low cost ultra wideband radar for human ...823001/fulltext01.pdfa simple low cost ultra wideband...

Maumlrlardalen UniversitySchool of Innovation Design and Engineering

Vaumlsterarings Sweden

Thesis for the Degree of Master of Science in Engineering - RoboticsDVA502 300 credits

LOW COST ULTRA WIDEBANDRADAR FOR HUMAN PROTECTION

Martina Oumlhlundmohlund0gmailcom

Hampus Carlssonhcn10002studentmdhse

Examiner Magnus OtterskogMaumllardalen University Vaumlsterarings Sweden

Supervisor Martin EkstroumlmMaumllardalen University Vaumlsterarings Sweden

June 12 2015

Maumllardalen University Master Thesis

AcronymsADC Analog-to-Digital Converter

BJT Bipolar Junction Transistor

CMOS Complementary Metal Oxide Semiconductor

EMC electromagnetic compatibility

EMI electromagnetic interference

ESS-H Embedded sensor systems for health

LNA Low Noise Amplifier

MCU Microcontroller Unit

MOSFET metal oxide semiconductor field effect transistors

PAM Pulse-amplitude modulation

PCB Printed Circuit Board

RF Radio Frequency

SRD Step Recovery Diode

UWB Ultra Wideband

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AbstractThe majority of the UWB radars available on the market today are expensive and often closed forfurther development due to proprietary rights Therefore it is difficult to fully understand and adaptthe functionality of an available UWB system to fit onersquos needs The consulting-firm Addiva purchasedan UWB radar to be used in a safety system However the radar had limitations and the functionalityof it was partly unknown This master thesis was inspired from this issue to examine the possibilitiesof developing a low-cost UWB radar with main focus on research of human detection The systemshould be easy to understand and modify as well as reporting reliable data from the scanning Theresults indicate that such a system can be developed However further development to the UWB radarneeds to be made in order to have a complete system

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SammanfattningMajoriteten av de befintliga UWB radarsystemen som finns paring marknaden idag aumlr dyra och oftabegraumlnsade foumlr viderutveckling paring grund av aumlganderaumltt Detta leder till komplikationer att faring enfull foumlrstaringelse oumlver funktionaliteten i ett befintligt UWB system och att anpassa den efter ens behovKonsultbolaget Addiva infoumlrskaffade en UWB radar foumlr anvaumlndning i ett saumlkerhetssystem Dennaradar hade dock begraumlnsningar och viss del av funktionaliteten var okaumlnd Det haumlr examensarbetetinspirerades utifraringn dessa problem att undersoumlka moumljligheterna foumlr att utveckla en laringgkostnads-UWBradar foumlr anvaumlndning fraumlmst inom forskning foumlr detektering av maumlnniskor Systemet skall vara laumlttatt foumlrst och modifiera samtidigt som det ska ge tillfoumlrlitlig data fraringn scanning Resultaten av dennarapport indikerar att ett saringdant system kan utvecklas Vidareutveckling av systemet behoumlvs dock foumlratt ett komplett fungerande system skall erharingllas

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Table of ContentsAcronyms 1

1 Hypothesis 5

2 Problem formulation 5

3 Introduction 6

4 Background 741 Pulse generator 842 Pulse shaper 843 Amplification transmitter 944 Antenna 945 Amplification receiver 946 Sampler Integrator 947 State of the art 10

5 Method 11

6 Hardware 1261 Testing 12

7 Transmitter 1371 Pulse Generator 13

711 Pulse Generator V10 13712 Pulse Generator V11 15713 Pulse Generator V12 16714 Miscellaneous Pulse Generators 16

8 Receiver 1981 Pulse Amplification 1982 Pulse matching 19

821 Advanced Gilbert Cell 20822 Basic Gilbert Cell 21

83 Pulse Extender 22

9 EMC 2391 EMC Issues in this project 24

10 Results 25101 Q1 What are the drawbacks of a low cost UWB radar 25102 Q2 Will the bottleneck be in software or hardware 30

11 Discussion 31

12 Future Work 33

References 38

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1 HypothesisThe hypothesis for this thesis work is as followsA simple low cost Ultra Wideband (UWB) radar for human detection can be developed

In order to develop a low cost system each module should be assessed and made from scratch toevaluate where the cost can be reduced This will result in the development of a simple and easilyunderstandable system which allows for further development of the UWB radar

The problem formulation (Section 2) evaluates this hypothesis and focuses on the possible challengeswith it

2 Problem formulationIn order to make a low cost UWB radar some questions need to be answered This section discussesthe main challenges that emerges when developing a UWB radar

Q1 What are the drawbacks of a low cost UWB radarOne of the challenges of making the system low cost is that it is time consuming As there isno low cost chip available see Section 47 for more information the electronics need to be builtfrom scratch This results in that each sub circuit needs to be tested thoroughly to ensure aproperly working systemThis leads to the question of whether or not it will be possible to decide if a reasonably low costUWB radar can be made within the given time frame A prototype will be developed during thisperiod where the quality of it may vary However there will at least be some groundwork onthe subject which can be further researched in the future It should also with this informationbe possible to roughly decide the probability of developing a successful low cost UWB radarAnother concern about making it low cost is if it heavily affects the precision of the UWB radarWill the function of some sub circuits be affected by the fact that it is low cost and thereforenot being able to perform as well as a more expensive solution The strength for high frequencysignals declines rapidly with longer distances on the circuit board Therefore this could be aproblem with a low cost solution as more components will be present on the circuit board

Q2 Will the bottleneck be in software or hardwareSome functionalities are better to implement in software other in hardware Some parts will berestricted due to the limited development time while other parts will restrict the final product Itis therefore difficult to pinpoint the bottleneck as it depends on how and what is being evaluatedFor example when developing it will most likely be the development of the hardware that ismost time consuming and therefore acts as the bottleneck In the final prototype however itmay be the software that slows down the system compared to the hardware part

This thesis does not consider the areas of health and safety as to limit the field of research

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3 IntroductionWhile UWB technology is not a new subject research in the field is still relatively limited Howeveradvances in high frequency electronics and an increasing demand for wireless technology have givenrise to an exploding interest in UWB UWB is generally defined as a wireless system that is operatingwith a bandwidth of at least 500 MHz [1]

To achieve a wideband signal most UWB systems are pulse based and tend to operate in higherfrequencies at around a few GHz To analyze these pulses on the receiver a common but expensivemethod is to have a very fast analog to digital converter (ADC) in the order of multi Gigasample persecond (Gsps) for digital analysis of the signal [2] To heavily reduce the cost this project will focuson doing most of the signal processing with analog electronics It will also remove the requirementfor a fast sampler which otherwise apart from being expensive also would produce a huge amount ofdata to be processed

Some areas of use for a UWB radar are within industry rescue work and healthcare In anindustrial environment the UWB radar could be used as part of a safety system for detection ofhumans approaching heavy machinery [3] In rescue work the system could be used for detecting livinghumans trapped under some debris [4] In healthcare the areas of use could be to monitor movementin senior homes without invasion of privacy as opposed to camera monitoring [5]

This master thesis has a main focus on research and not on development towards a commercialproduct However there is a collaboration with the company Addiva Addiva is a consulting-firm witha focus on product development and technology They acquired an UWB radar to be a part of a safetysystem where they were going to develop most of the software However it turned out that the UWBsystem itself has some limitations and acted as a black box

This was the inspiration for the thesis work on a low cost UWB radar The goal is to researchabout the possibilities to make a low cost UWB radar Apart from being low-cost the system shouldalso be easy to understand and manipulate so that further research on the radar can be done

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4 BackgroundThe research in UWB technology is not new but in recent time the field has had surge of interestas the demand for wireless technology together with the advances in high frequency electronics Thedefinition of UWB can generally be seen as a wireless technology that is operating with at least 500MHz bandwidth This is usually achieved with a pulse based system rather than manipulating a carrierwave which is what is done in more traditional wireless technologies [6]

Because UWB is operating over such a wide set of frequencies it can be made to not interfere withnarrowband signals operating within the same frequency band To achieve this the system distributesits energy over its entire frequency band making the energy very low at each frequency while thetotal energy can be similar as a narrowband signal Most other wireless technology perceives the weakwide band signal as some low powered noise If designed correctly this does also allow it to be robustagainst other narrowband wireless systems for similar reasons The UWB system can be made to onlycare a little about each frequency making narrowband signal to only slightly alter what the systemsees even though the signal strength at that frequency might spike [7 8]

UWB can be used either as a high bandwidth short range communication or as a high precisionshort range radar In the past most of the focus has been in communication which leaves the radarside even less explored [9] This makes it so there are very few established radar platforms to build onand those that do exists are quite expensive hard to use or have some limiting functionality Shortrange radar can be used in multiple applications ranging from detecting some simple life signal in asenior citizens home for health monitoring searching for humans in rescue work to detect a humanapproaching a heavy machine [10 3] Industries with heavy machinery can require some form of humanprotection It can be done by limiting the physical availability of the machine or where a machinecan automatically slow down if a human approaches Other types of radars exists to detect humansin these areas but UWB provides other sets of characteristics such as the low interference and thepossibility to see through walls acting as a complement to other technologies[4] Compared to otherdetection method like IR and camera UWB allows for the sensor to be omnidirectional making itpossible for one sensor to detect in all directions [11] But for it to be practical to be used in thoseareas the price tag of a radar system has to be reduced [12 13]

