Märlardalen University School of Innovation Design and Engineering Västerås, Sweden

Märlardalen University School of Innovation Design and Engineering Västerås, Sweden
Märlardalen University
School of Innovation Design and Engineering
Västerås, Sweden
Thesis for the Degree of Master of Science in Engineering - Robotics
DVA502 30.0 credits
LOW COST ULTRA WIDEBAND
RADAR FOR HUMAN PROTECTION
Martina Öhlund
[email protected]
Hampus Carlsson
[email protected]
Examiner: Magnus Otterskog
Mälardalen University, Västerås, Sweden
Supervisor: Martin Ekström
Mälardalen University, Västerås, Sweden
June 12, 2015
Mälardalen University
Master Thesis
Acronyms
ADC 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.
1
Mälardalen University
Master Thesis
Abstract
The majority of the UWB radars available on the market today are expensive and often closed for
further development due to proprietary rights. Therefore it is difficult to fully understand and adapt
the functionality of an available UWB system to fit one’s needs. The consulting-firm Addiva purchased
an UWB radar to be used in a safety system. However, the radar had limitations and the functionality
of it was partly unknown. This master thesis was inspired from this issue to examine the possibilities
of developing a low-cost UWB radar, with main focus on research of human detection. The system
should be easy to understand and modify, as well as reporting reliable data from the scanning. The
results indicate that such a system can be developed. However, further development to the UWB radar
needs to be made in order to have a complete system.
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Mälardalen University
Master Thesis
Sammanfattning
Majoriteten av de befintliga UWB radarsystemen som finns på marknaden idag är dyra och ofta
begränsade för viderutveckling på grund av äganderätt. Detta leder till komplikationer att få en
full förståelse över funktionaliteten i ett befintligt UWB system och att anpassa den efter ens behov.
Konsultbolaget Addiva införskaffade en UWB radar för användning i ett säkerhetssystem. Denna
radar hade dock begränsningar och viss del av funktionaliteten var okänd. Det här examensarbetet
inspirerades utifrån dessa problem att undersöka möjligheterna för att utveckla en lågkostnads-UWB
radar, för användning främst inom forskning för detektering av människor. Systemet skall vara lätt
att först� och modifiera, samtidigt som det ska ge tillförlitlig data från scanning. Resultaten av denna
rapport indikerar att ett sådant system kan utvecklas. Vidareutveckling av systemet behövs dock, för
att ett komplett fungerande system skall erhållas.
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Mälardalen University
Master Thesis
Table of Contents
Acronyms
1
1 Hypothesis
5
2 Problem formulation
5
3 Introduction
6
4 Background
4.1 Pulse generator . . . . . . .
4.2 Pulse shaper . . . . . . . .
4.3 Amplification / transmitter
4.4 Antenna . . . . . . . . . . .
4.5 Amplification / receiver . .
4.6 Sampler / Integrator . . . .
4.7 State of the art . . . . . . .
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7
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5 Method
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6 Hardware
12
6.1 Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7 Transmitter
7.1 Pulse Generator . . . . . . . . . . . . .
7.1.1 Pulse Generator V1.0 . . . . .
7.1.2 Pulse Generator V1.1 . . . . .
7.1.3 Pulse Generator V1.2 . . . . .
7.1.4 Miscellaneous Pulse Generators
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13
13
13
15
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16
8 Receiver
8.1 Pulse
8.2 Pulse
8.2.1
8.2.2
8.3 Pulse
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19
19
19
20
21
22
Amplification . . . . . .
matching . . . . . . . .
Advanced Gilbert Cell
Basic Gilbert Cell . .
Extender . . . . . . . .
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9 EMC
23
9.1 EMC Issues in this project . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
10 Results
25
10.1 Q1: What are the drawbacks of a low cost UWB radar? . . . . . . . . . . . . . . . . . . 25
10.2 Q2: Will the bottleneck be in software or hardware? . . . . . . . . . . . . . . . . . . . . 30
11 Discussion
31
12 Future Work
33
References
38
4
Mälardalen University
Master Thesis
1 Hypothesis
The hypothesis for this thesis work is as follows:
A 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 to
evaluate where the cost can be reduced. This will result in the development of a simple and easily
understandable system, which allows for further development of the UWB radar.
The problem formulation (Section 2) evaluates this hypothesis and focuses on the possible challenges
with it.
2 Problem formulation
In order to make a low cost UWB radar, some questions need to be answered. This section discusses
the main challenges that emerges when developing a UWB radar:
Q1 What are the drawbacks of a low cost UWB radar?
One of the challenges of making the system low cost is that it is time consuming. As there is
no low cost chip available, see Section 4.7 for more information, the electronics need to be built
from scratch. This results in that each sub circuit needs to be tested thoroughly to ensure a
properly working system.
This leads to the question of whether or not it will be possible to decide if a reasonably low cost
UWB radar can be made within the given time frame. A prototype will be developed during this
period, where the quality of it may vary. However, there will at least be some groundwork on
the subject, which can be further researched in the future. It should also with this information
be possible to roughly decide the probability of developing a successful low cost UWB radar.
Another concern about making it low cost is if it heavily affects the precision of the UWB radar.
Will the function of some sub circuits be affected by the fact that it is low cost and therefore
not being able to perform as well as a more expensive solution? The strength for high frequency
signals declines rapidly with longer distances on the circuit board. Therefore this could be a
problem with a low cost solution as more components will be present on the circuit board.
Q2 Will the bottleneck be in software or hardware?
Some functionalities are better to implement in software, other in hardware. Some parts will be
restricted due to the limited development time while other parts will restrict the final product. It
is therefore difficult to pinpoint the bottleneck as it depends on how and what is being evaluated.
For example, when developing, it will most likely be the development of the hardware that is
most time consuming and therefore acts as the bottleneck. In the final prototype, however, it
may 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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Mälardalen University
Master Thesis
3 Introduction
While UWB technology is not a new subject, research in the field is still relatively limited. However,
advances in high frequency electronics and an increasing demand for wireless technology have given
rise to an exploding interest in UWB. UWB is generally defined as a wireless system that is operating
with a bandwidth of at least 500 MHz [1].
To achieve a wideband signal, most UWB systems are pulse based and tend to operate in higher
frequencies at around a few GHz. To analyze these pulses on the receiver, a common but expensive
method is to have a very fast analog to digital converter (ADC) in the order of multi Gigasample per
second (Gsps) for digital analysis of the signal [2]. To heavily reduce the cost, this project will focus
on doing most of the signal processing with analog electronics. It will also remove the requirement
for a fast sampler which otherwise, apart from being expensive, also would produce a huge amount of
data to be processed.
Some areas of use for a UWB radar are within industry, rescue work and healthcare. In an
industrial environment, the UWB radar could be used as part of a safety system, for detection of
humans approaching heavy machinery [3]. In rescue work the system could be used for detecting living
humans trapped under some debris [4]. In healthcare the areas of use could be to monitor movement
in 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 commercial
product. However, there is a collaboration with the company Addiva. Addiva is a consulting-firm with
a focus on product development and technology. They acquired an UWB radar to be a part of a safety
system, where they were going to develop most of the software. However, it turned out that the UWB
system 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 research
about the possibilities to make a low cost UWB radar. Apart from being low-cost, the system should
also be easy to understand and manipulate so that further research on the radar can be done.
