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Ultra-Wideband Pulse Doppler Radar for Short-Range Targets

ProQuest Dissertations and Theses, 2011
Dissertation
Author: Nicola Jean Kinzie
Abstract:
This thesis addresses the design and characterization of a pulse Doppler radar designed to detect targets at short range (R ≤ 7 m). To minimize the shortest detectable range, a subnanosecond transmitted pulsewidth is desired. UWB design techniques were combined with a pulse Doppler radar architecture to demonstrate a full radar, including the transmitter, receiver, simulated channel, and post processor. The transmitted pulse train has a 2.5GHz carrier frequency, a 730 ps pulsewidth, and a 1 GHz 10 dB-bandwidth. The PRF of the radar is 20 MHz, which allows unambiguous range and Doppler detection with a single PRF. The peak transmitted power is 1.2W. The characteristics of the transmitted waveform provide fine range accuracy ( δR = ±0.03 m), facilitate a short minimum range, and allow for an efficient transmitter design. The receiver was designed to complement the transmitter; it has a homodyne architecture and is pulsed to isolate a specific detectable range. A closed-loop channel model was designed to simulate the range delay, Doppler shift, and channel attenuation of a moving target; the model is connected to the transmitter and receiver with coaxial cable, facilitating bench-top characterization of the radar and eliminating some effects of wireless transmission, such as multipath. Extensive closed-loop radar testing was performed, and the following radar characteristics were determined: (1) The minimum detectable SNR, assuming a 36.5 μs integration time, is 0 dB. (2) Assuming a transmitter-to-receiver isolation of 80 dB, the minimum range of the radar is Rmin = 1.3 m + Rlk , where Rlk is the apparent leakage range between the transmitter and receiver. Depending on the antenna system design, the radar can detect targets from 1.5 m [Special characters omitted.] R ≤ 7 m, meeting the original goal of this work. These results support the supposition that a UWB pulse Doppler radar architecture can be employed for short-range, moving target detection.

This thesis entitled: Ultra-Wideband Pulse Doppler Radar for Short-Range Targets written by Nicola Jean Kinzie has been approved for the Department of Electrical,Computer,and Energy Engineering Zoya Popovi´c Dejan Filipovi´c Date The final copy of this thesis has been examined by the signatories,and we find that both the content and the formmeet acceptable presentation standards of scholarly work in the above mentioned discipline.

Kinzie,Nicola Jean (Ph.D.,Electrical Engineering) Ultra-Wideband Pulse Doppler Radar for Short-Range Targets Thesis directed by Professor Zoya Popovi´c Abstract This thesis addresses the design and characterization of a pulse Doppler radar designed to detect targets at short range (R ≤ 7m).To minimize the shortest detectable range,a subnanosecond transmitted pulsewidth is desired.UWB design techniques were combined witha pulse Doppler radar architecture todemonstrate a full radar,includingthe transmitter, receiver,simulated channel,and post processor. The transmitted pulse train has a 2.5GHz carrier frequency,a 730ps pulsewidth,and a 1GHz 10dB-bandwidth.The PRF of the radar is 20MHz,which allows unambiguous range and Doppler detection with a single PRF.The peak transmitted power is 1.2 W.The characteristics of the transmitted waveformprovide fine range accuracy (δR = ±0.03m), facilitate a short minimumrange,and allowfor an efficient transmitter design.The receiver was designed to complement the transmitter;it has a homodyne architecture and is pulsed to isolate a specific detectable range. Aclosed-loop channel model was designed to simulate the range delay,Doppler shift, and channel attenuation of a moving target;the model is connected to the transmitter and receiver with coaxial cable,facilitating bench-top characterization of the radar and eliminating some effects of wireless transmission,such as multipath.Extensive closed-loop radar testing was performed,and the following radar characteristics were determined:(1) The minimumdetectable SNR,assuming a 36.5 µs integration time,is 0 dB.(2) Assuming a transmitter-to-receiverisolationof 80 dB,theminimumrangeof theradaris R min = 1.3 m+R lk , where R lk is the apparent leakage range between the transmitter and receiver.Depending on the antenna systemdesign,the radar can detect targets from1.5 m R ≤ 7 m,meeting iii

the original goal of this work.These results support the supposition that a UWB pulse Doppler radar architecture can be employed for short-range,moving target detection. iv

Dedication To Matthewand waiting for two cookies [1].