There are primarily two techniques used in UWB radar technology The most common methodis a Pulse-amplitude modulation (PAM) sending a known pulse train where the pulse strength isvaried The idea is that the environment is static enough so that each pulse is exposed to the sameenvironment The receiver tries to match the incoming pulses with the known sequence and theyshould all be affected in a similar way The other method involves repeatability sending pulses thatwill be integrated over time to remove most of the background noise This method also relies on astatic environment where multiple pulses can reflect in the same way to get a degree of certainty ona detected target however to achieve this the system requires some sort of a pulse matcher in thereceiver to be matched with a duplicate of the antenna pulse in the transmitter usually sent via adelay line [14 15] Both types use similar design overall but one key difference is on the receiver endas the PAM type needs some type of matcher that is able to tell the different pulses apart A commonmethod to do this is digitally This puts a heavy load on the analyzing hardware as UWB is oftenoperating in GHz frequencies requiring a powerful computer connected to a fast sampler circuit Thisdoes however make it relatively simple to calculate distance with the time of flight with a high degreeof certainty that it is not a random interfering signal from an external source The PAM is a similartechnique that is used in UWB communication allowing some solutions to be copied over and used inradar as the research in the communication field is more developed The method of integration canusually rely on more analog techniques to detect the pulses and also reduce the demand for the highcomputational demand as multiple pulses can be integrated into one output signal Combination ofthe two techniques is often used in a way to reduce the demand on a high speed Analog-to-DigitalConverter (ADC) or the potential of high complexity analog circuity Other methods are more commonin UWB communications

As most UWB systems operate with the same type of modules the following subsections will

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describe the general design approach for the hardware side of UWB Each key module and its functionwill be described Some modules can be combined and some might not be necessary depending on howthe system is implemented Additional support circuit will be needed and includes things like clockand power supply

41 Pulse generatorThe pulse generator can be seen as the heart of an UWB system as it dictates what frequency rangethe system will operate in As the name implies the task of this module is to generate narrow pulsesthe width can vary from a few hundred picoseconds up to a few nanoseconds The end goal for thepulses is to be radiated out through an antenna The pulse type is typically either Gaussian- or monocycle pulses The shape of the pulse can be altered to change the power distribution of the signal overthe frequencies Although very hard to achieve the ideal signal is generally homogeneously distributedover the entire operating frequency range to not disturb other electronics operating in that frequencyIn some applications it might be desired to have more power in some frequency to get a particularbehavior or it can be used to compensate from some losses due to miss matching components [16]

Researchers often build custom made pulse generators as an IC to fit some specified requirementoften with Complementary Metal Oxide Semiconductor (CMOS) technology This allows for a precisecircuit where it can be fine-tuned to function properly as the technique is very mature and is wildlyused in digital circuits like microprocessors It is very fast and it is common to use in other RadioFrequency (RF) applications It does however require a lot of knowledge and time as the entire chiphas to be remade when it requires something to be changed [15] IC has the additional benefit ofhaving a small size compared to if the circuit were to be built with traditional components which isa important aspect when it comes to RF Each trace length add impedance and can also act as anantenna altering the signal and making it more challenging to estimate the behavior

While building the generator in an IC has many advantages it makes it challenging to analyze inreal time as it does not have any easy way to probe the internal signal As it does also take a lot oftime for each iteration discrete components can be used instead A popular component in this caseis to use is a Step Recovery Diode (SRD) It got a special property when switching from a positivevoltage to a negative voltage it discharges a very small capacitance This can be used to generate veryshort pulses allowing a wide band signal The signal generated with a SRD does have very specificcharacteristics it generates many harmonic spikes over the frequency spectrum with equal spacingThis is called a comb generator [17]

The pulse length is very important as it sets the limit on the range resolution where a shorter pulseallows objects closer to each other to be detected as different entities The standard formula for thiscan be seen in equation 1 where c is the speed of light tau is the pulse width time and Sr is the rangeresolution This leads to a pulse width of 1 ns that will at best have the ability to see the differencebetween one object and another object that is 15 cm further away [18]

sr =c middot τ

2(1)

42 Pulse shaperDepending on how the pulse is generated the generated pulse might require to be manipulated to geta desired shape to better match an antenna It can be seen as a part of the pulse generator as it cancontain components to tweak the generated pulse To change the signal it can sharpen the edges ofthe pulse invert the signal or even make the pulse longer Thus the pulse shaper can account forlosses or filter unwanted frequencies although generally not in used with simple pulses as UWB oftenbenefit of using a wide set of frequencies It can be required if it is outside the allowed frequency band[19]

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43 Amplification transmitterWhich type of amplification is suitable depends on previous steps A good pulse is commonly justamplified with an RF amplifier which will keep most of the signal intact As the widely used wirelessLAN is operating around 24 GHz RF amplifiers around that frequency is relatively cheap and isaccessible There are some IC amplifiers that are specificity designed for a very wide band of frequencymostly based on CMOS technology Some systems use the amplification as a pulse shaper which canbe done with the help of a fast transistor which will both amplify and change the characteristics of thepulse As with all RF technology the output impedance should match the 50 ohm that is commonlyused for antennas [20 21] This can be very hard to achieve as continuous matching for a wide setof frequencies is very challenging if not impossible Some implementations of a generator generatespowerful enough pulses to go directly out to the antenna which does not require amplifications Someamplifiers do also split the signal going to the antenna into two signals which can later be used as atemplate or for further analyzing

44 AntennaAntenna designs for UWB is often designed very differently to a traditional narrowband antenna Asnarrowband antennas only require and even benefit from being good around a single frequency it istraditionally just a wire with a specific length specified by the wavelength The UWB radars haveto be good in multiple frequencies which often leads to designs to have rounded shapes and varyingpieces of length To achieve this most antennas are printed on copper laminates allowing for a morecomplex design It is however very challenging to achieve an antenna which is preforming uniformlyover the entire operating spectrum and the signal might get distorted This project will not deal withany development of an UWB antenna This will be researched and prototyped in parallel with thisproject and is done by doctoral student Melika Hozhabri who currently is working with Addiva andEmbedded sensor systems for health (ESS-H) [22 23 24]

45 Amplification receiverAs the returning signal will generally be very weak it requires amplification Most wireless systemsamplify the signal very close to the receiving antenna to reduce the loss of the signal microwavefrequencies have a high loss rate in coaxial cable To receive most of the signal impedance matching isvery important in the receiver more so than in the transmitter The signal will be amplified throughwhat most likely to be a Low Noise Amplifier (LNA) and it is the key in finding the weak responsesignal [25] This type of amplifier does have a static gain typically ranging from 6 dB to 30 dB If thesignal power is still not strong enough additional amplification stages can be added with the use ofmore traditional amplifiers after the LNA when the signal strength is much stronger than the internalnoise of an amplifier

Another possible approach is to integrate the input signal directly allowing multiple pulses to beaveraged resulting in the noise cancelling itself out while the pulses keep adding up The signal canthen be amplified with less regard to the noise figure of the amplifier

46 Sampler IntegratorMost radar systems today do the end analysis digitally which adds the requirement to convert theanalog signal to a representative signal digitally This can often be a challenging part in UWB dueto the high frequency components coupled with the wide band of frequencies On one extreme themost straightforward solution is to oversample the received signal and analyze the signal digitallyThis allows for frequency analysis and signal integrity without complex electronics This does howeverrequire a multi GHz ADC and it will produce massive amount of data to be processed with theobvious drawback of high cost The other extreme is to build most of the signal analysis with analogelectronics This can heavily reduce the cost due to much lower hardware demand on the digital side

9


but the added complexity can be more challenging than the rest of the entire system Solutions oftenlie in-between leaning towards a more digital analysis [15 25]

47 State of the artThe initial interest of UWB has been in communication while radar techniques has been lagging behindThis is why it is common in this field that many techniques in radar stem from the communication sideespecially IR communication The key component that is commonly used for a low cost transmitteris a SRD it allows for an extremely short pulse length of a few hundred picoseconds [26] This isrelevant as it is directly related to the range resolution where a shorter pulse gives a potentially betterresolution With this many systems today operate in a frequency band of a few GHz typically within05 GHz to 10 GHz [18]

The primary focus of a low cost UWB is in the receiver as most of the cost is generally connected to acomplex ADC together with powerful computational hardware for signal analysis Most methods usedthat lowers the cost does often require some sort of compromise such as loss of information reducedspeed or using very complex analog circuitry Methods used often include some sort of down conversionlike 1-bit sampling synchronous pulse matching and pulse detection triggering [27 28] Many of theanalog filters used in broadband signals are derived and adapted from narrowband applications Inmany applications analog filters are primary used for compliance with frequency regulations [29]