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Mälardalen University
Master Thesis
4 Background
The research in UWB technology is not new but in recent time the field has had surge of interest
as the demand for wireless technology together with the advances in high frequency electronics. The
definition of UWB can generally be seen as a wireless technology that is operating with at least 500
MHz bandwidth. This is usually achieved with a pulse based system rather than manipulating a carrier
wave 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 with
narrowband signals operating within the same frequency band. To achieve this, the system distributes
its energy over its entire frequency band, making the energy very low at each frequency while the
total energy can be similar as a narrowband signal. Most other wireless technology perceives the weak
wide band signal as some low powered noise. If designed correctly, this does also allow it to be robust
against other narrowband wireless systems for similar reasons. The UWB system can be made to only
care a little about each frequency, making narrowband signal to only slightly alter what the system
sees 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 precision,
short range radar. In the past, most of the focus has been in communication which leaves the radar
side even less explored [9]. This makes it so there are very few established radar platforms to build on
and those that do exists are quite expensive, hard to use or have some limiting functionality. Short
range radar can be used in multiple applications ranging from detecting some simple life signal in a
senior citizens home for health monitoring, searching for humans in rescue work to detect a human
approaching a heavy machine [10, 3]. Industries with heavy machinery can require some form of human
protection. It can be done by limiting the physical availability of the machine or where a machine
can automatically slow down if a human approaches. Other types of radars exists to detect humans
in these areas but UWB provides other sets of characteristics such as the low interference and the
possibility to see through walls, acting as a complement to other technologies[4]. Compared to other
detection method like IR and camera, UWB allows for the sensor to be omnidirectional making it
possible for one sensor to detect in all directions [11]. But for it to be practical to be used in those
areas, 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 method
is a Pulse-amplitude modulation (PAM), sending a known pulse train where the pulse strength is
varied. The idea is that the environment is static enough so that each pulse is exposed to the same
environment. The receiver tries to match the incoming pulses with the known sequence and they
should all be affected in a similar way. The other method involves repeatability sending pulses that
will be integrated over time to remove most of the background noise. This method also relies on a
static environment where multiple pulses can reflect in the same way to get a degree of certainty on
a detected target. however, to achieve this the system requires some sort of a pulse matcher in the
receiver to be matched with a duplicate of the antenna pulse in the transmitter, usually sent via a
delay line [14, 15]. Both types use similar design overall but one key difference is on the receiver end,
as the PAM type needs some type of matcher that is able to tell the different pulses apart. A common
method to do this is digitally. This puts a heavy load on the analyzing hardware as UWB is often
operating in GHz frequencies, requiring a powerful computer connected to a fast sampler circuit. This
does however make it relatively simple to calculate distance with the time of flight with a high degree
of certainty that it is not a random interfering signal from an external source. The PAM is a similar
technique that is used in UWB communication, allowing some solutions to be copied over and used in
radar as the research in the communication field is more developed. The method of integration can
usually rely on more analog techniques to detect the pulses and also reduce the demand for the high
computational demand as multiple pulses can be integrated into one output signal. Combination of
the two techniques is often used in a way to reduce the demand on a high speed Analog-to-Digital
Converter (ADC) or the potential of high complexity analog circuity. Other methods are more common
in UWB communications.
As most UWB systems operate with the same type of modules, the following subsections will
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Mälardalen University
Master Thesis
describe the general design approach for the hardware side of UWB. Each key module and its function
will be described. Some modules can be combined and some might not be necessary depending on how
the system is implemented. Additional support circuit will be needed and includes things like clock
and power supply.
4.1 Pulse generator
The pulse generator can be seen as the heart of an UWB system as it dictates what frequency range
the system will operate in. As the name implies, the task of this module is to generate narrow pulses,
the width can vary from a few hundred picoseconds up to a few nanoseconds. The end goal for the
pulses is to be radiated out through an antenna. The pulse type is typically either Gaussian- or mono
cycle pulses. The shape of the pulse can be altered to change the power distribution of the signal over
the frequencies. Although very hard to achieve, the ideal signal is generally homogeneously distributed
over the entire operating frequency range to not disturb other electronics operating in that frequency.
In some applications it might be desired to have more power in some frequency to get a particular
behavior 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 requirement,
often with Complementary Metal Oxide Semiconductor (CMOS) technology. This allows for a precise
circuit where it can be fine-tuned to function properly as the technique is very mature and is wildly
used in digital circuits like microprocessors. It is very fast and it is common to use in other Radio
Frequency (RF) applications. It does however require a lot of knowledge and time as the entire chip
has to be remade when it requires something to be changed [15]. IC has the additional benefit of
having a small size compared to if the circuit were to be built with traditional components which is
a important aspect when it comes to RF. Each trace length add impedance and can also act as an
antenna, 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 in
real time as it does not have any easy way to probe the internal signal. As it does also take a lot of
time for each iteration, discrete components can be used instead. A popular component in this case
is to use is a Step Recovery Diode (SRD). It got a special property when switching from a positive
voltage to a negative voltage, it discharges a very small capacitance. This can be used to generate very
short pulses, allowing a wide band signal. The signal generated with a SRD does have very specific
characteristics, it generates many harmonic spikes over the frequency spectrum with equal spacing.
This is called a comb generator [17].
The pulse length is very important as it sets the limit on the range resolution, where a shorter pulse
allows objects closer to each other to be detected as different entities. The standard formula for this
can be seen in equation 1 where c is the speed of light, tau is the pulse width time and Sr is the range
resolution. This leads to a pulse width of 1 ns that will at best have the ability to see the difference
between one object and another object that is 15 cm further away [18].
sr =
c·τ
2
(1)
4.2 Pulse shaper
Depending on how the pulse is generated, the generated pulse might require to be manipulated to get
a desired shape to better match an antenna. It can be seen as a part of the pulse generator as it can
contain components to tweak the generated pulse. To change the signal, it can sharpen the edges of
the pulse, invert the signal, or even make the pulse longer. Thus, the pulse shaper can account for
losses or filter unwanted frequencies although generally not in used with simple pulses as UWB often
benefit of using a wide set of frequencies. It can be required if it is outside the allowed frequency band
[19].
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Mälardalen University
Master Thesis
4.3 Amplification / transmitter
Which type of amplification is suitable depends on previous steps. A good pulse is commonly just
amplified with an RF amplifier which will keep most of the signal intact. As the widely used wireless
LAN is operating around 2.4 GHz, RF amplifiers around that frequency is relatively cheap and is
accessible. There are some IC amplifiers that are specificity designed for a very wide band of frequency,
mostly based on CMOS technology. Some systems use the amplification as a pulse shaper which can
be done with the help of a fast transistor which will both amplify and change the characteristics of the
pulse. As with all RF technology, the output impedance should match the 50 ohm that is commonly
used for antennas [20, 21]. This can be very hard to achieve as continuous matching for a wide set
of frequencies is very challenging if not impossible. Some implementations of a generator generates
powerful enough pulses to go directly out to the antenna which does not require amplifications. Some
amplifiers do also split the signal going to the antenna into two signals, which can later be used as a
template or for further analyzing.
4.4 Antenna
Antenna designs for UWB is often designed very differently to a traditional narrowband antenna. As
narrowband antennas only require and even benefit from being good around a single frequency, it is
traditionally just a wire with a specific length specified by the wavelength. The UWB radars have
to be good in multiple frequencies which often leads to designs to have rounded shapes and varying
pieces of length. To achieve this, most antennas are printed on copper laminates, allowing for a more
complex design. It is however very challenging to achieve an antenna which is preforming uniformly
over the entire operating spectrum and the signal might get distorted. This project will not deal with
any development of an UWB antenna. This will be researched and prototyped in parallel with this
project and is done by doctoral student Melika Hozhabri who currently is working with Addiva and
Embedded sensor systems for health (ESS-H) [22, 23, 24].