Personal Acknowledgments I have been fortunate to share the last four and a half years with some fantastic people, both in and out of the electromagnetics group.While many people have contributed to my experience at CU,I would like to thank a fewpeople in particular:Evan Cullens for bringing a little bit of Kansas to the mountains;Negar Ehsan and Mabel Rami´rez for some great girl talk;Kendra Kumley for much needed coffee breaks;Erez Falkenstein,Mike Roberg,and Rob Scheeler for bringing some...humor to the lab;Dr.Mike Elsbury,Dr.John Hoversten,Dr.Luke Sankey,andRebecca Sankey for making me feel welcome andincluded fromthe very beginning;Joseph Mruk for always making me smile;Dr.Randy Direen for enthusiastic lunch breaks;Dr.Charles Dietlein for teaching me the ropes;Jonathan Chisum for many great conversations about science fiction,fantasy,engineering,and life.I would like to thank Dan Kuester for helping me through my toughest semester at CUand being a fantastic friend ever since.Finally,I would like to thank Kirsten Farnsworth and Evan Sheehan for bringing some music,entertainment,and fun into my life. I would like to thank my parents who have always been supportive.Who bought me newbooks when I begged on every shopping trip.Who logged hours on the road driving me to music lessons,concerts,and camps.Who barely flinched when I announced I was going to attend that “other” university in Kansas and even put a purple tag on the front of my pickup.Who drove the Macksville–Boulder–Albuquerque triangle many,many times helping me move to and fromAlbuquerque each summer.My parents taught me to always do work I was proud of and to choose a career that I would enjoy doing.I would not be vi

who I amwithout their love and guidance.Thanks,Momand Dad. I would like to thank my little sister,Paige,for always being there to listen,to share a good book with,or to shop for black sheep.In many ways,no one understands me better than Paige.Many times graduate school left me wanting to scream,“That’s not fair!”,and Paige knows that the correct response is “Fair as chocolate cake and elephant’s knees.” Thanks,Paige,for always being there and for being such a great friend. Finally,I would like to thank MatthewMartin.Seven years ago we started dating and assumed the relationship would end when he left our alma mater for Cornell University; little did we knowthat we would be dating long distance for over six years.Despite the distance and the accompanying hardships,Matt has been incredibly supportive,both emotionally and professionally,throughout my tenure at CU.He’s encouraged me to challenge myself and helped me cope when I’ve taken on challenges that seemed too big. He’s my best friend,and I can’t wait to start the next chapter of my life with him.Thank you,Matt.The best is yet to come. vii

Professional Acknowledgments I would like to thank my advisor,Dr.Zoya Popovi´c,for the opportunity to be a part of her research group.She has brought together an excellent group of students and put together a fantastic microwave lab.I would especially like to thank her for taking a chance on a newstudent who came in with a potential project in mind.Prior to starting graduate school,I spent eight months interning in a radar group at Sandia National Laboratories. Fromday one I was hooked and knewI wanted to work there upon graduation,so when I learned they were interested in funding a graduate student to research ultra-wideband radar,I jumped at the opportunity.After hearing the details of the project,Zoya graciously agreed to advise me.Through the course of the project,I have had to the opportunity to spend my summers at Sandia,working on my thesis project.Had Zoya not been willing to take a chance on something new,I would not have had the opportunity to stay actively involved in the company I wanted to work for. I would also like to thank my other committee members:Dr.TomDeGrand,Dr.Dejan Filipovi´c,Dr.EdKuester,andDr.Chris Rodenbeck.I appreciate the time they have invested during these last months of my studies,reading and evaluating my thesis and making suggestions regarding my work.I also appreciate the time they have invested in my education,both in and out of the classroom.They have taught me invaluable technical skills and have fostered my desire to continually pursue newknowledge. viii

I would like to thank Sandia National Laboratories for funding my thesis project. 1 I would like to specifically thank a few individuals:John Dye for opening the door to a young engineer;Dr.Luke Feldner for introducing me to microwave engineering and helping me findmy path;TeddRohwer andDr.Chris Rodenbeck for having faith in me and supporting me throughout my graduate career;Clint Haslett,Ray Ortiz,and Dennis Wilder for technical support in the lab;and Brian Duverneay,AdamFerguson,Rick Heintzleman, Rick Knudson,Jesse Lai,and Jeff Pankonin for mentoring me and providing excellent technical advice and support. I would like to thank the administrative staff at CU for their time and assistance, especially Jarka Hladisova and AdamSadoff.I would like to thank the U.S.Department of Education for funding Prof.Popovi´c’s Graduate Assistance In Areas of National Need (GAANN) fellowship proposal,which funded my first year of graduate research.Finally,I would like to thank Lincoln Laboratory for providing additional funding through their graduate fellowship. 1 Sandia National Laboratories is a multi-programlaboratory managed and operated by Sandia Corpo- ration,a wholly owned subsidiary of Lockheed Martin Corporation,for the U.S.Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. ix