Low cost antennas are very common in the UWB field as complex designs can be created fromsimple copper laminates and a circuit mill Different antenna designs have been proposed but one ofthe most recurring design that is used is variances of the Vivaldi antenna It provides good propertiesregarding a wide bandwidth for emission absorption and low signal distortion The Vivaldi antennais generally operating in planar operation and can be arranged in an array [30 31] To standardizethe evaluation of the characteristics for wide bandwidth antennas some methods have been proposed[32 33]

Most of the existing UWB radar IC chips available does only act as a transmitter of a radar Theygenerate UWB pulses that are usually strong enough to not need any further amplification Manychips are configurable to some extent like changing the pulse frequency and center frequency of thepulse No suitable receiver IC chip is currently available Part of the reason is due to how the receiveris often tied to the transmitter [14 25 34 35]

There are few low cost UWB radar products on the market today There are some existing radarcircuits on the market today with a lower price around $18 00 [36] developed by KBOR This radaris not a complete system just a transceiver The most common scenario is that the prices are notavailable as public information The Swedish company Radarbolaget provides a product for stationarymonitoring of the inside of a furnace detecting defects in the manufacturing process [37] NoveldaAS got a product called Xethru which allow human interaction with the system able to controlsoftware with hand motions and breath [38] Timedomain has got the PulsON 410 platform which isa versatile platform for UWB applications [39] Geozondas offers different UWB radar kits designedfor tracking objects through walls or rubble stating it to be a cheap equipment set [40] The pricesfor all theses products are however unlisted There are multiple scientific papers describing differentimplementations of low cost UWB transceivers [41] or modules [42 43] However few offer a completesystem with both software and hardware

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5 MethodMost complete systems can be divided into sub-circuits where generally each have a specific functionTo develop a low cost version of UWB radar this project will adapt and configure different modulesfrom different existing solutions When a general design is made each module will be built andevaluated before moving to the next one This will ease the procedure of choosing sub-circuits withgood performance for low cost Each module that is built will increase the understanding of the systemand allows for a better approach when designing and building the next module

The process of achieving a finished product will be according to an iterative design acting as themethodology The theory of each circuit will be based on scientific papers and existing systems If aparticular design is considered applicable in this system it will be designed and adapted to achieve aspecific function When the circuit is built it will be evaluated if performed as expected Papers thatproposes designs which are described to have desired functionality for this system but lack properexplanation will be evaluated if it can be understood with the help of a simulation or when built Eachcircuit chosen will initially be built with the specified components or if the components is unavailablecomparable components will be chosen If the result from a circuit is decent it can be modifiedto improve the results This process will be repeated until satisfactory results for each module areachieved

All circuits will almost exclusively only use surface mounted component as the legs of throughhole components tends to act like antennas The circuits will be on a printed circuit board (PCB)using 35 microm thick copper laminate They will then be evaluated and when possible adapted to getthe desired result Different circuits will be built and evaluated to achieve an understanding of howdifferent implementations of the same function as well as the PCB layout changes the characteristicsEach circuit will start off with a quick and simple design without much consideration of the PCBlayout Circuits that are very unstable with a crude PCB design will not be further developed Thisis partly to save time as it speeds up the process of evaluating many different circuits and it makesit easier to replicate and reuse the final design from this report When each module has a suitablecandidate they will be put together into a transmitter or a receiver system for further testing Theinformation on how each part works separately can help a great deal if problems occur in the completesystem Most of the system will not be dependent on a specific implementation of a single moduleIn essence the pulse generator can be changed to generate another type of pulse while the rest of thesystem should not require much change if any at all The point is to allow the system to be furtherdeveloped to increase the functionality reliability andor precision with less limitations When therequired modules are finished following the procedure they will be connected into one system

In conclusion the process for each module will follow these points

1 Design

2 Implement

3 Evaluate

4 If results are unsatisfactory repeat step 1-3

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6 HardwareDuring this thesis all designs and circuits were made from scratch and produced first hand Toachieve this a number of tools and practices were used All the circuits were designed using thesoftware programs Multisim 130 and Ultiboard 130 [44 45] The PCBs were made with a ProtoMatS62 circuit mill [46] The components were soldered by hand and in some cases also with the help ofa LPKF ProtoPlace S pick and place machine [47]

61 TestingDuring the testing phase of the circuits a HMC 8043 regulated power supply HMF2525 functiongenerator TDS 3012 oscilloscope and a multimeter were used as needed [48 49] The function generatorwas used for easily generating input signals in order to obtain the preferred signal for each input

An FSP spectrum analyzer and ZVB8 vector network analyzer has also been used during theimplementation for analysis of the transmitter [50 51] The spectrum analyzer has been used toinvestigate the frequency range of the system The network analyzer was used for displaying Schmittdiagrams

A block diagram of the system can be seen in Figure 1 Here the method chosen for the functionalityof the UWB radar can be observed The following two sections will describe the implementation ofthe hardware and the design chosen

Figure 1 Block diagram of the UWB radar design

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7 TransmitterAn essential part in a UWB is its transmitter The main task of the transmitter in any UWB system isto generate and send out a short pulse in the order of nanoseconds This is done with the use of somesort of pulse generator Also there is usually a pulse forming step and a transmission line present inthe transmitter In this master thesis the formed pulse is sent out to the antenna and a delayed pulseis sent to the receiver in order to match the incoming pulse See Section 4 for more information Thissection describes the development of the UWB transmitter and the implementation of its sub-circuits

71 Pulse GeneratorDuring this master thesis a number of different pulse generators were created and tested to evaluatewhich type would produce the most suitable pulses Mainly one pulse generator was made anddeveloped into many versions

A predominant key component in many low cost pulse generators is a SRD Due to limited avail-ability of this component it was not used in this project A PIN diode in certain conditions is describedto have similar characteristics as a SRD when used in a comb generator which is a common type ofpulse generator [52] PIN diodes are more available than SRD it was used as a replacement in circuitsthat required it as a prototype It was however noted that there might be some limitations in higherfrequencies compared to a SRD

To evaluate if Multisim were able to simulate the effect a PIN diode can preform it was simulatedin Multisim and the same circuit was physically tested in order to compare the outputs This wasdone to examine whether it was reasonable to test whole circuit modules by simulation first or if thebehaviour was too different for a simulation to be reliable The outputs from the two circuits aredepicted in Figure 2 the circuit was a diode with a load and a sine wave as input The two signals wasdeemed to not correlate enough to satisfy that the simulation data would represent an entire modulecontaining a PIN diode good enough Some circuits does also rely on a physical distance of traces aso called transmission line where the distance of a specific track is very important as it decides thepulse width These circuits were not simulated in Multisim

This section is divided into two subsections First the main pulse generator with its iterations isdescribed and secondly the alternative pulse generators tested are discussed

711 Pulse Generator V10

The first pulse generator built was based on mainly two reports on UWB pulse generators [53 54]This type of pulse generator has been developed throughout the whole master thesis It was createdin three different versions where each version has a number of patches

(a) Output behaviour of a PIN diode (b) Output from Multisim simulation of a PIN diode

Figure 2

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The schematics and main building blocks of the pulse generator can be seen in Figure 3 Thepurpose of the driver is to create a triggering pulse for the transmission line The driver containsa speed-up step a delay step and two transistors for creating the pulse When the signal from theinverter enters the driver it will go through the speed-up step as well as through the delay line Thesignal from the speed-up will reach the transistor first opening it up When the delayed signal reachesthe other transistor connected to ground it will short circuit the first transistor thus ending the pulseThe fall time of the driver output needs to be short enough for the transmission line to be triggered

The purpose of the transmission line is to convert a fast falling edge from the driver into a narrowpulse Originally in the design it contained a SRD As there is limited availability it was replacedwith a PIN diode After the driver there is a bias current added to the system This bias keeps the PINdiode forward biased when no pulse is present There is also a Schottky diode on the transmission lineand it is reverse biased in this state When a driver pulse reaches the transmission line the PIN diodewill turn off creating a negative falling edge which goes both directly to the capacitor and outputand to the now forward biased Schottky diode The Schottky diodes short-circuits the system and theinverted signal is reflected back to the output The unchanged falling edge and the inverted waveformare then summed up to a pulse by the help of a capacitor at the output [54]

Figure 3 Schematics of the main pulse generator and its sub-circuits

Implementation of pulse generator V10The driver circuit was simulated in Multisim as there were no special components included in this stepThe output from the simulated circuit can be seen in Figure 4 The output fall time is at 1 ns andaccording to the report that the circuit is based on this time should be at 600-700 ps The simulationresult was considered reasonably close enough to the expected value and the circuit was constructedfor further testingThis pulse generator was the first circuit built At this early stage into the project the length of cablesand tracks were not optimized as the main priority was to get the circuit to work even if poorly Inorder to have the ability to change the length of the transmission line for longer or shorter pulse lengtha socket strip was added between the two diodes on the transmission line A cable of desired lengthwas then added to the socket strip acting as a microstrip