4.5 Amplification / receiver
As the returning signal will generally be very weak, it requires amplification. Most wireless systems
amplify the signal very close to the receiving antenna to reduce the loss of the signal, microwave
frequencies have a high loss rate in coaxial cable. To receive most of the signal, impedance matching is
very important in the receiver, more so than in the transmitter. The signal will be amplified through
what most likely to be a Low Noise Amplifier (LNA) and it is the key in finding the weak response
signal [25]. This type of amplifier does have a static gain, typically ranging from 6 dB to 30 dB. If the
signal power is still not strong enough, additional amplification stages can be added with the use of
more traditional amplifiers after the LNA when the signal strength is much stronger than the internal
noise of an amplifier.
Another possible approach is to integrate the input signal directly, allowing multiple pulses to be
averaged resulting in the noise cancelling itself out while the pulses keep adding up. The signal can
then be amplified with less regard to the noise figure of the amplifier.
4.6 Sampler / Integrator
Most radar systems today do the end analysis digitally which adds the requirement to convert the
analog signal to a representative signal digitally. This can often be a challenging part in UWB due
to the high frequency components coupled with the wide band of frequencies. On one extreme, the
most straightforward solution is to oversample the received signal and analyze the signal digitally.
This allows for frequency analysis and signal integrity without complex electronics. This does however
require a multi GHz ADC and it will produce massive amount of data to be processed, with the
obvious drawback of high cost. The other extreme is to build most of the signal analysis with analog
electronics. This can heavily reduce the cost due to much lower hardware demand on the digital side
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Mälardalen University
Master Thesis
but the added complexity can be more challenging than the rest of the entire system. Solutions often
lie in-between, leaning towards a more digital analysis [15, 25].
4.7 State of the art
The initial interest of UWB has been in communication while radar techniques has been lagging behind.
This is why it is common in this field that many techniques in radar stem from the communication side,
especially IR communication. The key component that is commonly used for a low cost transmitter
is a SRD, it allows for an extremely short pulse length of a few hundred picoseconds [26]. This is
relevant as it is directly related to the range resolution where a shorter pulse gives a potentially better
resolution. With this, many systems today operate in a frequency band of a few GHz, typically within
0.5 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 a
complex ADC together with powerful computational hardware for signal analysis. Most methods used
that lowers the cost does often require some sort of compromise such as loss of information, reduced
speed or using very complex analog circuitry. Methods used often include some sort of down conversion
like 1-bit sampling, synchronous pulse matching and pulse detection triggering [27, 28]. Many of the
analog filters used in broadband signals are derived and adapted from narrowband applications. In
many 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 from
simple copper laminates and a circuit mill. Different antenna designs have been proposed but one of
the most recurring design that is used is variances of the Vivaldi antenna. It provides good properties
regarding a wide bandwidth for emission, absorption and low signal distortion. The Vivaldi antenna
is generally operating in planar operation and can be arranged in an array [30, 31]. To standardize
the 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. They
generate UWB pulses that are usually strong enough to not need any further amplification. Many
chips are configurable to some extent, like changing the pulse frequency and center frequency of the
pulse. No suitable receiver IC chip is currently available. Part of the reason is due to how the receiver
is 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 radar
circuits on the market today with a lower price around $18 00 [36], developed by KBOR. This radar
is not a complete system, just a transceiver. The most common scenario is that the prices are not
available as public information. The Swedish company Radarbolaget provides a product for stationary
monitoring of the inside of a furnace, detecting defects in the manufacturing process [37]. Novelda
AS got a product called Xethru which allow human interaction with the system, able to control
software with hand motions and breath [38]. Timedomain has got the PulsON 410 platform which is
a versatile platform for UWB applications [39]. Geozondas offers different UWB radar kits designed
for tracking objects through walls or rubble, stating it to be a cheap equipment set [40]. The prices
for all theses products are however unlisted. There are multiple scientific papers describing different
implementations of low cost UWB transceivers [41] or modules [42, 43]. However, few offer a complete
system with both software and hardware.
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Mälardalen University
Master Thesis
5 Method
Most complete systems can be divided into sub-circuits where generally each have a specific function.
To develop a low cost version of UWB radar this project will adapt and configure different modules
from different existing solutions. When a general design is made, each module will be built and
evaluated before moving to the next one. This will ease the procedure of choosing sub-circuits with
good performance for low cost. Each module that is built will increase the understanding of the system
and 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 the
methodology. The theory of each circuit will be based on scientific papers and existing systems. If a
particular design is considered applicable in this system, it will be designed and adapted to achieve a
specific function. When the circuit is built, it will be evaluated if performed as expected. Papers that
proposes designs which are described to have desired functionality for this system, but lack proper
explanation will be evaluated if it can be understood with the help of a simulation or when built. Each
circuit chosen will initially be built with the specified components or if the components is unavailable,
comparable components will be chosen. If the result from a circuit is decent, it can be modified
to improve the results. This process will be repeated until satisfactory results for each module are
achieved.
All circuits will almost exclusively only use surface mounted component as the legs of through
hole components tends to act like antennas. The circuits will be on a printed circuit board (PCB)
using 35 µm thick copper laminate. They will then be evaluated and when possible adapted to get
the desired result. Different circuits will be built and evaluated to achieve an understanding of how
different implementations of the same function, as well as the PCB layout, changes the characteristics.
Each circuit will start off with a quick and simple design without much consideration of the PCB
layout. Circuits that are very unstable with a crude PCB design will not be further developed. This
is partly to save time as it speeds up the process of evaluating many different circuits and it makes
it easier to replicate and reuse the final design from this report. When each module has a suitable
candidate they will be put together into a transmitter or a receiver system for further testing. The
information on how each part works separately can help a great deal if problems occur in the complete
system. Most of the system will not be dependent on a specific implementation of a single module.
In essence, the pulse generator can be changed to generate another type of pulse while the rest of the
system should not require much change if any at all. The point is to allow the system to be further
developed to increase the functionality, reliability and/or precision with less limitations. When the
required 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 Hardware
During this thesis, all designs and circuits were made from scratch and produced first hand. To
achieve this, a number of tools and practices were used. All the circuits were designed using the
software programs Multisim 13.0 and Ultiboard 13.0 [44, 45]. The PCBs were made with a ProtoMat
S62 circuit mill [46]. The components were soldered by hand and in some cases also with the help of
a LPKF ProtoPlace S pick and place machine [47].
6.1 Testing
During the testing phase of the circuits, a HMC 8043 regulated power supply, HMF2525 function
generator, TDS 3012 oscilloscope and a multimeter were used as needed [48, 49]. The function generator
was 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 the
implementation for analysis of the transmitter [50, 51]. The spectrum analyzer has been used to
investigate the frequency range of the system. The network analyzer was used for displaying Schmitt
diagrams.
A block diagram of the system can be seen in Figure 1. Here, the method chosen for the functionality
of the UWB radar can be observed. The following two sections will describe the implementation of
the hardware and the design chosen.
Figure 1: Block diagram of the UWB radar design.
