Design och implementering av enhandsburen RFID-läsare

DESIGN OCH IMPLEMENTERING AV EN

HANDBUREN RFID-LÄSARE

Edward Nordström

Johan Hollander

EXAMENSARBETE 2007

ELEKTROTEKNIK

DESIGN OCH IMPLEMENTERING AV EN

HANDBUREN RFID-LÄSARE

DESIGN AND IMPLEMENTATION OF A HANDHELD

RFID-READER

Edward Nordström Johan Hollander

Detta examensarbete är utfört vid Tekniska Högskolan i Jönköping inom ämnesområdet Elektroteknik. Arbetet är ett led i teknologie magisterutbildningen med inriktning inbyggda elektronik- och datorsystem. Författarna svarar själva för framförda åsikter, slutsatser och resultat.

Handledare: Alf Johansson Examinator: Youzhi Xu

Omfattning: 30 högskolepoäng (D-nivå) Datum:

Abstract

Radio frequency identification (RFID) is a versatile wireless technology used worldwide. The fields of applications are many and its popularity constantly grows due to smaller in size, better and less expensive components. RFID is used to identify, track or share information about an object using radio waves. This master thesis describes the process of designing and implementing a handheld UHF RFID reader. The goal was to, based on a UHF RFID-chip design a fully functional, small in size and power efficient device. A microcontroller provides the user interface and is also used to control the RFID-chip and a Bluetooth device. A Bluetooth- and GPRS-compatible mobile phone will be used to forward data to a server connected to the Internet. All parts of the design are described, such as the printed circuit board design as well as the software for the micro controller and the mobile phone.

Because the extent of this thesis it is neither possible nor necessary to dig too deep into the Bluetooth- or GPRS-protocol. The focus will be on designing software and hardware for the handheld unit.

Sammanfattning

Radio frekvens identifiering (RFID) är en mångsidig trådlös teknik som

används över hela världen. Områdena där tekniken används är många och dess popularitet växer konstant tack vare mindre storlek, bättre och billigare

komponenter. RFID används för att identifiera, spåra eller dela med sig information om ett objekt med radiovågor.

Det här examensarbetet beskriver processen av design och implementering av en handburen UHF RFID läsare. Målet har varit att, baserat på ett UHF-RFID chip, designa en fullt fungerande, liten och strömsnål enhet. En micro

controller förser dels användaren med ett användargränssnitt och sköter dels kommunikationen med RFID chip och en blåtandsmodul. En blåtands- och GPRS- eller 3G-kompatibel mobiltelefon används for att skicka vidare data till en server kopplad till Internet. Alla delar av designen är beskrivna, så som PCB design, mjukvara för micro controllern och mobiltelefonen.

På grund av omfattningen av det här examensarbetet så har det inte varit möjligt eller nödvändigt att gräva för djupt i Blåtands- eller

GPRS/3G-protokollen. Fokus är på att designa hårdvara och mjukvara för den handhållna enheten.

Acknowledgements

We want to thank Alf Johansson at JTH for providing us with the contact to Combiport AB.

We also want to thank Werner Hilliges and Torbjörn Birging at Combiport AB for being very friendly, helpful and for assisting us with the project.

We also want to thank Rickard Nordström for helping us with the J2ME proplematics.

Keywords

Wireless Communication Radio frequency (RF)

Radio frequency identification (RFID)

Passive radio frequency identification tags (tags) Bluetooth

Java 2 Platform, Micro Edition (J2ME) C++

Micro Controller

General Packet Radio Service (GPRS) 3G (Third generation)