Contents 1 Introduction 1 1.1 Radar Systems 1 1.1.1 Transmitter 2 1.1.2 Antenna System 8 1.1.3 Receiver 11 1.2 Radar Range Equation 12 1.2.1 MaximumRange,Receiver Sensitivity,andDynamic Range 13 1.2.2 Receiver SNR 14 1.2.3 Radar Cross Section 15 1.2.4 Noise Figure 17 1.3 Radar Applications 18 2 Short-Range Radar 21 2.1 Short-Range Radar Parameters 21 2.1.1 Range Accuracy and Resolution 21 2.1.2 Doppler Accuracy and Resolution 22 2.1.3 Radar Uncertainty Principle 24 2.2 Short-Range Radar Architectures 24 2.2.1 Frequency-Modulated Continuous-Wave Radar 24 2.2.2 Pulse Doppler Radar 27 2.2.3 Pulse Compression Radar 29 x

2.2.4 Short-Range Radar Architecture Trade-Offs 30 2.3 Short-Pulse Doppler Radar Parameters 33 2.3.1 Ultra-Wideband Systems 37 2.4 ANote On Units 42 3 UWB Pulse Doppler Radar Architecture 43 3.1 Transmitter Architecture 45 3.2 Receiver Architecture 48 3.3 Antenna System 49 3.4 Digital Control 49 3.5 Post Processor 51 3.5.1 Number of Samples 53 3.5.2 Number of Samples and Integration Bandwidth 55 3.5.3 Integration Bandwidth 56 3.5.4 Number of Samples and Signal Frequency 56 3.5.5 Number of Sample Sets 57 3.6 Channel 58 3.6.1 Limitations of Closed- and Open-Loop Radar Testing 60 3.7 Theoretical Radar Characteristics 61 3.7.1 Sensitivity 62 3.7.2 MinimumRange and MinimumTX-RX Isolation 68 3.7.3 MinimumTX-RX Isolation 69 3.7.4 Out-of-Range Ambiguity Resolution 70 xi

4 UWB Pulse Generator 71 4.1 Pulse Generator Requirements 71 4.1.1 Output Pulse Shapes 71 4.1.2 Pulse Doppler Radar Requirements 73 4.1.3 Circuit Technology 73 4.2 UWB Pulse Generator Technologies 74 4.2.1 SRDPulse Generators 74 4.2.2 Passive Pulse Generators and Pulse-Shaping Circuits 78 4.2.3 Digital Pulse Generators 81 4.2.4 Transistor-Based Pulse Generators 83 4.2.5 Nonlinear Transmission Lines 84 4.2.6 UWB Pulse Generator Trade-Offs 85 4.3 Varactor-Diode PCC Design 89 4.3.1 Pulse Shape 91 4.3.2 PCC Design 92 4.4 Varactor-Diode PCC Characterization 93 4.4.1 PCC Operation for UWB Radar 95 5 UWB Transmitter 98 5.1 UWB Transmitter Components 99 5.1.1 PCC and Driver Circuitry 100 5.1.2 VCOand Modulation 102 5.1.3 Upconverter and Switched PA 108 5.2 Transmitter Simulation Model 112 5.3 Transmitter Performance 115 6 UWB Receiver 117 xii

6.1 UWB Receiver Components 117 6.1.1 Range Gate 118 6.1.2 RF LNA 120 6.1.3 Downconverter and IF LNA 121 6.1.4 Matched Filter 121 6.2 Receiver Simulation Model 122 6.3 Receiver Performance 124 6.4 Receiver and Post Processor 127 7 UWB Antenna System 129 7.1 UWB Antenna Types 129 7.2 UWB Antenna System Considerations 131 7.2.1 Transfer Function and Impulse Response 131 7.2.2 Temporal Differentiation of V TX 138 7.2.3 Compensation Techniques 139 7.3 UWB Antenna System 141 7.3.1 Antenna Design 141 7.3.2 Measured Antenna Pattern 143 7.3.3 Measured Antenna SystemIsolation 147 7.3.4 Measured Transfer Function 148 7.3.5 Measured Time-Domain Behavior 150 7.3.6 Improvements to Antenna System 150 7.3.7 Antennas in SystemModel 152 8 Closed-Loop UWB Radar Testing 153 8.1 Closed-Loop Channel Model 153 8.1.1 Components 154 xiii

8.1.2 Channel Losses 156 8.2 UWB Radar Setup Considerations 158 8.2.1 Timing 158 8.2.2 Leakage Signals 160 8.3 Radiative and Channel Model Feed-Through Leakage 165 8.4 Single-Pulse SNRs and Radar Losses 168 8.4.1 Signal Power 169 8.4.2 Noise Power 172 8.5 Coherent Processing Interval 175 8.6 Sensitivity and MinimumDetectable signal-to-noise ratio (SNR) 177 8.6.1 Radar Simulation Model 182 8.7 MinimumDetectable Range and MinimumTX-RX Isolation 184 8.8 Range Ambiguity Resolution 191 8.9 Summary 194 9 Future Work 195 9.1 Open-Loop Radar Testing 195 9.1.1 Open-Loop Test Setup 1 196 9.1.2 Open-Loop Test Setup 2 200 9.1.3 Future Open-Loop Testing 204 9.2 Potential SystemImprovements 205 9.2.1 Coherent Processing Interval and local oscillator (LO) Isolation 205 9.2.2 Efficiency and direct current (DC) Power Consumption 206 9.2.3 Sensitivity 208 xiv