14


Figure 4 Schematics and output of the driver in the main pulse generator

As presented in Section 101 the output pulse was not satisfactory In an attempt to improve thecircuit one by one the components were changed to different values First the driver was examinedas the issue seemed to be that the driver pulse fall time was not short enough To decrease the falltime it was believed that either the transistor or the speed-up step had to be faster The speed-upwas modified by decreasing the capacitor value so that it would de-charge faster and thereby speed upthe driver fall time Different values were tested but no noticeable change was observed The resistorvalue in the speed-up was modified but like the capacitor it made no major difference to the driveroutput In total the fall time was shortened down by a couple of nanoseconds from about 15 ns to10 ns

The driver transistor was replaced (transistor Q3 in Fig 3) The transistor MMBT3904 wasreplaced with a transistor of model BFG135 which should be faster [55 56] However this did notaffect the system remarkably At this point the output pulse width had decreased from the initial 30ns to about 20 ns After soldering off and on components many times the PCB was worn down andtherefore a new PCB was made to clean it up This new PCB is described in the following section


In this version the PCB layout was altered This was done by mainly shortening the PCB tracks andreplacing components of the first pulse generator The overall placement of the components stayed thesame The microstrip line between the Schottky diode and PIN diode was redesigned by removal ofthe socket strip to reduce the distance between the two diodes Two vertical lines were added to thePCB so that the physical distance between the diodes could be changed by adding a microstrip overthe two lines at a desired distance from the diodes The design can be seen in Figure 5

The bottleneck in this circuit appeared to be that the transistors were not fast enough as the driverfall time was not noticeably affected when manipulating the circuit The only significant change wasintroduced ringing most likely from the self-frequency of the capacitors in the system The transistorBFG135 (Q3) was replaced with BFG591 [56] After the replacement no remarkable difference in theoutput was seen It was concluded that not enough current was delivered to the transistors which couldbe crucial as they are of the type BJT and therefore current controlled Thus the inverter 74HCT04

15


(U1) with an output current of 68 mA was exchanged to an inverter of model SN74LVC1G04 with a32mA output [57 58] No significant changes to the output signal after this modification were madeThe last capacitor in the driver just before the bias was changed in value from 10 nF to 180 pF SeeSection 101 for the results

The circuit was further tested in a network analyzer and spectrum analyzer The network analyzerdid not give any results A test in the spectrum analyzer gave a response which was observed at thereceived signal from the transmitter as the signal changed along with the generated pulse

Figure 5 Patched pulse generator V11 with a pulse length of 20ns


In this version of the pulse generator the placement of the components was changed The circuit becamemore compact and track lengths were minimized This was an attempt to decrease the interferencefrom other appliances in the surrounding environment and also to decrease the risk of self-resonancein the system The circuit is depicted in Figure 8

The 1k Ω potentiometer was replaced with a 200 Ω potentiometer This was to obtain a higheraccuracy as it was observed that the potentiometer gave satisfactory results at 0-200 Ω As the newpotentiometer had more turns available than the previous component it could be more fine-tunedIf the resistance would need to be higher one can easily add a resistor of suitable size One couldalso change the capacitor in series with the potentiometer However it seemed more practical to havea high resolution potentiometer for tuning than having to replace the capacitor for a suitable valueduring testing

The inductor was replaced from 100 microH to 2 nH in order to see what effect this would have on thesystem The Schottky and PIN diodes were replaced with a component containing two PIN diodes inseries The results are presented in Section 101

714 Miscellaneous Pulse Generators

Some other pulse generators apart from the main one were made Theses pulse generators are builtup differently and uses other technologies to generate pulses These circuits will be described in thefollowing section

16


Figure 6 Pulse generator V12

Tunable Pulse GeneratorA tunable pulse generator made can be seen in Figure 7 where Figure 7a shows the circuit built andFigure 7b depicts the corresponding schematics This version is based on a report about a tunablepulse generator [59] The idea of this pulse generator is to have transmission lines of different lengthdepending on how wide pulse is desired It can then be controlled which path to use and therebydetermining the pulse width In this circuit the diode D2 in Figure 7b is originally a SRD but wasreplaced with a PIN diode during testing The other three diodes are PIN diodes as should be accordingto the schematics

The functionality of this circuit is that a reversed pulse shuts down the charged SRD which createsa sharp falling edge as the SRD becomes discharged This falling edge travels through the systemdirectly to the output creating the start of the pulse and also through the PIN diode configurationwhich acts as a delay step There is a transmission line between diode D3 and D4 which decidesthe pulse width The polarity of the pulse is reversed through the use of a short-circuit and the twocomponents are summed up to create one pulse

The pulse generator of this type was made with only one transmission line as the main goal wasto examine how well this type worked This circuit was not simulated as it contained both PIN diodesand a transmission line which as stated in Section 71 was difficult to simulate in Multisim The pulsegenerator worked at the first try however poorly See Section 101 for the results Increased voltageof the square wave resulted in more ringing of the output This circuit was not further developedafter the first version due to very poor results It was believed that one of the reasons why the circuitworked poorly was due to the usage of a PIN diode instead of an SRD Another theory is that it wasdue to the transmission line being too long If this was the case then likely a modified version withshorter delay line would produce a shorter pulse

17


(a) Tunable pulse generator circuit (b) Schematics of the tunable pulse generator

Figure 7

Comparator based pulse generatorAnother pulse generator was created based upon Williams work presented in rdquoSimple nanosecond-width pulse generator provides high performancerdquo [60] The idea behind this version is to have twodelayed signals one with a small offset with respect to the other This is achieved by a small offsetin value of two resistors which are placed in parallel at the start of the circuit The schematics canbe seen in Figure 8 Each signal goes through a comparator and then to an AND gate The firstsignal will reach the comparator and produce a low output Right after the second signal will reachits comparator and produce a high output The first signal will then end and switch the comparator tohigh while the second signal is still high This opens the AND gate and creates the start of a pulse untilthe second signal goes back to low again and closes the gate along with the pulse The input signalto this circuit is a sine wave from the function generator which converts to a square wave through acomparator

Compared to the results of the main pulse generator V12 (Section 101) this generator gives out aweaker but about as wide pulse As the AND gate did not go all the way up to 5 V before switchingit was considered that the limitation was due to a too slow logic gate This circuit did not performbetter or as good as the main pulse generator and therefore it was not further developed

Figure 8 Schematics of the comparator based pulse generator

18


8 ReceiverOne main part of a UWB radar is its receiver In this master thesis the receiver is defined as beingresponsible for collecting a signal through an antenna and match it with the delayed pulse It thenprocesses the matching data in order to make an assumption of the potential object found In thissection the receiver step its sub-circuits and design will be discussed A block diagram of the receivercan be seen in Figure 1

81 Pulse AmplificationAfter the antenna has received a signal the signal amplitude will have decreased compared to theoriginal pulse sent out of the transmitter As the energy level of a UWB pulse is already very lowand the received pulse will have lost some of its amplitude an amplification of the received pulse isnecessary The first step after reaching the antenna is therefore through an LNA The characteristicsof an LNA makes it very suitable for UWB applications It is very good at amplifying weak signalswhile keeping the noise level low [61] However as an UWB signal is at noise level it is necessary tocollect and add a number of pulses so that the noise cancels out while the signal grows

There were no simulations made of the LNA circuits tested during this project The reason forthis was because it was assumed that the LNA test circuits which were taken and made from theirrespective data sheets were correct Also as the LNArsquos are very sensitive to interference it was thoughtthat a representative simulation would be difficult to achieve

There were five LNA circuits made during this master thesis The first LNA [62] version 1 (V1)purchased was about half the price compared to the other LNAs found which costed in the rangeof 60-70 SEK A suggested circuit from its data sheet was made in order to test the performance ofthe LNA However as mentioned in result 101 it did not preform well and after some further testingand modification that was suggested in the data sheet it was discarded as it was never providing anyuseful output

The second LNA used was SPF5189Z a more expensive version than the first one [63] The datasheet provided two different configurations one optimized for 900 MHz and the other for 1900 MHzThe data sheet also provided an evaluation board which was used as a base for the PCB layout Theoutput of the 900 MHz version is described in result 101 it proved to be very unstable and wastherefore not used The second configuration adapted for 1900 MHz proved to even more unstableThe PCB layout was redesigned to match the layout of a evaluation board of the 1900 MHz test circuit[63] After these changes the system became more stable and was not affected by the surroundingsas easily However the signal response itself did not improve and no amplification was present Thecable lengths and track lengths were shortened down and SMA connectors were added to the outputand input of the circuit This modification made the self-resonance disappear

Two designs were made for two similar LNAs BGA420 and BGA616 [64 65] Both were onlytested briefly as similar behavior as the previous LNAs was observed BGA420 was discarded as itwas unstable and BGA616 did not amplify the signal enough Both used schematics from respectivedata sheet but no PCB layout were available The layout was instead based on a evaluation boardSPF5189Z [63]

The final LNA tested in this project was MGA30889 which is of type gain block [66] The datasheet provided test circuits which the PCB layout was based on To reduce the risk of issues such asself-resonance together with SMA connectors for the input and output the connectors to the supplyvoltage and ground were also more carefully designed This LNA circuit gave an usable output whichis described in 101