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7 Transmitter
An essential part in a UWB is its transmitter. The main task of the transmitter in any UWB system is
to generate and send out a short pulse in the order of nanoseconds. This is done with the use of some
sort of pulse generator. Also, there is usually a pulse forming step and a transmission line present in
the transmitter. In this master thesis, the formed pulse is sent out to the antenna and a delayed pulse
is sent to the receiver in order to match the incoming pulse. See Section 4 for more information. This
section describes the development of the UWB transmitter and the implementation of its sub-circuits.
7.1 Pulse Generator
During this master thesis a number of different pulse generators were created and tested to evaluate
which type would produce the most suitable pulses. Mainly, one pulse generator was made and
developed into many versions.
A predominant key component in many low cost pulse generators is a SRD. Due to limited availability of this component, it was not used in this project. A PIN diode in certain conditions is described
to have similar characteristics as a SRD when used in a comb generator, which is a common type of
pulse generator [52]. PIN diodes are more available than SRD, it was used as a replacement in circuits
that required it as a prototype. It was however noted that there might be some limitations in higher
frequencies compared to a SRD.
To evaluate if Multisim were able to simulate the effect a PIN diode can preform, it was simulated
in Multisim and the same circuit was physically tested in order to compare the outputs. This was
done to examine whether it was reasonable to test whole circuit modules by simulation first or if the
behaviour was too different for a simulation to be reliable. The outputs from the two circuits are
depicted in Figure 2, the circuit was a diode with a load and a sine wave as input. The two signals was
deemed to not correlate enough to satisfy that the simulation data would represent an entire module
containing a PIN diode good enough. Some circuits does also rely on a physical distance of traces, a
so called transmission line, where the distance of a specific track is very important as it decides the
pulse width. These circuits were not simulated in Multisim.
This section is divided into two subsections. First, the main pulse generator with its iterations is
described and secondly the alternative pulse generators tested are discussed.
7.1.1 Pulse Generator V1.0
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 created
in 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. The
purpose of the driver is to create a triggering pulse for the transmission line. The driver contains
a speed-up step, a delay step and two transistors for creating the pulse. When the signal from the
inverter enters the driver, it will go through the speed-up step as well as through the delay line. The
signal from the speed-up will reach the transistor first, opening it up. When the delayed signal reaches
the other transistor connected to ground it will short circuit the first transistor, thus ending the pulse.
The 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 narrow
pulse. Originally in the design, it contained a SRD. As there is limited availability, it was replaced
with a PIN diode. After the driver there is a bias current added to the system. This bias keeps the PIN
diode forward biased when no pulse is present. There is also a Schottky diode on the transmission line
and it is reverse biased in this state. When a driver pulse reaches the transmission line, the PIN diode
will turn off, creating a negative falling edge which goes both directly to the capacitor and output
and to the now forward biased Schottky diode. The Schottky diodes short-circuits the system and the
inverted signal is reflected back to the output. The unchanged falling edge and the inverted waveform
are 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 V1.0
The driver circuit was simulated in Multisim as there were no special components included in this step.
The output from the simulated circuit can be seen in Figure 4. The output fall time is at 1 ns and
according to the report that the circuit is based on this time should be at 600-700 ps. The simulation
result was considered reasonably close enough to the expected value and the circuit was constructed
for further testing.
This pulse generator was the first circuit built. At this early stage into the project, the length of cables
and tracks were not optimized as the main priority was to get the circuit to work, even if poorly. In
order to have the ability to change the length of the transmission line for longer or shorter pulse length,
a socket strip was added between the two diodes on the transmission line. A cable of desired length
was then added to the socket strip, acting as a microstrip.
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Figure 4: Schematics and output of the driver in the main pulse generator.
As presented in Section 10.1, the output pulse was not satisfactory. In an attempt to improve the
circuit, one by one the components were changed to different values. First, the driver was examined,
as the issue seemed to be that the driver pulse fall time was not short enough. To decrease the fall
time it was believed that either the transistor or the speed-up step had to be faster. The speed-up
was modified by decreasing the capacitor value so that it would de-charge faster and thereby speed up
the driver fall time. Different values were tested but no noticeable change was observed. The resistor
value in the speed-up was modified, but like the capacitor, it made no major difference to the driver
output. In total, the fall time was shortened down by a couple of nanoseconds, from about 15 ns to
10 ns.
The driver transistor was replaced (transistor Q3 in Fig. 3). The transistor MMBT3904 was
replaced with a transistor of model BFG135, which should be faster [55, 56]. However, this did not
affect the system remarkably. At this point, the output pulse width had decreased from the initial 30
ns to about 20 ns. After soldering off and on components many times, the PCB was worn down and
therefore a new PCB was made to clean it up. This new PCB is described in the following section.
7.1.2 Pulse Generator V1.1
In this version the PCB layout was altered. This was done by mainly shortening the PCB tracks and
replacing components of the first pulse generator. The overall placement of the components stayed the
same. The microstrip line between the Schottky diode and PIN diode was redesigned by removal of
the socket strip to reduce the distance between the two diodes. Two vertical lines were added to the
PCB so that the physical distance between the diodes could be changed by adding a microstrip over
the 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 driver
fall time was not noticeably affected when manipulating the circuit. The only significant change was
introduced ringing, most likely from the self-frequency of the capacitors in the system. The transistor
BFG135 (Q3) was replaced with BFG591 [56]. After the replacement no remarkable difference in the
output was seen. It was concluded that not enough current was delivered to the transistors, which could
be crucial as they are of the type BJT and therefore current controlled. Thus, the inverter 74HCT04
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(U1) with an output current of 6.8 mA was exchanged to an inverter of model SN74LVC1G04 with a
32mA output [57, 58]. No significant changes to the output signal after this modification were made.
The last capacitor in the driver, just before the bias, was changed in value from 10 nF to 180 pF. See
Section 10.1 for the results.
The circuit was further tested in a network analyzer and spectrum analyzer. The network analyzer
did not give any results. A test in the spectrum analyzer gave a response which was observed at the
received signal from the transmitter as the signal changed along with the generated pulse.
Figure 5: Patched pulse generator V1.1 with a pulse length of 20ns.
7.1.3 Pulse Generator V1.2
In this version of the pulse generator the placement of the components was changed. The circuit became
more compact and track lengths were minimized. This was an attempt to decrease the interference
from other appliances in the surrounding environment and also to decrease the risk of self-resonance
in 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 higher
accuracy, as it was observed that the potentiometer gave satisfactory results at 0-200 Ω. As the new
potentiometer had more turns available than the previous component, it could be more fine-tuned.
If the resistance would need to be higher, one can easily add a resistor of suitable size. One could
also change the capacitor in series with the potentiometer. However, it seemed more practical to have
a high resolution potentiometer for tuning than having to replace the capacitor for a suitable value
during testing.
The inductor was replaced from 100 µH to 2 nH in order to see what effect this would have on the
system. The Schottky and PIN diodes were replaced with a component containing two PIN diodes in
series. The results are presented in Section 10.1.
7.1.4 Miscellaneous Pulse Generators
Some other pulse generators apart from the main one were made. Theses pulse generators are built
up differently and uses other technologies to generate pulses. These circuits will be described in the
following section.
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Figure 6: Pulse generator V1.2.