9.3 Application-Specific System Improvements 209 9.3.1 Advanced Receiver and Post Processor Architectures 209 9.3.2 Antenna System 211 9.4 Radar Testing 211 9.5 RF Circuit Integration 214 9.5.1 Short-Pulse UWB Circuits 214 9.5.2 UWB Circuits 220 9.5.3 Narrowband Circuits 222 9.5.4 Packaging 225 9.6 Summary and Contributions 226 Bibliography 229 Acronyms and Abbreviations 251 Appendix A:MATLAB Signal Processing Script 256 xv

List of Tables 1.1 IEEE Standard RF Letter-Band Nomenclature 3 2.1 Doppler Accuracy 23 2.2 Short-Range Radar Trade-Offs 32 3.1 FPGAOutputs 50 3.2 Signal Processing Variables 54 3.3 Desired UWB Pulse Doppler Radar Parameters 63 3.4 Measured UWB Transmitter Parameters 64 3.5 Measured UWB Receiver Parameters 64 3.6 Expected UWB Pulse Doppler Radar Parameters 65 3.7 Measured UWB Pulse Doppler Radar Parameters 65 4.1 UWB Pulse Generator and Pulse-Shaping Circuits 86 4.2 UWB Pulse Generator and Pulse-Shaping Circuits 88 4.3 PCC LC Sections 94 4.4 PCC Pulsewidth 95 4.5 PCC Peak Voltage 95 5.1 Turn-Off Characteristics of Transmitter 115 7.1 Single-Element UWB Antennas 130 7.2 Antenna Design 141 7.3 TX-RX Isolation of Antenna System 148 xvi

8.1 Channel Losses:Spherical Target with Radius D max /2 157 8.2 Channel Losses:MetallicPlateTarget withWidthandHeight of D max 157 8.3 Channel Attenuation and Full-Scale Voltage of Digitizer 167 8.4 RadiativeandChannel-Model Feed-ThroughLeakageMeasurements 167 8.5 Theoretical and Measured Single-Pulse SNRs (N = 1E6) 169 8.6 Theoretical and Measured Signal Power (N = 1E6) 170 8.7 Sampling Loss 171 8.8 Theoretical and Measured Noise Power (N = 1E6) 172 8.9 Processing SNR Gain 177 8.10 Measured Single-Pulse SNRs,Signal Power,and Noise Power 178 8.11 Processed SNRs,Signal Power,and Noise Power (N = 730) 182 8.12 Single-Pulse SNR with Range Gate Closed (N = 730) 191 8.13 Out-of-Range Ambiguity Rejection Ratio (N = 730) 192 9.1 Theoretical and Measured Results for Open-Loop Test Setup 1 199 9.2 Theoretical and Measured Results for Open-Loop Test Setup 2 204 9.3 UWB Pulse Doppler Radar Parameters 226 xvii

List of Figures 1.1 Radar SubsystemDiagram 2 1.2 Bandwidth Definitions 5 1.3 Pulsed Signal 6 1.4 Power Amplifier Transmitter 9 1.5 Antenna Systems 10 1.6 Basic Receiver Architecture 12 1.7 Scattering Mechanisms 17 2.1 Range Resolution 23 2.2 Basic FMCWArchitecture and Modulation 26 2.3 Basic Pulse Doppler Architecture and Signal 28 2.4 Basic Pulse Compression Radar Architectures 30 2.5 Pulse Doppler Radar Design Parameters 35 2.6 Pulse Doppler Radar Blind Zone 38 3.1 System-Level Block Diagram 44 3.2 Transmitter Block Diagram 45 3.3 Receiver Block Diagram 49 3.4 Impact of Number of Cycles on PSDCalculation 57 3.5 Closed-Loop Channel Model 59 3.6 Open-Loop Channel Model 59 xviii

3.7 Single-Pulse SNR,Probability of False Alarm,and Probability of Detec- tion 66 4.1 Pulse Shapes 72 4.2 SRDPulse Sharpener 75 4.3 SRDPulse Sharpener Signals 76 4.4 SRDPulse Generator 77 4.5 SRDPulse Generator 77 4.6 Passive Pulse-Shaping Circuit 79 4.7 Passive Pulse-Shaping Circuit 80 4.8 Digital Pulse Generator 81 4.9 Digital Pulse Generator 82 4.10 Digital Pulse Generator 83 4.11 Digital Pulse Generator 84 4.12 NLTL Model 85 4.13 PCC 90 4.14 Varactor and Inductor Models 94 4.15 PCC DC Bias 96 4.16 PCC Output for UWB Radar 97 5.1 Transmitter Block Diagram 98 5.2 PCC Driver Circuitry 101 5.3 VCOCircuitry 103 5.4 RX VCOPath Output 104 5.5 BPSK Modulator Circuitry 106 5.6 BPSK Modulator Output 107 5.7 Switch Output 109 5.8 Upconverter Output 111 xix