82 Pulse matchingAfter the received signal has been amplified it needs to be matched with the delayed pulse from thetransmitter in order to check the similarity and whether a match has been found or not This step was

19


done using a four quadrant Gilbert cell Two kinds of Gilbert cells were made a basic version and amore advanced Gilbert cell Two circuits of each version were implemented and tested

Gilbert cells are commonly used in IC as a frequency mixer to shift a data signal in or out froman RF signal This is done with the help of a local oscillator as one of the inputs to the cell but canused as a signal multiplier instead The four quadrant is able to handle four different input signalsand generates two output signals The inputs are coupled two and two where one dealing with thepositive part and one dealing with the negative part of the signal The negative should be invertedto a representative positive signal to work The output signal represents a multiplication of the twosignals as it is four quadrant one of the output represent a negative answer and the other a positiveA Gilbert cell is usually either a linear multiplier or logarithmic but linearization requires additionalcomponents over the basic version and are not a necessity in this application[67]

The Gilbert cell needs a template input and an input from the signal to be matched In this casethe template signal is the delayed pulse from the transmitter and the other input is the received pulsefrom the antenna During the testing of the different Gilbert cells a sine wave from the functiongenerator was used as a test signal As template a DC signal was used When the sine wave matchedwith the DC level the output dropped respectively The more the output level dropped the bettermatch was obtained If the DC signal is very low then the matching level will be very weak and theoutput will not drop as much as for a higher DC value It is also important that the signals are highenough for the transistors to open up properly

Figure 9 Schematics of a basic Gilbert cell

821 Advanced Gilbert Cell

The first Gilbert cell to be built was a multiplier based Gilbert cell being founded on another UWBreceiver project [68] This Gilbert cell like most Gilbert cells multiplies currents Two advantageswith this design were the ability to integrate multiple pulses by controlling when the integration shouldbe reset and that the integration converts the current output into voltage output which is easier toanalyze Both of these additional features are desirable and needs to be implemented in some way oranother in the system The multiple pulse integration is used to increase the certainty of a correctlydetected target

20


This circuit was first simulated in Multisim However the simulation did not give expected outputsand also there was an issue in the simulation which resulted in the simulation constantly crashing Itwas decided that a circuit should be made despite a non-functional simulation The reason for this wasbecause it was thought that the circuit could be more easily evaluated and manipulated when havinga physical circuit to test Also as the behaviour of the advanced Gilbert cell was not fully understooda physical circuit was considered to help the understanding of the functionality

The first version used BFG591 Bipolar Junction Transistor (BJT)s [69] However with this con-figuration no output was obtained The circuit seemed to become short-circuited when starting up Ifonly the supply voltage was applied then the system worked However when sending in the templateand input signal the system drew a high amount of current Whenever this happened the system hadto be reset The reset was made by removing a transistor and then solder it back on again The causeof this behaviour was thought to be either because the capacitors did not discharge or that the kickstart effect that should take place in order to start the system did not function

After some research it was discovered that metal oxide semiconductor field effect transistors (MOS-FET) should be used for this design which is presented in the paper on a UWB receiver [68] Thereforethe circuit was modified and produced to be used with MOSFETs instead This version did not giveany expected outcome The output did not correlate with any kind of multiplication The circuit acteddifferently depending on the clock frequency and the inputs did not affect the system as they shouldAlso the clock was present in the output signal The PCB is depicted in Figure 10

Figure 10 The second version of the advanced Gilbert cell using MOSFETs

822 Basic Gilbert Cell

As the advanced Gilbert cell did not work as expected it was decided that a more basic Gilbert cellshould be built The main idea of making this version was to achieve a better understanding of how aGilbert cell works This would also result in better manipulation of the Gilbert cell in order to add orchange functions for it to be tailor-made for the receiver step For the schematics of the basic Gilbertcell see Figure 9

The first version of the Gilbert cell can be seen in Figure 11a The transistors used for this versionwere of type MOSFET This circuit had similar issues as the first advanced Gilbert cell as it wasshort circuited after start-up The transistors were examined and it was discovered that they brokeeasily presumably because they were not powerful enough Therefore another circuit was made usingBFG591 BJTs instead [69] This version worked as expected A picture of the second version PCBcan be seen in Figure 11b For the results of this circuit see Section 101

21


(a) The first version of the basic Gilbert cell using MOS-FETs

(b) The second version of the basic Gilbert cell usingBJTs

Figure 11 The two basic Gilbert cell circuits

83 Pulse ExtenderAfter the two pulses have been multiplied the data is to be sampled and processed in order to drawa conclusion of the detected object in question This step is very critical when designing a low-costsystem as the simple solution is to use a fast ADC in the order of about 20 Gigasample per second(Gsps) As fast ADCs are very expensive this is not a reasonable solution for the goal of this masterthesis The need for such a fast ADC is to oversample the pulse enough for retrieving a satisfactoryrepresentation of the appearance of the nanosecond short pulses In order to remove the need for afast ADC the pulse is sampled and extended The method used for this project is based on holdingthe pulse before sampling it to the software The idea is to hold the nanosecond pulses for abouta microsecond This would decrease the sampling speed thus allowing for a slower less expensivesampler to be used The circuit is based on a pulse stretcher [70] The schematics is shown in Figure12a

This circuit also has an adjustable object detector function built into it This part is built up of aSchmitt trigger and a digital resistor Tuning of the resistor changes the threshold for the object sizeto be detected The purpose is to have the ability to change the threshold for the energy level fromthe pulse matching at the Gilbert cell This allows for detection of objects of desired size When thethreshold has been reached the sample and hold function will trigger elongating the pulse

For the development phase the digital resistor was replaced with a potentiometer in order to testthe circuit without software After some modifications this circuit worked as expected Depending onthe value of the resistor the delay increases or decreases where higher values increase the delay timeThe results are presented in Section 101

22


(a) Schematics of the pulse extender (b) Pulse extender circuit

Figure 12 Pulse extender

9 EMCAs soon as high speed digital or high frequency analog electronics is designed electromagnetic compat-ibility (EMC) has to be taken into consideration with great care Electromagnetic interference (EMI)is a core part of EMC as it describes the phenomenon of disturbance while EMC is how to control itEMC is not excluded to high frequency electronics but it is much more prevalent in that field Eachpart in a circuit is susceptible to the problems that can occur including self-resonance loss of powerinterference emittance and interference susceptibility The received interference is commonly pickedup in the system with cables long traces or from the power source As a radar system often deal withweak signals the introduced noise can be as strong as the signal itself if it is introduced in the wrongplace Similarly fast internal switching can leak out radiation via cables or long tracers and if thecircuit is not properly shielded [71]

Impedance matching is also a very common issue when it comes to wireless technology often whendealing with an antenna Matching impedance will allow for a better transfer of power between twonodes for example between an signal amplifier and an antenna The power that is not transferredcan bounce back into the circuit which can create standing waves or worse damage some sensitivecomponents Radar and other wireless technology often use 50 Ω as the default impedance Using oneof the standard impedances make it easier to buy components or connectors that are matching Theideal scenario is when the impedance between two stages is 50 Ω without any inductance or capacitanceover the entire frequency range which is very hard to achieve [72] Capacitors and inductors changestheir behavior with changing frequency and can even swap behavior an inductor can act as a capacitorand vise versa This is due to the parasitic properties of real life components The characteristics ofthe impedance can be measured with a network analyzer where the most common parameter is the Sparameter The S parameter describes how much power is lost at specific frequency and the responseimpedance giving the complex impedance where the imaginary part describes the capacitance orinductance Impedance matching is often achieved with small circuits called L-networks or Π-networktransformers or with a tunable IC Some ICs can even automatically tune detecting signal bouncesand altering the impedance accordingly to get the maximal power transfer However most matchingtechniques are matching for just a few frequencies or are only applicable on lower frequency whichmakes it hard to match for such a wide band in which UWB operates in [73]

All these problems are no less of a problem when dealing with UWB radar where high frequencysignal is present As the wavelength of a GHz signal where UWB often operate is close to the size ofa PCB the trace design is important Controlling the trace length can reduce the risk of generating

23


standing waves within the circuit or absorbing signals of specific frequencies Many types of pulsegenerators do also contain closed loops adding a risk of self-resonance with capacitors A typicalsource of EMI comes from ICs anda way of dealing with it is by having decoupling capacitors as closeas possible to the supply pin

91 EMC Issues in this projectThis project started off with little consideration about EMI as the initial goal was to get the circuitsto just work before improving and matching the circuits The idea was to speed up the process ofevaluating different circuits The first instance where it became an obvious issue were with the verysensitive LNAs The initial thought was that the LNA circuit would work even if poorly but wereproven not to give any proper response Issues that arose with the LNAs were self-resonance ringingsignals and flat out dead signals To solve these issues new PCB designs were made where the tracelength component placement cable length and connectors where more carefully considered on thedifferent LNA circuits When all those problems were reduced the act of using an oscilloscope probeproved to be enough to disturb the system enough to generate self-resonance To solve this SMAconnectors were used on key points to connect to the oscilloscope directly with SMA coaxial cableskeeping the impedance at 50 Ω In figure 13 one of the improvement on one LNA can be seen This isthe difference that shortening the cables made from just creating a self-resonance signal to an impulseresponse