Tunable Pulse Generator
A tunable pulse generator made can be seen in Figure 7, where Figure 7a shows the circuit built and
Figure 7b depicts the corresponding schematics. This version is based on a report about a tunable
pulse generator [59]. The idea of this pulse generator is to have transmission lines of different length
depending on how wide pulse is desired. It can then be controlled which path to use and thereby
determining the pulse width. In this circuit the diode D2 in Figure 7b is originally a SRD, but was
replaced with a PIN diode during testing. The other three diodes are PIN diodes as should be according
to the schematics.
The functionality of this circuit is that a reversed pulse shuts down the charged SRD which creates
a sharp falling edge as the SRD becomes discharged. This falling edge travels through the system
directly to the output, creating the start of the pulse, and also through the PIN diode configuration,
which acts as a delay step. There is a transmission line between diode D3 and D4 which decides
the pulse width. The polarity of the pulse is reversed through the use of a short-circuit and the two
components are summed up to create one pulse.
The pulse generator of this type was made with only one transmission line, as the main goal was
to examine how well this type worked. This circuit was not simulated as it contained both PIN diodes
and a transmission line, which as stated in Section 7.1 was difficult to simulate in Multisim. The pulse
generator worked at the first try, however poorly. See Section 10.1 for the results. Increased voltage
of the square wave resulted in more ringing of the output. This circuit was not further developed
after the first version due to very poor results. It was believed that one of the reasons why the circuit
worked poorly was due to the usage of a PIN diode instead of an SRD. Another theory is that it was
due to the transmission line being too long. If this was the case, then likely a modified version with
shorter delay line would produce a shorter pulse.
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(a) Tunable pulse generator circuit.
(b) Schematics of the tunable pulse generator.
Figure 7
Comparator based pulse generator
Another pulse generator was created, based upon Williams work presented in ”Simple nanosecondwidth pulse generator provides high performance” [60]. The idea behind this version is to have two
delayed signals, one with a small offset with respect to the other. This is achieved by a small offset
in value of two resistors which are placed in parallel at the start of the circuit. The schematics can
be seen in Figure 8. Each signal goes through a comparator and then to an AND gate. The first
signal will reach the comparator and produce a low output. Right after, the second signal will reach
its comparator and produce a high output. The first signal will then end and switch the comparator to
high while the second signal is still high. This opens the AND gate and creates the start of a pulse until
the second signal goes back to low again and closes the gate along with the pulse. The input signal
to this circuit is a sine wave from the function generator which converts to a square wave through a
comparator.
Compared to the results of the main pulse generator V1.2 (Section 10.1), this generator gives out a
weaker but about as wide pulse. As the AND gate did not go all the way up to 5 V before switching,
it was considered that the limitation was due to a too slow logic gate. This circuit did not perform
better or as good as the main pulse generator and therefore it was not further developed.
Figure 8: Schematics of the comparator based pulse generator.
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8 Receiver
One main part of a UWB radar is its receiver. In this master thesis, the receiver is defined as being
responsible for collecting a signal through an antenna and match it with the delayed pulse. It then
processes the matching data in order to make an assumption of the potential object found. In this
section the receiver step, its sub-circuits and design will be discussed. A block diagram of the receiver
can be seen in Figure 1.
8.1 Pulse Amplification
After the antenna has received a signal, the signal amplitude will have decreased compared to the
original pulse sent out of the transmitter. As the energy level of a UWB pulse is already very low
and the received pulse will have lost some of its amplitude, an amplification of the received pulse is
necessary. The first step after reaching the antenna is therefore through an LNA. The characteristics
of an LNA makes it very suitable for UWB applications. It is very good at amplifying weak signals
while keeping the noise level low [61]. However, as an UWB signal is at noise level, it is necessary to
collect 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 for
this was because it was assumed that the LNA test circuits which were taken and made from their
respective data sheets were correct. Also, as the LNA’s are very sensitive to interference it was thought
that 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 LNA:s found, which costed in the range
of 60-70 SEK. A suggested circuit from its data sheet was made in order to test the performance of
the LNA. However, as mentioned in result 10.1 it did not preform well and after some further testing
and modification that was suggested in the data sheet, it was discarded as it was never providing any
useful output.
The second LNA used was SPF5189Z, a more expensive version than the first one [63]. The data
sheet provided two different configurations, one optimized for 900 MHz and the other for 1900 MHz.
The data sheet also provided an evaluation board which was used as a base for the PCB layout. The
output of the 900 MHz version is described in result 10.1, it proved to be very unstable and was
therefore not used. The second configuration, adapted for 1900 MHz, proved to even more unstable.
The 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 surroundings
as easily. However, the signal response itself did not improve and no amplification was present. The
cable lengths and track lengths were shortened down and SMA connectors were added to the output
and input of the circuit. This modification made the self-resonance disappear.
Two designs were made for two similar LNA:s, BGA420 and BGA616 [64, 65]. Both were only
tested briefly as similar behavior as the previous LNA:s was observed. BGA420 was discarded as it
was unstable and BGA616 did not amplify the signal enough. Both used schematics from respective
data sheet, but no PCB layout were available. The layout was instead based on a evaluation board
SPF5189Z [63].
The final LNA tested in this project was MGA30889, which is of type gain block [66]. The data
sheet provided test circuits which the PCB layout was based on. To reduce the risk of issues such as
self-resonance, together with SMA connectors for the input and output, the connectors to the supply
voltage and ground were also more carefully designed. This LNA circuit gave an usable output which
is described in 10.1.
8.2 Pulse matching
After the received signal has been amplified it needs to be matched with the delayed pulse from the
transmitter in order to check the similarity and whether a match has been found or not. This step was
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done using a four quadrant Gilbert cell. Two kinds of Gilbert cells were made, a basic version and a
more 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 from
an RF signal. This is done with the help of a local oscillator as one of the inputs to the cell but can
used as a signal multiplier instead. The four quadrant is able to handle four different input signals
and generates two output signals. The inputs are coupled two and two where one dealing with the
positive part and one dealing with the negative part of the signal. The negative should be inverted
to a representative positive signal to work. The output signal represents a multiplication of the two
signals, as it is four quadrant, one of the output represent a negative answer and the other a positive.
A Gilbert cell is usually either a linear multiplier or logarithmic but linearization requires additional
components 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 case,
the template signal is the delayed pulse from the transmitter and the other input is the received pulse
from the antenna. During the testing of the different Gilbert cells, a sine wave from the function
generator was used as a test signal. As template, a DC signal was used. When the sine wave matched
with the DC level, the output dropped respectively. The more the output level dropped, the better
match was obtained. If the DC signal is very low, then the matching level will be very weak and the
output will not drop as much as for a higher DC value. It is also important that the signals are high
enough, for the transistors to open up properly.
Figure 9: Schematics of a basic Gilbert cell.
8.2.1 Advanced Gilbert Cell
The first Gilbert cell to be built was a multiplier based Gilbert cell being founded on another UWB
receiver project [68]. This Gilbert cell, like most Gilbert cells, multiplies currents. Two advantages
with this design were the ability to integrate multiple pulses by controlling when the integration should
be reset and that the integration converts the current output into voltage output, which is easier to
analyze. Both of these additional features are desirable and needs to be implemented in some way or
another in the system. The multiple pulse integration is used to increase the certainty of a correctly
detected target.