5.9 Switched PACircuitry 112 5.10 Time-Domain Transmitter Output 113 5.11 Frequency-Domain Transmitter Output 114 6.1 Receiver Block Diagram 118 6.2 Range Gate Circuitry 119 6.3 Matched Filter 123 6.4 Receiver Gain 125 7.1 Antenna SystemSetup 132 7.2 Practical Transfer Function 137 7.3 Temporal Differentiation of Transmitted Waveform 140 7.4 Elliptically-Tapered Antipodal Slot Antenna 142 7.5 Copole Antenna Patterns 145 7.6 45 ◦ Antenna Patterns 146 7.7 Antenna Coupling Measurement Setup 147 7.8 Transfer Function and Return Loss 149 7.9 Time-Domain Antenna SystemBehavior 151 7.10 Antenna Model 152 8.1 Closed-Loop Channel Model 153 8.2 Physical Closed-Loop Channel Model 155 8.3 Range Gate Timing 159 8.4 Closed-Loop Radar SystemTest Bench 164 8.5 Radiative Leakage 166 8.6 Radar Loss Test Setup 171 8.7 PSDs for L ch = 70 dB and L ch = 90 dB 174 8.8 PSDfor L ch = 95 dB 175 8.9 Measured and Simulated PSDs for L ch = 70 dB and L ch = 90 dB 180 xx

8.10 Measured and Simulated PSDs for L ch = 105 dB 181 8.11 Time-Domain SNR Response Measurements 186 8.12 Time-Domain SNR Response Measurements 187 8.13 Time-Domain SNR Response 189 8.14 Time-Domain SNR Response 190 8.15 PSDfor In- and Out-of-Range Targets with L ch = 70 dB 193 9.1 Open-Loop Test Setup 1 197 9.2 PSDfor Open-Loop Test Setup 1,L ch = 77.1 dB,f D = 50 kHz 200 9.3 Open-Loop Test Setup 2 202 9.4 Updated Transmitter and Receiver Block Diagram 207 9.5 Channelized Post-Processor Design 212 9.6 Channelized Receiver Design 213 9.7 Range-Tracking Post Processor 213 9.8 Closed-Loop Channel Model with Multiple Channels 215 9.9 Integrated NLTL-Based PCC 217 9.10 Stepped-Impedance,Coupled-Line Filters 223 9.11 Stepped-Impedance,Coupled-Line Filter S-Parameters 224 9.12 Stepped-Impedance,Coupled-Line Filter Group Delay 225 xxi

Chapter 1 Introduction This thesis presents a pulse Doppler radar that utilizes ultra-wideband (UWB) techniques to facilitate short-range target detection.To better present the requirements that drive the design,we begin with an overviewof radar. 1.1 Radar Systems Radar,which was originally an acronymfor RAdio Detection And Ranging,has a rich history dating back to Heinrich Hertz’s classical experiments in the 1880’s [2].Today,radar systems exist for a variety of applications fromweather observation to guidance systems and lawenforcement.In its simplest form,a radar systemconsists of three subsystems:a transmitter,a receiver,and an antenna system,as illustrated in Figure 1.1 1 .The transmitter generates an electrical signal that is radiated by the antenna system.If the signal is incident on a target,such as an airplane,rain,or a bird,it will be partially reflected back to the radar systemand incident on the antenna system.The received signal will be routed by the antenna system to the receiver.The receiver processes the signal to determine the presence of a target,as well as target characteristics,such as range and velocity.Avariety 1 The nomenclature TX and RX will be used in this thesis for the transmitter and receiver subsystems, respectively. 1

Figure 1.1:Radar Subsystem Diagram.A radar system consists of three subsystems:a transmitter,a receiver,and an antenna system.The radar system is used to detect the presence of a target,as well as characteristics of the target. of design choices exist for each subsystem,and the primary subsystemcharacteristics are summarized in the following sections. 1.1.1 Transmitter The transmitter’s purpose is to generate an electrical signal that in transmitted by the antenna system,reflected froma target,and received by the antenna system.It can then be processed by the receiver to determine target characteristics,such as range and velocity. As such,the transmitter specifications focus on the desired transmitted waveform,and the transmitter hardware is designed to generate the specified waveform.Waveform characteristics and transmitter technologies are presented in the following sections. Transmitted Waveform:Frequency Domain Radar systems operate over a wide range of frequencies in the microwave regime,often considered to be between 300 MHz and 300 GHz [3].In the past,most operational systems were designed in the 100MHz to 36GHz range;however,systems exist that operate at frequencies as low as a few megahertz and up to the millimeter-wave regime,where 2