Contents

1 Introduction ... 10

1.1 PROLOGUE... 10

1.2 BACKGROUND... 11

1.3 PROBLEM DESCRIPTION... 12

1.4 PURPOSE AND GOALS... 13

1.5 LIMITATIONS... 14

1.6 DISPOSITION... 15

2 Data Communication ... 16

2.1 WIRELESS COMMUNICATION... 16

2.1.1 Radio frequency basics (RF)... 16

2.1.2 Radio frequency identification (RFID) ... 19

2.1.3 Antennas ... 27

2.1.4 Circulator ... 33

2.1.5 Bluetooth... 33

2.1.6 General Packet Radio Service (GPRS) / 3G... 36

2.2 WIRED COMMUNICATION... 37

2.2.1 UART and RS-232... 37

2.2.2 Serial Peripheral Interface (SPI)... 37

3 Design decisions ... 39

3.1 HARDWARE... 39

3.1.1 Principal solution for MobiReader™ ... 40

3.1.2 Calculation of the reading distance ... 41

3.1.3 ATmega64(L) – Micro controller ... 46

3.1.4 IDS_R900 – RFID chip... 47

3.1.5 OEM-SPA310 – Bluetooth module ... 49

3.1.6 MAX863 – Voltage Step up device... 50

3.1.7 Fractus chip antenna ... 53

3.1.8 Circulator ... 54

3.1.9 Piezo Buzzer ... 56

3.1.10 Microchip SPI Potentiometer... 56

3.1.11 Display ... 56 3.1.12 Diodes ... 56 3.1.13 Batteries ... 57 3.1.14 Buttons ... 57 3.2 SOFTWARE DESIGN... 61 3.2.1 ATmega software ... 61 3.2.2 Java software ... 62 4 Implementation... 64 4.1 HARDWARE... 64