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This circuit was first simulated in Multisim. However, the simulation did not give expected outputs
and also there was an issue in the simulation which resulted in the simulation constantly crashing. It
was decided that a circuit should be made despite a non-functional simulation. The reason for this was
because it was thought that the circuit could be more easily evaluated and manipulated when having
a physical circuit to test. Also, as the behaviour of the advanced Gilbert cell was not fully understood,
a 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 configuration no output was obtained. The circuit seemed to become short-circuited when starting up. If
only the supply voltage was applied, then the system worked. However, when sending in the template
and input signal, the system drew a high amount of current. Whenever this happened, the system had
to be reset. The reset was made by removing a transistor and then solder it back on again. The cause
of this behaviour was thought to be either because the capacitors did not discharge or that the kick
start 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 (MOSFET) should be used for this design, which is presented in the paper on a UWB receiver [68]. Therefore,
the circuit was modified and produced to be used with MOSFETs instead. This version did not give
any expected outcome. The output did not correlate with any kind of multiplication. The circuit acted
differently depending on the clock frequency and the inputs did not affect the system as they should.
Also, 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 MOSFET:s.
8.2.2 Basic Gilbert Cell
As the advanced Gilbert cell did not work as expected, it was decided that a more basic Gilbert cell
should be built. The main idea of making this version was to achieve a better understanding of how a
Gilbert cell works. This would also result in better manipulation of the Gilbert cell in order to add or
change functions for it to be tailor-made for the receiver step. For the schematics of the basic Gilbert
cell, see Figure 9.
The first version of the Gilbert cell can be seen in Figure 11a. The transistors used for this version
were of type MOSFET. This circuit had similar issues as the first advanced Gilbert cell as it was
short circuited after start-up. The transistors were examined and it was discovered that they broke
easily, presumably because they were not powerful enough. Therefore, another circuit was made using
BFG591 BJTs instead [69]. This version worked as expected. A picture of the second version PCB
can be seen in Figure 11b. For the results of this circuit, see Section 10.1.
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(a) The first version of the basic Gilbert cell using MOS-(b) The second version of the basic Gilbert cell using
FET:s.
BJTs.
Figure 11: The two basic Gilbert cell circuits.
8.3 Pulse Extender
After the two pulses have been multiplied, the data is to be sampled and processed in order to draw
a conclusion of the detected object in question. This step is very critical when designing a low-cost
system, 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 master
thesis. The need for such a fast ADC is to oversample the pulse enough for retrieving a satisfactory
representation of the appearance of the nanosecond short pulses. In order to remove the need for a
fast ADC, the pulse is sampled and extended. The method used for this project is based on holding
the pulse before sampling it to the software. The idea is to hold the nanosecond pulses for about
a microsecond. This would decrease the sampling speed, thus allowing for a slower, less expensive,
sampler to be used. The circuit is based on a pulse stretcher [70]. The schematics is shown in Figure
12a.
This circuit also has an adjustable object detector function built into it. This part is built up of a
Schmitt trigger and a digital resistor. Tuning of the resistor changes the threshold for the object size
to be detected. The purpose is to have the ability to change the threshold for the energy level from
the pulse matching at the Gilbert cell. This allows for detection of objects of desired size. When the
threshold 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 test
the circuit without software. After some modifications, this circuit worked as expected. Depending on
the value of the resistor the delay increases or decreases, where higher values increase the delay time.
The results are presented in Section 10.1.
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(a) Schematics of the pulse extender.
(b) Pulse extender circuit.
Figure 12: Pulse extender.
9 EMC
As soon as high speed digital or high frequency analog electronics is designed, electromagnetic compatibility (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 it.
EMC is not excluded to high frequency electronics but it is much more prevalent in that field. Each
part in a circuit is susceptible to the problems that can occur including self-resonance, loss of power,
interference emittance and interference susceptibility. The received interference is commonly picked
up in the system with cables, long traces or from the power source. As a radar system often deal with
weak signals, the introduced noise can be as strong as the signal itself if it is introduced in the wrong
place. Similarly, fast internal switching can leak out radiation via cables or long tracers and if the
circuit is not properly shielded [71].
Impedance matching is also a very common issue when it comes to wireless technology, often when
dealing with an antenna. Matching impedance will allow for a better transfer of power between two
nodes, for example between an signal amplifier and an antenna. The power that is not transferred
can bounce back into the circuit which can create standing waves or worse, damage some sensitive
components. Radar and other wireless technology often use 50 Ω as the default impedance. Using one
of the standard impedances make it easier to buy components or connectors that are matching. The
ideal scenario is when the impedance between two stages is 50 Ω without any inductance or capacitance
over the entire frequency range, which is very hard to achieve [72]. Capacitors and inductors changes
their behavior with changing frequency and can even swap behavior, an inductor can act as a capacitor
and vise versa. This is due to the parasitic properties of real life components. The characteristics of
the impedance can be measured with a network analyzer where the most common parameter is the S
parameter. The S parameter describes how much power is lost at specific frequency and the response
impedance, giving the complex impedance where the imaginary part describes the capacitance or
inductance. Impedance matching is often achieved with small circuits called L-networks or Π-network,
transformers or with a tunable IC. Some ICs can even automatically tune, detecting signal bounces
and altering the impedance accordingly to get the maximal power transfer. However most matching
techniques are matching for just a few frequencies or are only applicable on lower frequency which
makes 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 frequency
signal is present. As the wavelength of a GHz signal, where UWB often operate, is close to the size of
a PCB, the trace design is important. Controlling the trace length can reduce the risk of generating
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standing waves within the circuit or absorbing signals of specific frequencies. Many types of pulse
generators do also contain closed loops, adding a risk of self-resonance with capacitors. A typical
source of EMI comes from IC:s anda way of dealing with it is by having decoupling capacitors as close
as possible to the supply pin.
9.1 EMC Issues in this project
This project started off with little consideration about EMI, as the initial goal was to get the circuits
to just work before improving and matching the circuits. The idea was to speed up the process of
evaluating different circuits. The first instance where it became an obvious issue were with the very
sensitive LNAs. The initial thought was that the LNA circuit would work, even if poorly, but were
proven not to give any proper response. Issues that arose with the LNAs were self-resonance, ringing
signals and flat out dead signals. To solve these issues, new PCB designs were made where the trace
length, component placement, cable length and connectors where more carefully considered on the
different LNA circuits. When all those problems were reduced, the act of using an oscilloscope probe
proved to be enough to disturb the system enough to generate self-resonance. To solve this, SMA
connectors were used on key points to connect to the oscilloscope directly with SMA coaxial cables,
keeping the impedance at 50 Ω. In figure 13, one of the improvement on one LNA can be seen. This is
the difference that shortening the cables made, from just creating a self-resonance signal to an impulse
response.
The probes used for measuring circuit signals on the rest of the system had to be re-evaluated
as it was discovered that they caused self-resonance in the system. This lead to small modifications
of adding SMA connectors to some of the existing circuits, including pulse generators, to be able to
better see a more representative signal in the oscilloscope as they also deal with the high frequency
signals. The circuits are not as affected with an oscilloscope probe everywhere, but it is generally good
to make sure how the probes affects the system. Also, if possible using a probe with a high multiplier
is 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 any
component changes close to the matching. The primary focus of the impedance matching is around the
antennas, to be able to send and receive as good signal as possible. There exists equations to estimate
impedances and how to match it, but they can quickly become a highly non-linear, multivariate system
even in basic cases. The approach was to solve it through empirical research with different networks,
aiming to match for a center frequency while trying to minimize the mismatch for the rest. However
due to time limitation, the impedance matching was not finished in the final circuit design and just
contains 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.