Table 1.1:IEEE Standard RF Letter-Band Nomenclature.(Adapted from[2]) Band Designation Nominal Frequency Range HF 3–30MHz VHF 30–300 MHz UHF 300–1000MHz L 1–2GHz S 2–4GHz C 4–8GHz X 8–12GHz K u 12–18 GHz K 18–27 GHz K a 27–40 GHz V 40–75 GHz W 75–110 GHz mm 110-300GHz wavelengths are on the order of a millimeter [2].Impulse,or carrier-free,radars operate down to frequencies on the order of 1MHz [4] and light detection and ranging (LIDAR) systems operate in the optical regime [5]. The microwave spectrumis subdividedinto bands,as notedinTable 1.1.Transmissionin the electromagnetic (EM) spectrumis regulated by government bodies,such as the Federal Communications Commission (FCC) in the United States.A radio license is required to operate a microwave systemin most of the EMspectrum;notable exceptions are the industrial,scientific,and medical (ISM) bands,which are 902–928 MHz,2.400–2.484 GHz, and 5.725–5.850 GHz in the United States,and the UWB band,which is 3.1-10.6 GHz in the United States [3].While a licence is not required,it is important to note that explicit rules exist for transmission in the ISMand UWB bands,especially related to the allowed power densities. Microwave signals can be characterized by their carrier,or center,frequency and bandwidth.The carrier frequency is often defined as the frequency in the middle of the transmission band.For example,the carrier frequency could be 1.5GHz for a radar operatinginthe 1–2 GHz L-band.The bandwidthdescribes the range of frequencies covered 3

by the microwave signal and can be defined in a variety of ways: • 3-dBBandwidth.The 3-dBbandwidthof a bandpass signal is definedby the half-power points of the signal spectrum.If f h is the upper half-power corner frequency and f l is the lower half-power corner frequency of the spectrum,then the 3-dB bandwidth is β 3dB = f h − f l ,as illustrated in Figure 1.2a.In this thesis,the 3-dB bandwidth of a low-pass signal will be given based on the double-sided signal spectrumor the full-width half-maximum (FWHM) bandwidth.In other words,if the half-power corner frequency is f l ,then β 3dB = 2f l ,as illustrated in Figure 1.2b. • 10-dB Bandwidth.The 10-dB bandwidth is defined like the 3-dB bandwidth,except the corner frequencies are taken at the -10 dB points of the normalized signal power spectrum. • Fractional Bandwidth.The fractional bandwidth of a bandpass signal is defined as ( f h − f l )/f c ,where f c is the center frequency of the signal and is defined as ( f h + f l )/2. The corner frequencies can be selected as desired.In this thesis,the 3-dB and 10-dB fractional bandwidths will be used,where the corner frequencies are selected as the -3 dB and -10dB points,respectively. • Bandwidth Ratio.The bandwidth of a bandpass signal can be defined as the ratio of the upper to lower corner frequency,or f h /f l :1.Abandwidth ratio of 2:1 corresponds to an octave;a bandwidth of 10:1 corresponds to a decade. • Effective Bandwidth.The effective bandwidth,or the root meansquare (rms) bandwidth, is defined as β 2 e f f =

∞ −∞ (2πf ) 2 |S( f )| 2 df

∞ −∞ |S( f )| 2 df (1.1) where β e f f is the effective bandwidth,f is frequency,and S( f ) is the double-sided, baseband signal spectrum [6].It is used when calculating radar accuracies,as in Section 2.1.1. 4

(a) (b) Figure 1.2:Bandwidth Definitions.The 3-dB bandwidth of a bandpass and low-pass signal are illustrated in (a) and (b),respectively. Transmitted signals are classified as narrowband,wideband,or UWB based the signal bandwidth.Anarrowband signal has up to 1%10-dB fractional bandwidth;a wideband signal has between 1% and 20% 10-dB fractional bandwidth [7];and a UWB signal has greater than 20%fraction bandwidth [8].Most conventional radar systems are narrowband [9]. 5

Figure 1.3:Pulsed Signal.Apulsed signal is defined by its carrier frequency,PRF,and duty cycle,where the PRF equals 1/T and the duty cycle equals τ/T. Transmitted Waveform:Time Domain It is also important to consider the time-domain characteristics of the transmitted radar signal.The radar signal can be a continuous-wave (CW) waveformor a pulsed waveform. ACWtransmitter broadcasts a continuous radio frequency or radar frequency (RF) signal, while a pulsed transmitter broadcasts a train of RF pulses with a system-specific carrier frequency,pulse repetition frequency (PRF),and duty cycle.The PRF is the frequency at which the RF pulses are transmitted and is equal to 1/T,where T is the time between transmitted pulses,as shown in Figure 1.3.The duty cycle is defined as the ratio τ:T,where τ is the transmitted pulsewidth. CW radar systems are generally simpler than pulsed radars in terms of hardware and signal control since they are always on.However,the design of CWradars is complicated due to significant disparity between the transmitted and received power levels;the power ratio can be can be on the order of P TX :P RX = 10 9 ,making detection difficult.In a pulsed system,the transmitter and receiver are never on simultaneously,making it easier to detect a target return at the expense of increased hardware and signal complexity. A pulsed radar signal can be incoherent or coherent.To be coherent there must be a 6

