated Circuit Symposium, 2010 69 ... types of phased arrays: passive and active phased arrays In passive phased arrays, antenna elements have a central transmitter/receiver (T/R) and a power distribution network There is generally... diagram of phased array demonstrator and the 360º VSPS .48 Figure 5-3: Photo of the phased array demonstrator .49 Figure 5-4: Photo of the DAC AD5390 control of analog voltages for beam... analysis of broadband phase shifters like the vector- sum phase shifter which allows high phase resolution with low RMS phase errors Such broadband vector- sum phase shifters are then designed and improved