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10 Results
In this section the results of the master thesis will be discussed. The results will be based on the
problem formulation questions as to give a representation of how well answered these questions are.
10.1 Q1: What are the drawbacks of a low cost UWB radar?
As discussed in Q1 (2), many aspects of designing an UWB were time consuming and different circuits
showed a big variation in performance. Most of the circuits were built from primary basic components
such as diodes, transistors, resistors, inductors and capacitors. Most of the duration during this
project was spent on researching, building and evaluating different UWB modules. An IC module
usually comes with a datasheet complete with information on how to balance the circuitry around it,
requiring less time spent on repetitious work regarding filtering and component compatibility.
The following modules have mostly just been tested separately and the behavior described here
was in that single module configuration. The components that is dealing the high frequency signal
are limited to 2.6 GHz as it is the maximum operating frequency for some of the components in the
system. Each circuit lacks the proper support components like power regulators, polarity protection,
clocks, shielding, spike protection and other common safety circuitry. All the different supply voltages
were provided by a power cube. Similarly, all the clocks or input pulses were made using a function
generator.
Main Pulse Generator
The first module to be built was the pulse generator, the key component of most low cost alternatives used a step recovery diode (SRD). SRD:s are however not widely available so the component
was replaced with the more common PIN diode, which shares some of the properties used for pulse
generation [52]. None of the pulse generators that were built could achieve the same short pulse width
as what was reported in the articles which the circuit designs were based on. The first Printed Circuit
Board (PCB) made of pulse generator V1.0 (Section 7.1.1) gave a very weak pulse, at about 100 mV,
with a pulse length of 25 to 30 ns. With some minor tweaking on the circuits the pulse width was
shortened down to 20 ns. The input to the system is the clock, supply voltages and a bias voltage.
For this version, a bias voltage of 0.7 V gave a stable Gaussian pulse.
In pulse generator V1.1 a decrease in fall time from the transistors, from 20 ns to 14 ns, throughout
the system was obtained. The output pulse was between 15 ns and 25 ns wide depending on the value
the potentiometer, although a change in bias voltage to 0.8V resulted in a 10 ns wide pulse. The
output peak voltage of the pulse was increased to 1.5 V.
The output from the driver of Main Pulse Generator V1.2 (see Section 7.1.3) was improved compared to the previous versions, with a fall time of 10 ns. The output could be reduced to a 8-10 ns wide
pulse with carefully tuned potentiometer value. The peak voltage dropped down to 1 V. A slightly
wider pulse output pulse can be seen in Figure 14. The main pulse generator showed inconsistency
in the pulse strength between pulses, which was apparent in all of the versions. The final version
contained the widest frequency spectrum (figure 17a). The final version was tested in the network
analyzer and from Figure 17b, it can be observed that the trace follows the 50 Ω resistance circle. The
trace lies within the inductive area, so if the circuit would be made more conductive the trace would
naturally stabilize around 50 Ω.
The frequency response characteristics of the pulse generators varied greatly, even between different
versions of the same base design. All generators had a low minimum frequency close to 1 KHz but the
maximum frequency ranged from 100 MHz to around 2.5 GHz. The pulse generator that was chosen
to be used is described in the section 7.1.3.
Tunable Pulse Generator
The tunable pulse generator in section 7.1.4 gave the widest pulses out of the three generators. The
output gave 100 ns long pulses with a peak voltage of 200 mV. The input to this system was a 6 V peak
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Figure 14: Output pulse from pulse generator V1.2
to peak square wave and a supply voltage of 0.6 V. Increasing the supply voltage gave an increased
peak 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 Generator
The comparator based pulse generator in section 7.1.4 (figure 15) outputted pulses ranging from 20 ns
to 40 ns with an amplitude of 300 mV. The input to this generator requires only supply the IC:s and
an input clock. A generated pulse from the comparator based pulse generator is depicted in Figure 16.
Gilbert cell
The second module was the pulse comparator, where the designs are based on a four quadrant Gilbert
cell multiplier. Two different designs were made. A more complex cell was made which had more
functionality in the design. It allowed the output current to be converted into output voltage via
integration and it also supported resetting of the integration, allowing multiple pulses to be integrated
in the Gilbert itself [68]. Two different version of this circuit were made, only differentiating with
different types of transistors. BJT was swapped out for MOSFET. Neither version of this design
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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 waves
and pulses. The second design was based on a basic four quadrant Gilbert cell which does nothing
else but act as a multiplier. This was also made into two versions, one with MOSFET and one with
BJT. The transistors of MOSFET version was destroyed under testing. The version with BJT proved
to be more robust. The simplified design worked as intended but lacks the functionality of the more
advanced version. Part of the analog analysis is in the pulse matcher. The dropping output voltage
correlates to the simultaneous high voltages of the inputs. The inputs to the basic Gilbert cell were
a DC signal at 1 V and an AC signal, as described in Section 8.2. The Gilbert cell showed a much
greater sensitivity when an offset of 650 mV was applied to the input signals. The output from this
circuit can be seen in Figure 18. Here, the maximum value means the smallest match. The lower the
value, the higher the match. Lowering the DC input reduced the voltage drop in the output. At the
lowest point the multiplication of the two signals gives the highest match. This still gives a short pulse
as an output which can be even shorter than the initial pulse. The chosen Gilbert cell is described in
the section 8.2.2.
Pulse Extender
The short output pulse from the Gilbert cell is the input to the pulse extender. It provided two
function, the first one was to extend a few nanosecond pulse to around a microsecond and the other
functionality was the ability to change the trigger level out from the Gilbert cell. The pulse extender is
a modified Schmitt trigger with a latch function. The extended pulse length is based on a capacitance
and a bleed resistor, where increasing the value of the resistor makes the pulse longer. However, if
the pulse is too long, it will interfere with the next pulse. The test input to the pulse extender was a
pulse generated from the function generator, with a pulse width of 15 ns. The output from the system
is depicted in Figure 19. As the output triggers high and stays high until a given threshold and then
turns low, the curve looks very similar to a PWM square wave. It can be observed that the 15 ns input
pulse has extended to about 4.3 µs. More detail on the pulse extender can be read in the section 8.3.
LNA
The last required module was the amplifier on the receiver, which is a LNA connected with an antenna.
This type of amplifier is very sensitive to incorrect component matching and to the PCB layout. This
lead to the making of a total of five different LNA circuits, each with different LNA. The first LNA
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(a) The output from the spectrum analyzer.
Master Thesis
(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 as
matching input.
circuit was built based on circuit design provided by the manufacturer [62]. With different pulses as an
input, there were no amplification of the input signal, but rather a de-amplification. The output signal
of the LNA acted very poorly and did not seem correlate directly with the input signal. The circuit
was modified by adding a capacitor between the LNA ground pin and ground, for the DC to be blocked
instead of going directly to ground. However, no noticeable change was observed. It was believed that
due to the low cost of the the LNA, it was more difficult to get it to work as more components for the
test circuit was needed than for a more expensive LNA. The EMC was not particularly considered in
this circuit.
Two slightly different configurations of the second LNA was tested. One optimized for 900 MHz, the
initial circuit generated very unstable peaks from an input pulse and was very prone to self-resonance.
The test circuit was modified into another very similar test circuit optimized for 1900 MHz by replacing
values of some capacitors and removing an inductor. However, this version generated an even more
unstable outputs than the previous circuit configuration and both sine waves and pulses as input often
created self-resonance of higher frequency. The final design used proper SMA connectors for input and
output, this eliminated the issue with self-resonance but did not provide an amplified signal and the
output signal did not follow the shape of the input. This was true for both a sine wave and a pulse as
an input.