deterministic phase relationshipfor the carrier frompulse topulse.This canbe accomplished by switching a CWcarrier on and off. Transmitted Waveform:Power The transmitted power level is application dependant.The required power level will depend on a variety of criteria including:the selected duty cycle,the range to the target, the radar cross section (RCS) of the target,the antenna system,the receiver characteristics, and the transmission environment.The relationship between the transmitted power level and these criteria will be discussed in Section 1.2 in the context of the radar range equation. Transmitted Waveform:Modulation Both CWand pulsed transmitters can include waveformmodulation,which can be phase modulation,frequency modulation,amplitude modulation,or a combination of modulation types.For pulsed systems,the modulation can be applied within each pulse over the time period τ,so the modulation varies throughout the pulse.Alternately,the modulation can be constant over each individual pulse;in this case,the modulation is often referred to as pulse tagging.Waveformmodulation will be discussed in more detail in Chapter 2 as it relates to the radar described in this work. Transmitter Technologies Transmitter architectures can be divided into two categories:power oscillator transmitters and power amplifier transmitters [10].Power oscillator transmitters typically employ a magnetron,or similar device,to generate the transmitted signal directly.Apower amplifier transmitter generates the RF signal at low power using an oscillator and amplifies the signal with a power amplifier or a set of power amplifiers.Power amplifier transmitters can be constructed using vacuumtubes or solid-state devices. 7

Power amplifier transmitters exhibit advantages over power oscillator transmitters in terms of stability,since a lower-power LOcombined with a power amplifier (PA) can be designed with better stability than a high power oscillator.As such,power amplifier transmitters are better suitedfor coherent radar systems [11].Apower amplifier transmitter will be used in this thesis work. The component technology also impacts the capabilities of the transmitter.Tube-based devices are often used for high power applications,as they can produce 1kWto 1MW average power.Solid-state devices,such as transistors,are typically used for lower power applications.Single transistors can achieve up to few hundred watts at S-band,and transistor amplifier arrays have been demonstrated at kilowatt power levels.Solid state devices are of particular interest in radar transmitters because the devices have a long mean time between failure,leading to higher systemreliability.In addition,solid-state design lends itself to modular construction,simplifying the initial systemdesign and allowing for easy systemmaintenance.Finally,solid state devices operate at lower voltages than tube-based devices and have lownoise and good stability [10].This thesis work focuses on a low-power design (P TX p 1 W),so solid-state devices will be used. Two examples of simple power amplifier transmitter architectures are illustrated in Figure 1.4.Part (a) illustrates a homodyne architecture where a baseband signal modulates the RF LO signal through a mixing stage.The prefix “homo-” indicates that a single upconversion stage is utilized.Part (b) illustrates a heterodyne architecture where the baseband signal undergoes two stages of upconversion,with two different LOs,resulting in an RF carrier equal to the sumof the LOfrequencies.The prefix “hetero-” indicates that two or more upconversion stages are utilized. 1.1.2 Antenna System There is a great deal of variety in antenna systemdesign,and the antenna systemspecifica- tions depend on the application.Here we will focus on following variables:the antenna 8

(a) (b) Figure 1.4:Power Amplifier Transmitter.A homodyne transmitter is shown in (a),and a heterodyne transmitter is shown in (b). pattern,the number of antennas,and the antenna locations. Antennas can be directional or omnidirectional.Directional antennas radiate energy more effectively in some directions than in others [12];examples include horn antennas, tapered slot antennas,spiral antennas,and Yagi-Uda antennas.Omnidirectional antennas radiate energy uniformly in one plane and are directional in perpendicular planes [12]; the doughnut-shaped pattern of a dipole antenna is an excellent example.Directional and omnidirectional antennas are often specified relative to isotropic antennas.An isotropic antenna is a hypothetical antenna which radiates equally in all directions [12]. Radar antenna systems can consist of a single TX/RX antenna,a pair of antennas for transmission and reception,or an array of antennas.Asingle TX/RX antenna can be used for pulsed systems,but is normally avoided in CWconfigurations [13];a single antenna system is illustrated in Figure 1.5a.When a single antenna is employed,a circulator is 9