The probes used for measuring circuit signals on the rest of the system had to be re-evaluatedas it was discovered that they caused self-resonance in the system This lead to small modificationsof adding SMA connectors to some of the existing circuits including pulse generators to be able tobetter see a more representative signal in the oscilloscope as they also deal with the high frequencysignals The circuits are not as affected with an oscilloscope probe everywhere but it is generally goodto make sure how the probes affects the system Also if possible using a probe with a high multiplieris preferable to lessen the load on the device under testing (DUT)

The impedance matching has been one of the last steps to be considered as it will be affected by anycomponent changes close to the matching The primary focus of the impedance matching is around theantennas to be able to send and receive as good signal as possible There exists equations to estimateimpedances and how to match it but they can quickly become a highly non-linear multivariate systemeven in basic cases The approach was to solve it through empirical research with different networksaiming to match for a center frequency while trying to minimize the mismatch for the rest Howeverdue to time limitation the impedance matching was not finished in the final circuit design and justcontains an inverted Π-network to the antenna

Figure 13 To the left Self-resonance of the LNA To the right The output signal after modifications

24


10 ResultsIn this section the results of the master thesis will be discussed The results will be based on theproblem formulation questions as to give a representation of how well answered these questions are

101 Q1 What are the drawbacks of a low cost UWB radarAs discussed in Q1 (2) many aspects of designing an UWB were time consuming and different circuitsshowed a big variation in performance Most of the circuits were built from primary basic componentssuch as diodes transistors resistors inductors and capacitors Most of the duration during thisproject was spent on researching building and evaluating different UWB modules An IC moduleusually comes with a datasheet complete with information on how to balance the circuitry around itrequiring less time spent on repetitious work regarding filtering and component compatibility

The following modules have mostly just been tested separately and the behavior described herewas in that single module configuration The components that is dealing the high frequency signalare limited to 26 GHz as it is the maximum operating frequency for some of the components in thesystem Each circuit lacks the proper support components like power regulators polarity protectionclocks shielding spike protection and other common safety circuitry All the different supply voltageswere provided by a power cube Similarly all the clocks or input pulses were made using a functiongenerator

Main Pulse GeneratorThe first module to be built was the pulse generator the key component of most low cost alterna-tives used a step recovery diode (SRD) SRDs are however not widely available so the componentwas replaced with the more common PIN diode which shares some of the properties used for pulsegeneration [52] None of the pulse generators that were built could achieve the same short pulse widthas what was reported in the articles which the circuit designs were based on The first Printed CircuitBoard (PCB) made of pulse generator V10 (Section 711) gave a very weak pulse at about 100 mVwith a pulse length of 25 to 30 ns With some minor tweaking on the circuits the pulse width wasshortened down to 20 ns The input to the system is the clock supply voltages and a bias voltageFor this version a bias voltage of 07 V gave a stable Gaussian pulse

In pulse generator V11 a decrease in fall time from the transistors from 20 ns to 14 ns throughoutthe system was obtained The output pulse was between 15 ns and 25 ns wide depending on the valuethe potentiometer although a change in bias voltage to 08V resulted in a 10 ns wide pulse Theoutput peak voltage of the pulse was increased to 15 V

The output from the driver of Main Pulse Generator V12 (see Section 713) was improved com-pared to the previous versions with a fall time of 10 ns The output could be reduced to a 8-10 ns widepulse with carefully tuned potentiometer value The peak voltage dropped down to 1 V A slightlywider pulse output pulse can be seen in Figure 14 The main pulse generator showed inconsistencyin the pulse strength between pulses which was apparent in all of the versions The final versioncontained the widest frequency spectrum (figure 17a) The final version was tested in the networkanalyzer and from Figure 17b it can be observed that the trace follows the 50 Ω resistance circle Thetrace lies within the inductive area so if the circuit would be made more conductive the trace wouldnaturally stabilize around 50 Ω

The frequency response characteristics of the pulse generators varied greatly even between differentversions of the same base design All generators had a low minimum frequency close to 1 KHz but themaximum frequency ranged from 100 MHz to around 25 GHz The pulse generator that was chosento be used is described in the section 713

Tunable Pulse GeneratorThe tunable pulse generator in section 714 gave the widest pulses out of the three generators Theoutput gave 100 ns long pulses with a peak voltage of 200 mV The input to this system was a 6 V peak

25


Figure 14 Output pulse from pulse generator V12

to peak square wave and a supply voltage of 06 V Increasing the supply voltage gave an increasedpeak vale but also added some ringing The output can be seen in figure 15

Figure 15 Output pulse of tunable pulse generator with a pulse length of 100 ns

Comparator based Pulse GeneratorThe comparator based pulse generator in section 714 (figure 15) outputted pulses ranging from 20 nsto 40 ns with an amplitude of 300 mV The input to this generator requires only supply the ICs andan input clock A generated pulse from the comparator based pulse generator is depicted in Figure 16

Gilbert cellThe second module was the pulse comparator where the designs are based on a four quadrant Gilbertcell multiplier Two different designs were made A more complex cell was made which had morefunctionality in the design It allowed the output current to be converted into output voltage viaintegration and it also supported resetting of the integration allowing multiple pulses to be integratedin the Gilbert itself [68] Two different version of this circuit were made only differentiating withdifferent types of transistors BJT was swapped out for MOSFET Neither version of this design

26


Figure 16 An output pulse from the comparator based pulse generator

worked as intended they showed no output response with varying inputs including DC sine wavesand pulses The second design was based on a basic four quadrant Gilbert cell which does nothingelse but act as a multiplier This was also made into two versions one with MOSFET and one withBJT The transistors of MOSFET version was destroyed under testing The version with BJT provedto be more robust The simplified design worked as intended but lacks the functionality of the moreadvanced version Part of the analog analysis is in the pulse matcher The dropping output voltagecorrelates to the simultaneous high voltages of the inputs The inputs to the basic Gilbert cell werea DC signal at 1 V and an AC signal as described in Section 82 The Gilbert cell showed a muchgreater sensitivity when an offset of 650 mV was applied to the input signals The output from thiscircuit can be seen in Figure 18 Here the maximum value means the smallest match The lower thevalue the higher the match Lowering the DC input reduced the voltage drop in the output At thelowest point the multiplication of the two signals gives the highest match This still gives a short pulseas an output which can be even shorter than the initial pulse The chosen Gilbert cell is described inthe section 822

Pulse ExtenderThe short output pulse from the Gilbert cell is the input to the pulse extender It provided twofunction the first one was to extend a few nanosecond pulse to around a microsecond and the otherfunctionality was the ability to change the trigger level out from the Gilbert cell The pulse extender isa modified Schmitt trigger with a latch function The extended pulse length is based on a capacitanceand a bleed resistor where increasing the value of the resistor makes the pulse longer However ifthe pulse is too long it will interfere with the next pulse The test input to the pulse extender was apulse generated from the function generator with a pulse width of 15 ns The output from the systemis depicted in Figure 19 As the output triggers high and stays high until a given threshold and thenturns low the curve looks very similar to a PWM square wave It can be observed that the 15 ns inputpulse has extended to about 43 micros More detail on the pulse extender can be read in the section 83

LNAThe last required module was the amplifier on the receiver which is a LNA connected with an antennaThis type of amplifier is very sensitive to incorrect component matching and to the PCB layout Thislead to the making of a total of five different LNA circuits each with different LNA The first LNA

27


(a) The output from the spectrum analyzer (b) The impedance matching in the network analyzer

Figure 17 Results of the pulse generator in the complete circuit

Figure 18 Output from the basic Gilbert cell with a DC signal as template input and AC signal asmatching input

circuit was built based on circuit design provided by the manufacturer [62] With different pulses as aninput there were no amplification of the input signal but rather a de-amplification The output signalof the LNA acted very poorly and did not seem correlate directly with the input signal The circuitwas modified by adding a capacitor between the LNA ground pin and ground for the DC to be blockedinstead of going directly to ground However no noticeable change was observed It was believed thatdue to the low cost of the the LNA it was more difficult to get it to work as more components for thetest circuit was needed than for a more expensive LNA The EMC was not particularly considered inthis circuit

Two slightly different configurations of the second LNA was tested One optimized for 900 MHz theinitial circuit generated very unstable peaks from an input pulse and was very prone to self-resonanceThe test circuit was modified into another very similar test circuit optimized for 1900 MHz by replacingvalues of some capacitors and removing an inductor However this version generated an even moreunstable outputs than the previous circuit configuration and both sine waves and pulses as input oftencreated self-resonance of higher frequency The final design used proper SMA connectors for input andoutput this eliminated the issue with self-resonance but did not provide an amplified signal and theoutput signal did not follow the shape of the input This was true for both a sine wave and a pulse asan input