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Figure 19: Output from the pulse extender.
Two similar LNA:s 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 sine
wave was used as an input. The circuit with BGA616 showed a proper correlation between input and
output 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 selfresonance and provide a negative gain of 6. The signal had some slight ringing after passing the
amplifier. The output signal from the LNA is depicted in Figure 20, where a 15 ns wide Gaussian
pulse was used as input and the LNA had a supply voltage of 4.8 V. A common output from the
unstable LNA’s is depicted in Figure 21.
Figure 20: An amplified output from an stable LNA circuit.
10.2 Q2: Will the bottleneck be in software or hardware?
In Q2 (2), the location of the bottleneck is discussed, whether it will be in software or hardware. As
the software was not developed during this thesis due to time constraints, this question cannot be
answered as of now.
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Figure 21: Output from an unstable LNA test circuit.
The hardware contains several bottlenecks which are related to the delay signal, relative long pulse
width and the down conversion of the received signal. A pulse width of 10 ns gives a very low precision
as it would make it challenging to distinguish between two objects 1.5 m apart according to equation
1. The delayed template pulse limits the physical distance between the transmitter and receiver as the
signal contains high frequency components. The longer the signal travels, the more of the microwave
frequencies are lost resulting in a distorted signal as a template. Longer distances makes it also more
susceptible to external noise being absorbed. The lowest limit on the components in the circuits is not
rated for frequencies greater than 2.6 GHz. This prevents the system take advantage of an improved
pulse 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 which
is tied to a specific distance. But as the signal is so long, the end part of a pulse can be enough to
match the beginning of the expected pulse giving a match for a closer object. For the same reason,
the minimum detection range with this pulse width is 1.5 m. The down conversion introduces two
limitations, it limits the pulse repetition time as two pulses will overlap if the extended pulse is
longer than the time between two pulses. The second limitation with the pulse extender is the loss of
information. The pulse matcher produces an output signal depending on how well the received signal
matches with the template signal, giving an indication on how the signal has been altered. Most of
that information is lost in the pulse extender as it only acts like a peak detection.
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11 Discussion
In 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 results
indicates that the approach that was used would allow for such a system. The method of researching
and picking different modules to implement proved to contain both benefits and drawbacks. It allowed
for simpler testing environment of each subsystem without any dependency on the rest of the system.
The need of matching modules together and making them more standalone costs a lot of time. This
was one of the biggest drawbacks. The current design does however contain multiple flaws where some
of them could be resolved with further development. The characteristics of the circuits presented in
10 is not as good as desired. The current pulse generator generates pulses with the width of around 10
ns. This is a bit too wide and a shorter pulse increases the bandwidth[74]. To achieve a wider set of
frequencies in the gigahertz spectrum and a range resolution of less than 50 cm, a width of 0.2 to 3 ns
is required. The possibility to achieve 0.2 ns is reported in reports that was used as base for multiple
pulse generators, but has not been achieved in this project. This might be due to the SRD being
replaced with a PIN diode, which discussion online indicated would work for frequencies below 5 GHz.
This should not affect the other modules too much as they were kept not to be strictly dependent on
the characteristic of the signal. On some of the pulse generators, the power was high enough out from
the generators to not require any amplification before transmission.
The signal splitter which goes to the delay line from the transmitter antenna is not properly
designed. It acts like a buffer step while at the same time affecting the matching to the transmitting
antenna. Ideally, it should have no impact when splitting the signal. The complete impedance matching
was not finished for either the receiver nor the transmitter, which was apparent for the transmitter
as the antenna did not contribute much to the emitted signal. The frequency range of the generated
signal is not tuned to any specific range regarding human reflection which can limit the functionality
of the system.
The receiver does have a major drawback with the approach of using a Gilbert cell together with
the pulse extender. As of the current design, all that is required to trigger a pulse is a high enough
peak from the Gilbert cell to register as a pulse, leaving a lot of room for potential false positives. But
it does reduce the required sample rate by a great deal. To be able to analyze a pulse in software, a
fast ADC would require a high sample rate of a few Gsps while this design only requires a sample rate
of a few times greater than the pulse repetition frequency. This allows the control system to be as
slow as desired as long as it is compensated with a lower pulse repetition frequency. Another thing to
consider is the pulse extender. After detection it will hold high which will not allow for other objects
to be detected behind the target. This has to be done in software controlling the delay step, allowing
objects at different distances to be identified. However, the output signal can be directly read as a
digital input to a Microcontroller Unit (MCU) or an embedded computer. While this reduces the
requirement on an ADC, the output of the pulse extender does only give the output HIGH and LOW,
it 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 the
system. The delay step has to be controlled and calibrated to match each distance with a specific
delay where a longer delay allows the signal to propagate a longer distance before the system expects
the signal to return. The second key point to control is the trigger level of the pulse extender. This
will allow the system to change the voltage level required to trigger a detected signal which can be
used to account for loss of signal strength for objects further away and also to iterate different voltage
levels to approximate the size of the object. These two aspects can be altered with the help of a digital
resistor 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 human
body, which frequencies generally are reflected and which are absorbed by the body. This information
is important as it changes how the template pulse should be altered to be as similar as possible to the
expected return signal. Furthermore, it dictates which frequencies the pulse generator should generate
for better performance, if for example multiple but spread out frequencies work better than a coherent
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set of frequencies.
The 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 be
done in software. The system does also require adapting the trigger level for the pulse extender as
the voltage input will be inversely correlated to the distance to the target. This can also be used to
determine the size of the detected object, where a lower threshold level allow for bigger objects to be
detected. The software does also need to change the delay timer to change the current search distance
from the radar. The software is where the integration should take place, deciding how many detected
pulses 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 from
this project leaves human detection out as no information was gained to how well a system like this
could detect humans. Other projects does report that human detection is possible with UWB radar
technology [4, 75]. The radar system was not finished in this project but the results indicates that
most of the required pieces can be made with low cost components while also keeping the complexity
down. Most of the components in this design can be swapped to similar components, just with better
performance while still keeping the cost relatively low. One key component missing in this project is
the SRD which, if present, could allow for a better pulse generation. The current design does also lack
some more advanced frequency analysis on the received signal which loses some of the benefits with the
UWB technology. With a complementing software calibrated for this system and some improvements
in the overall circuitry design, the radar system might be able to scan an area and be able to detect
human presence.
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12 Future Work
Due to time constraints, it was not possible to make a complete, functional prototype of a low-cost
UWB radar during this master thesis. As described in Section 10, all the modules building up the
system give satisfactory results. However, a design of the complete system has not successfully been
made.
The main goal of the project was to detect humans so the system has to be tweaked around
that. One key aspect is the frequency of the generated pulse has to be optimize for frequencies that
reflects well on human tissue. Similarly, the receiver could be modified to be more sensitive to specific
frequencies or split the received signal into channels with different filters to get a more detailed analog
analysis. Another important aspect to be improved is the impedance matching through the entire
system. This area has only briefly been examined during the project and needs to be addressed in
order 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 master
thesis. To work with the current hardware design, the software has to have the ability to control the
delay step, change the voltage level of the pulse extender, read the digital signal out from the system
and process that information. The software has to be calibrated to match different delay times to the
expected energy received to achieve any form of radar control.
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38
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