(a) (b) Figure 1.5:Antenna Systems.Antenna systems can include one or two antennas or an array of antennas.Asingle antenna systemdesign is illustrated in (a);a two-antenna systemis illustrated in (b). used to connect the transmitter,antenna system,and receiver.Assuming an ideal circulator, the transmitted signal passes fromport 1 to 2 of the circulator,but not port 3;as such,the transmitted signal does not reach the receiver.If a target is present,the reflected signal is received by the antenna systemand passed fromport 2 to 3 of the circulator,but not port 1; as such,the received signal is passed to the receiver but not the transmitter.Realistically, there will be finite isolation between the circulator ports,leading to finite isolation between the transmitter and receiver.The ratio of the transmitted to received power is normally several orders of magnitude,so it is vital that the circulator provide sufficient isolation between ports 1 and 3 to allowthe receiver to detect the received signal without being jammed by the transmitted signal that leaks through the circulator. If separate transmit and receive antennas are used,the circulator of Figure 1.5a can be eliminated in favor of the setup in Figure 1.5b.In this case two antennas are used,and the intrinsic isolation between the antennas is leveraged to minimize the leakage fromthe transmitter to the receiver through the antenna system[13]. Finally,the antenna systemcan be comprised of an array of antennas.The transmitter and receiver can share an antenna array or use separate arrays.Antenna arrays are often used to achieve high directivity and are used extensively in radio astronomy and synthetic aperture radar (SAR) applications [14]. 10

If separate transmit and receive antennas are used,the antenna systemcan use either a monostatic or bistatic setup.In a monostatic setup,the transmit and receive antennas are close together.For first order approximations,monostatic antennas are assumed to be colocated.In a bistatic setup,the transmit and receive antennas are far apart,allowing for increased isolation between the transmitter and receiver through the antenna system. However,the separation distance must be accounted for when processing received target returns to ensure correct calculation of range or other target characteristics. 1.1.3 Receiver The radar receiver must amplify,filter,and downconvert the received target echo in such a way that the resulting intermediate frequency (IF) or baseband signal can be processed to discriminate between the desired echo and any interferers,including noise,clutter,etc. [15].The functionality of the receiver is accomplished by two receiver subsections,the RF front-end and the IF block,as shown in Figure 1.6.The RF front-end is comprised of an lownoise amplifier (LNA),a bandpass filter,and a downconverter.As the first stage in the receiver,the LNAshould exhibit high gain and a lownoise figure to maintain a lownoise figure for the overall receiver.The bandpass filter sets the RF bandwidth of the receiver and limits the receiver noise.The downconverter converts the received signal frequency to the IF band by mixing the received signal with the LO.In a coherent radar system,the receiver’s LOis synchronized with the transmitter’s LO;coherent systems are common in modern radar systems.In a heterodyne architecture,the downconverted signal is centered around the first LOfrequency;in a homodyne architecture,the downconverted signal is centered at DC.Upon downconversion to the IF band,the signal is filtered and amplified. The IF filter sets the final noise bandwidth of the receiver.The output of the receiver is then digitized,and digital signal processing is applied. The signal processing can be facilitated by including I/Q channels.I/Q channels can be set up in multiple ways.The downconverter can be replaced by an I/Qdemodulator, 11

Full document contains 306 pages
Abstract: This thesis addresses the design and characterization of a pulse Doppler radar designed to detect targets at short range (R ≤ 7 m). To minimize the shortest detectable range, a subnanosecond transmitted pulsewidth is desired. UWB design techniques were combined with a pulse Doppler radar architecture to demonstrate a full radar, including the transmitter, receiver, simulated channel, and post processor. The transmitted pulse train has a 2.5GHz carrier frequency, a 730 ps pulsewidth, and a 1 GHz 10 dB-bandwidth. The PRF of the radar is 20 MHz, which allows unambiguous range and Doppler detection with a single PRF. The peak transmitted power is 1.2W. The characteristics of the transmitted waveform provide fine range accuracy ( δR = ±0.03 m), facilitate a short minimum range, and allow for an efficient transmitter design. The receiver was designed to complement the transmitter; it has a homodyne architecture and is pulsed to isolate a specific detectable range. A closed-loop channel model was designed to simulate the range delay, Doppler shift, and channel attenuation of a moving target; the model is connected to the transmitter and receiver with coaxial cable, facilitating bench-top characterization of the radar and eliminating some effects of wireless transmission, such as multipath. Extensive closed-loop radar testing was performed, and the following radar characteristics were determined: (1) The minimum detectable SNR, assuming a 36.5 μs integration time, is 0 dB. (2) Assuming a transmitter-to-receiver isolation of 80 dB, the minimum range of the radar is Rmin = 1.3 m + Rlk , where Rlk is the apparent leakage range between the transmitter and receiver. Depending on the antenna system design, the radar can detect targets from 1.5 m [Special characters omitted.] R ≤ 7 m, meeting the original goal of this work. These results support the supposition that a UWB pulse Doppler radar architecture can be employed for short-range, moving target detection.