28


Figure 19 Output from the pulse extender

Two similar LNAs were used BGA420 and BGA616 for the third and forth circuits [64 65]BGA420 provided no output response for input pulses and outputted mostly noise when with a sinewave was used as an input The circuit with BGA616 showed a proper correlation between input andoutput from the LNA and were not prone to self-resonance but with a negative gain of 1

A circuit based on the LNA MGA30889 were made It proved to be both stable resilient to self-resonance and provide a negative gain of 6 The signal had some slight ringing after passing theamplifier The output signal from the LNA is depicted in Figure 20 where a 15 ns wide Gaussianpulse was used as input and the LNA had a supply voltage of 48 V A common output from theunstable LNArsquos is depicted in Figure 21

Figure 20 An amplified output from an stable LNA circuit

102 Q2 Will the bottleneck be in software or hardwareIn Q2 (2) the location of the bottleneck is discussed whether it will be in software or hardware Asthe software was not developed during this thesis due to time constraints this question cannot beanswered as of now

29


Figure 21 Output from an unstable LNA test circuit

The hardware contains several bottlenecks which are related to the delay signal relative long pulsewidth and the down conversion of the received signal A pulse width of 10 ns gives a very low precisionas it would make it challenging to distinguish between two objects 15 m apart according to equation1 The delayed template pulse limits the physical distance between the transmitter and receiver as thesignal contains high frequency components The longer the signal travels the more of the microwavefrequencies are lost resulting in a distorted signal as a template Longer distances makes it also moresusceptible to external noise being absorbed The lowest limit on the components in the circuits is notrated for frequencies greater than 26 GHz This prevents the system take advantage of an improvedpulse generator which can generate signals with higher frequency components

The system is designed to only trigger to a received signal together with the delayed signal whichis tied to a specific distance But as the signal is so long the end part of a pulse can be enough tomatch the beginning of the expected pulse giving a match for a closer object For the same reasonthe minimum detection range with this pulse width is 15 m The down conversion introduces twolimitations it limits the pulse repetition time as two pulses will overlap if the extended pulse islonger than the time between two pulses The second limitation with the pulse extender is the loss ofinformation The pulse matcher produces an output signal depending on how well the received signalmatches with the template signal giving an indication on how the signal has been altered Most ofthat information is lost in the pulse extender as it only acts like a peak detection

30


11 DiscussionIn this section the goal of the master thesis will be discussed

In this paper the possibility of a simple low cost UWB radar system was evaluated The resultsindicates that the approach that was used would allow for such a system The method of researchingand picking different modules to implement proved to contain both benefits and drawbacks It allowedfor simpler testing environment of each subsystem without any dependency on the rest of the systemThe need of matching modules together and making them more standalone costs a lot of time Thiswas one of the biggest drawbacks The current design does however contain multiple flaws where someof them could be resolved with further development The characteristics of the circuits presented in10 is not as good as desired The current pulse generator generates pulses with the width of around 10ns This is a bit too wide and a shorter pulse increases the bandwidth[74] To achieve a wider set offrequencies in the gigahertz spectrum and a range resolution of less than 50 cm a width of 02 to 3 nsis required The possibility to achieve 02 ns is reported in reports that was used as base for multiplepulse generators but has not been achieved in this project This might be due to the SRD beingreplaced with a PIN diode which discussion online indicated would work for frequencies below 5 GHzThis should not affect the other modules too much as they were kept not to be strictly dependent onthe characteristic of the signal On some of the pulse generators the power was high enough out fromthe generators to not require any amplification before transmission

The signal splitter which goes to the delay line from the transmitter antenna is not properlydesigned It acts like a buffer step while at the same time affecting the matching to the transmittingantenna Ideally it should have no impact when splitting the signal The complete impedance matchingwas not finished for either the receiver nor the transmitter which was apparent for the transmitteras the antenna did not contribute much to the emitted signal The frequency range of the generatedsignal is not tuned to any specific range regarding human reflection which can limit the functionalityof the system

The receiver does have a major drawback with the approach of using a Gilbert cell together withthe pulse extender As of the current design all that is required to trigger a pulse is a high enoughpeak from the Gilbert cell to register as a pulse leaving a lot of room for potential false positives Butit does reduce the required sample rate by a great deal To be able to analyze a pulse in software afast ADC would require a high sample rate of a few Gsps while this design only requires a sample rateof a few times greater than the pulse repetition frequency This allows the control system to be asslow as desired as long as it is compensated with a lower pulse repetition frequency Another thing toconsider is the pulse extender After detection it will hold high which will not allow for other objectsto be detected behind the target This has to be done in software controlling the delay step allowingobjects at different distances to be identified However the output signal can be directly read as adigital input to a Microcontroller Unit (MCU) or an embedded computer While this reduces therequirement on an ADC the output of the pulse extender does only give the output HIGH and LOWit loses a lot of information in the signal which cannot be analyzed in the software

Some of the information can still be gained if the software controls some of the key points of thesystem The delay step has to be controlled and calibrated to match each distance with a specificdelay where a longer delay allows the signal to propagate a longer distance before the system expectsthe signal to return The second key point to control is the trigger level of the pulse extender Thiswill allow the system to change the voltage level required to trigger a detected signal which can beused to account for loss of signal strength for objects further away and also to iterate different voltagelevels to approximate the size of the object These two aspects can be altered with the help of a digitalresistor which can be easily controlled by an MCU

This paper has not gathered any new real life data on how different frequencies react on a humanbody which frequencies generally are reflected and which are absorbed by the body This informationis important as it changes how the template pulse should be altered to be as similar as possible to theexpected return signal Furthermore it dictates which frequencies the pulse generator should generatefor better performance if for example multiple but spread out frequencies work better than a coherent

31


set of frequenciesThe entire system requires a control system to be useful which was not implemented As the

analog circuit will not integrate multiple pulses to assert confidence in a true match this has to bedone in software The system does also require adapting the trigger level for the pulse extender asthe voltage input will be inversely correlated to the distance to the target This can also be used todetermine the size of the detected object where a lower threshold level allow for bigger objects to bedetected The software does also need to change the delay timer to change the current search distancefrom the radar The software is where the integration should take place deciding how many detectedpulses is required in order to register it as a detected object

The conclusion of this paper is that a low cost UWB radar is fully possible The results fromthis project leaves human detection out as no information was gained to how well a system like thiscould detect humans Other projects does report that human detection is possible with UWB radartechnology [4 75] The radar system was not finished in this project but the results indicates thatmost of the required pieces can be made with low cost components while also keeping the complexitydown Most of the components in this design can be swapped to similar components just with betterperformance while still keeping the cost relatively low One key component missing in this project isthe SRD which if present could allow for a better pulse generation The current design does also lacksome more advanced frequency analysis on the received signal which loses some of the benefits with theUWB technology With a complementing software calibrated for this system and some improvementsin the overall circuitry design the radar system might be able to scan an area and be able to detecthuman presence

32


12 Future WorkDue to time constraints it was not possible to make a complete functional prototype of a low-costUWB radar during this master thesis As described in Section 10 all the modules building up thesystem give satisfactory results However a design of the complete system has not successfully beenmade

The main goal of the project was to detect humans so the system has to be tweaked aroundthat One key aspect is the frequency of the generated pulse has to be optimize for frequencies thatreflects well on human tissue Similarly the receiver could be modified to be more sensitive to specificfrequencies or split the received signal into channels with different filters to get a more detailed analoganalysis Another important aspect to be improved is the impedance matching through the entiresystem This area has only briefly been examined during the project and needs to be addressed inorder to achieve satisfactory results of a complete UWB radar

As the hardware is not complete the area of software has not been implemented during this masterthesis To work with the current hardware design the software has to have the ability to control thedelay step change the voltage level of the pulse extender read the digital signal out from the systemand process that information The software has to be calibrated to match different delay times to theexpected energy received to achieve any form of radar control

33


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[11] Ali M Niknejad Stanley B T Wang and Robert W Brodersen Circuit modeling methodologyfor uwb omnidirectional small antennas

[12] C E Romero J Watson F J Pearce N Levin C N Paulson J T Chang Ultra-wideband radarmethods and techniques of medical sensing and imaging httpse-reports-extllnlgovpdf325992pdf 2005 [Online accessed 9-February-2015]

[13] Greg Barrie Uwb impulse radar characterization and processing techniques httpwwwdticmilcgi-binGetTRDocLocation=U2ampdoc=GetTRDocpdfampAD=ADA437380 2004 [Onlineaccessed 17-February-2015]

[14] Huang Xiao-tao Liu Wen-yan Ding Hong Enhanced toa estimation in ir - uwb ranging via bakercoded pulse trains httpieeexploreieeeorgxplloginjsptp=amparnumber=60617552011 [Online accessed 7-February-2015]

[15] Enrico M Staderini Everything you always wanted to know about uwb radar a practicalintroduction to the ultra wideband technology httpwwwmikrocontrollernetattachment27468oseepdf [Online accessed 7-February-2015]

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[17] Anxue Zhang Anfu Zhu Fu Sheng An implementation of step recovery diode-based uwb pulsegenerator httpieeexploreieeeorgxplarticleDetailsjsparnumber=5614726 2010[Online accessed 7-February-2015]

[18] Radar fundamentals mocmodule1814190_ch1pdf [Online accessed 17-May-2015]

[19] John Barrett Pawel Rulikowski Ultra-wideband pulse shaping using lossy and dispersivenonuniform transmission lines httpieeexploreieeeorgxplloginjsptp=amparnumber=6015504 2011 [Online accessed 7-February-2015]

[20] Petr CERNY Zbynek SKVORI Jan VANCLI Vratislav SOKOL The uwb amplifier 31-106ghz httpieeexploreieeeorgxplloginjsptp=amparnumber=4569953 2008 [Online ac-cessed 7-February-2015]

[21] Michael Shaw Chao Lu Anh-Vu Pham A cmos power amplifier for full-band uwb transmit-ters httpieeexploreieeeorgxplloginjsptp=amparnumber=1651175 2006 [Onlineaccessed 7-February-2015]

[22] Hans Gregory Schantz Introduction to ultra-wideband antennas httpwwwresearchgatenetprofileHans_Schantzpublication4056610_Introduction_to_ultra-wideband_antennaslinks00b7d52a8a5d128046000000pdf] 2003 [Online accessed 9-February-2015]

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[25] Kangmin Hu Huaping Liu Rahul Khanna Jay Nejedlo Changhui Hu Patrick Y Chiang A90nm-cmos 500mbps fully-integrated ir-uwb transceiver using pulse injection-locking for receiverphase synchronization httpeecsoregonstateeduresearchvlsipublicationsPUBSUWB_RFIC2010_hupdf [Online accessed 7-February-2015]

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[27] Benoit Miscopein Joseph J Boutros Sami Mekki Jean-Luc Danger Em channel estimation ina low-cost uwb receiver based on energy detection httpbibliotelecom-paristechfrcgi-bindownloadcgiid=8545 [Online accessed 17-May-2015]

[28] Jimyung Kang Kwan-Ho Kim Soon-Woo Lee Young-Jin Park An ir-uwb receiver design for lowcost applications httpieeexploreieeeorgstampstampjsptp=amparnumber=4381025[Online accessed 17-May-2015]

[29] Philipp Walk Elena Pancera-Thomas Zwick Jens Timmermann Alireza Ajami Rashidi Applica-tion of optimal pulse design in non-ideal ultra-wideband transmission httpcdnintechopencompdfs-wm6880pdf 2009 [Online accessed 21-February-2015]

[30] A E Fathy Y Yang Y Wang Design of compact vivaldi antenna arrays for uwb see through wallapplications httpwwwjpierorgPIERpier822608040601pdf 2008 [Online accessed18-May-2015]

[31] Christian Sturm Werner Wiesbeck Grzegorz Adamiuk Basic properties and design principles ofuwb antennas httpwwweeoulufi~kkdtsptutoriaalitWiesbeckpdf 2009 [Onlineaccessed 18-May-2015]

[32] E Pancera T Zwick and W Wiesbeck Differentially fed array for uwb radar applicationshttpieeexploreieeeorgxplsabs_alljsparnumber=5067742amptag=1 2009 [Onlineaccessed 21-February-2015]

35


[33] Everett G Farr Extending the definitions of antenna gain and radiation pattern intothe time domain httpwwwresearchgatenetpublication237239804_Extending_the_Definitions_of_Antenna_Gain_and_Radiation_Pattern_Into_the_Time_Domain 1992 [On-line accessed 25-February-2015]

[34] Fellow Chun-Huat Heng Lei Wang Yong Lian 3ndash5 ghz 4-channel uwb beamforming trans-mitter with 1 scanning resolution through calibrated vernier delay line in 013-mcmos httpieeexploreieeeorgxplloginjsptp=amparnumber=6329989 2012 [Online accessed 7-February-2015]

[35] Cam Nguyen-Fellow Rui Xu Yalin Jin Power-efficient switching-based cmos uwb transmittersfor uwb communications and radar systems httpieeexploreieeeorgxplloginjsptp=amparnumber=1668344 2006 [Online accessed 7-February-2015]

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[42] Jeong Soo Lee and Cam Nguyen Novel low-cost ultra-wideband ultra-short-pulse transmitterwith mesfet impulse-shaping circuitry for reduced distortion and improved pulse repetition ratehttpwcspengusfeduuwb_testbed_docslow_costpdf 2001 [Online accessed 11-June-2015]

[43] Cemin Zhang and Aly E Fathy Reconfigurable pico-pulse generator for uwb applications httpwebeecsutkedu~fathyfathypulse_generator_utkpdf [Online accessed 11-June-2015]

[44] httpwwwnicommultisimhttpwwwnicommultisim [Online accessed 11-June-2015]

[45] httpwwwnicomultiboard [Online accessed 11-June-2015]

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[52] httpwwwqslnetn9ziawirelesspdfan922pdfl [Online accessed 11-June-2015]

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[53] Jan Machaacuteč Pavel Protiva Jan Mrkvica A compact step recovery diode subnanosecond pulsegenerator httponlinelibrarywileycomdoi101002mop24945pdf 2015 [Online ac-cessed 9-February-2015]

[54] Jan Machaacuteč Pavel Protiva Jan Mrkvica Universal generator of ultra-wideband pulses httpwwwradioengczfulltexts200808_04a_074_078pdf 2015 [Online accessed 7-February-2015]

[55] httpwwwnxpcomdocumentsdata_sheetMMBT3904pdf

[56] httpwwwnxpcomdocumentsdata_sheetBFG135pdf

[57] httpwwwnxpcomdocumentsdata_sheet74HC_HCT04pdf

[58] httpwwwticomlitdssymlinksn74lvc1g04pdf

[59] Ibrahim Tekin Sertac Yilmaz Ultra-wideband n-bit digitally tunable pulse generator httpieeexploreieeeorgxplsabs_alljsparnumber=1570027amptag=1 2005 [Online accessed20-February-2015]

[60] Jim Williams Simple nanosecond-width pulse generator pro-vides high performance httpwwwedncomdesignanalog4329164Simple-nanosecond-width-pulse-generator-provides-high-performance [Online ac-cessed 10-March-2015]

[61] M Edwall Low-noise amplifier design and optimization httpepublltuse1402-16172008230LTU-EX-08230-SEpdf 2008 [Online accessed 18-May-2015]

[62] httpwwwfarnellcomdatasheets66318pdf [Online accessed 11-June-2015]

[63] httpwwwfarnellcomdatasheets1848785pdf

[64] httpswwwinfineoncomdgdlInfineon-BGA420-DS-v01_01-enpdffileId=db3a304314dca3890115418cec781637

[65] httpswwwinfineoncomdgdlInfineon-BGA616-DS-v02_01-enpdffileId=db3a304314dca3890115419102bb163b

[66] httpwwwavagotechcompagesenrf_microwaveamplifiersgain_block_and_driversmga-30889

[67] Sowmya Madhavan Nandini AS and Dr Chirag Sharma Design and implementation of analogmultiplier with improved linearity httpairccseorgjournalvlsipapers3512vlsics08pdf 2012 [Online accessed 11-June-2015]

[68] Min-Suk Kang Sang-Gug Lee Anh Tuan Phan Ronan Farrell Low-power sliding correlationcmos uwb pulsed radar receiver for motion detection httpeprintsmaynoothuniversityie14551UWB_Radar_Phan_2475pdf 2015 [Online accessed 7-February-2015]


[70] Cheng-Wei Pei Fast simple one-shot pulse stretcher detects nanosecond events httpwwwplanetanalogcomdocumentaspdoc_id=527407 2015 [Online accessed 30-Mars-2015]

[71] Basics in emc and power quality schaffnercomcndownloadsfile-downloadfilebasics-in-emc-and-power-qualitypdf 2013 [Online accessed 30-Mars-2015]

37


[72] Umar Khayam Primas Emeraldi Design of matching impedance for ultra wideband partialdischarge detection httpieeexploreieeeorgstampstampjsptp=amparnumber=66762662013 [Online accessed 30-Mars-2015]

[73] B Becciolini Impedance matching networks applied to rf power transistors httpwwwplanetanalogcomdocumentaspdoc_id=527407 2005 [Online accessed 24-Mars-2015]

[74] httpwwwradartutorialeu09receiversrx10enhtml [Online accessed 10-June-2015]

[75] Marta Cavagnaro Erika Pittella and Stefano Pisa Uwb pulse propagation into human tissueshttpstacksioporg0031-915558i=24a=8689 2013 [Online accessed 10-June-2015]

38


AcronymsADC Analog-to-Digital Converter

BJT Bipolar Junction Transistor

CMOS Complementary Metal Oxide Semiconductor

EMC electromagnetic compatibility

EMI electromagnetic interference

ESS-H Embedded sensor systems for health

LNA Low Noise Amplifier

MCU Microcontroller Unit

MOSFET metal oxide semiconductor field effect transistors

PAM Pulse-amplitude modulation

PCB Printed Circuit Board

RF Radio Frequency

SRD Step Recovery Diode

UWB Ultra Wideband

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low cost ultra wideband radar for human ...823001/fulltext01.pdfa simple low cost ultra wideband...

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