4.1.1 Mobireader Main Board rev.1... 64

4.1.2 Mobireader power supply board rev.1 ... 66

4.1.3 Mobireader Main Board rev.2... 67

4.1.4 Mobireader Power Supply Board rev.2 ... 68

4.1.5 Test set up for measuring reading distance ... 68

4.2 SOFTWARE... 70

4.2.1 ATmega software ... 71

5 Results ... 85

5.1 HARDWARE... 85

5.1.1 Antenna... 85

5.1.2 Circulator ... 86

5.1.3 Reading distance measure results... 86

5.1.4 Summary ... 90

5.2 SOFTWARE... 91

5.2.1 ATmega 16 code ... 91

5.2.2 Mobile phone Java code ... 91

6 Conclusion and discussion ... 92

7 References... 93

8 Search Words ... 94

9 Appendix ... 95

9.1 APPENDIX A:CIRCUIT DIAGRAM MOBIREADER POWER SUPPLY BOARD... 95

9.2 APPENDIX B:CIRCUIT DIAGRAM MOBIREADER MAIN BOARD... 97

9.3 APPENDIX C:PCB LAYOUT MOBIREADER MAIN BOARD... 101

List of figures

FIGURE 1-1– MOBIREADER BLOCK DIAGRAM 12

FIGURE 2-1– POWER SUPPLY TO A PASSIVE TAG WITH INDUCTIVE COUPLING. 21

FIGURE 2-2– CLOSE COUPLING TRANSPONDER IN AN INSERTION READER WITH MAGNETIC

COUPLING COILS. 22

FIGURE 2-3– TAG STATE DIAGRAM [8] 25

FIGURE 2-4–EQUIVALENT CIRCUIT OF A ONE HALF WAVELENGTH DIPOLE. 27

FIGURE 2-5–EQUIVALENT CIRCUIT OF A ONE WAVELENGTH DIPOLE. 27

FIGURE 2-6– 4 PORT TRANSMISSION LINE 29

FIGURE 2-7– LINEAR, CIRCULAR AND ELLIPTICAL POLARIZATION. 30

FIGURE 2-8– PATCH ANTENNA RADIATION PATTERN 32

FIGURE 2-9− 3-PORT CLOCKWISE CIRCULATOR. 33

FIGURE 2-10– THE BLUETOOTH PROTOCOL STACK 34

FIGURE 2-11– A BLUETOOTH SCATTERNET [5] 35

FIGURE 2-12– SPI – ONE MASTER, THREE DAISY-CHAINED COUPLED SLAVES. 37

FIGURE 2-13– SPI – ONE MASTER, THREE INDIVIDUAL SLAVES. 38

FIGURE 3-1– PRINCIPAL SOLUTION FOR MOBIREADER 40

FIGURE 3-2– TRANSMITTED POWER (MW) VS. DISTANCE (M) 42

FIGURE 3-3– TRANSMITTED POWER (MW) VS. DISTANCE (1-3) M 43

FIGURE 3-4– TRANSMITTED POWER FROM TAG (MW) VS DISTANCE (1–3)M 43

FIGURE 3-5– TRANSMITTED POWER (MW) VS DISTANCE (0.5 - 1.8)M 44

FIGURE 3-6–TRANSMITTED POWER VS DISTANCE (0–0,75)M 44

FIGURE 3-7–TRANSMITTED POWER VS DISTANCE (0–0,35)M 45

FIGURE 3-8– BLOCK DIAGRAM IDS-R900G 48

FIGURE 3-9– OEM-SPA310 49

FIGURE 3-10– DISCHARGE TIME VERSUS VOLTAGE IN 2700MA AA BATTERY 53

FIGURE 3-11– 7MM SQUARE CIRCULATOR SC-7TN0859M 55

FIGURE 3-12– CIRCULATOR INSERTION LOSS 55

FIGURE 3-13– CIRCULATOR ISOLATION 56

FIGURE 3-14– HIGH VOLTAGE PEAK PROTECTION AND CONTACT BOUNCE PROTECTION. 58

FIGURE 3-15– CONTACT BOUNCE 58

FIGURE 3-16– VOLTAGE(V) VERSUS TIME(S) ON INPUT TO MICRO CONTROLLER 59

FIGURE 3-17– ATMEGA 16 STATE CHART 62

FIGURE 4-1– EAGLE CAD 2 LAYER PCB 65

FIGURE 4-2– SIDE VIEW OF MAIN AND POWER SUPPLY BOARDS 66

FIGURE 4-3– EAGLE CAD 4 LAYERS PCB 67

FIGURE 4-4– THE DIFFERENT PLACEMENTS OF THE RFID TAG VERSUS THE ANTENNA. 69

FIGURE 4-5– THE MODULES USED FOR THE PROJECT. 70

FIGURE 4-6– MOBIREADER BLOCK DIAGRAM WITH THE EVOLUTION BOARDS 71

FIGURE 4-7– ATMEGA 16 FLOWCHART PAGE 1 72

FIGURE 4-8– ATMEGA 16 FLOWCHART PAGE 2 73

FIGURE 4-9– ATMEGA 16 FLOWCHART PAGE 3 74

FIGURE 4-10– MIDLET GUI SCREENS 76

FIGURE 4-11– MOBILE PHONE SOFTWARE FLOWCHART PAGE 1 78

FIGURE 4-12– MOBILE PHONE SOFTWARE FLOWCHART PAGE 2 79

FIGURE 4-13– MOBILE PHONE SOFTWARE FLOWCHART PAGE 3 80

FIGURE 4-14– MOBILE PHONE SOFTWARE FLOWCHART PAGE 4 81

FIGURE 4-15– MOBILE PHONE SOFTWARE FLOWCHART PAGE 5 82

FIGURE 4-16– COMMUNICATION EVENTS OF MOBIREADER, MOBILE PHONE AND SERVER 84

FIGURE 5-1– MEASURE RESULTS AT PLACEMENT 1 87

FIGURE 5-2- MEASURE RESULTS AT PLACEMENT 2 87

FIGURE 5-3– MEASURE RESULTS AT PLACEMENT 3 88

FIGURE 5-4– MEASURE RESULTS AT PLACEMENT 4 88

FIGURE 5-5– MEASURE RESULTS AT PLACEMENT 5 89

List of tables

TABLE 2-1– ALLOCATION OF THE RF SPECTRUM. 19

TABLE 2-2– FREQUECY RANGES USED FOR RFID-SYSTEMS (AUGUST 2006) [10] 20

TABLE 2-3– COMPARISON DIFFERENT ANTENNAS 32

TABLE 3-1– NOTES ON THE PRINCIPAL SOLUTION 41

TABLE 3-2– READING DISTANCE 45

TABLE 3-3– POWER CONSUMPTION ATMEGA 64(L) 47

TABLE 3-4– POWER CONSUMPTION IDS-R900G 48

TABLE 3-5– POWER CONSUMPTION OEM-SPA310 50

TABLE 3-6– 7MM SQUARE CIRCULATOR SC-7TN0859M SPECIFICATION 55

List of abbreviations

PWM – Pulse Width Modulation

ADC – Analogue to Digital Conversation

UHF – Ultra High Frequency (300MHz…3 GHz) RF – Radio Frequency

RFID – Radio Frequency Identification ASK – Amplitude Shift Keying

AM – Amplitude Modultation PM – Phase Modulation

VCO – Voltage Controlled Oscillator PLL – Phase Locked Loop

GPRS - General Packet Radio Service PCB – Printed Circuit Board

1

Introduction

1.1

Prologue

The possibility to communicate wirelessly has opened up for new amazing technologies. Only the beginning of the wireless era has seen the light of day. Imagine all the everyday-use products that are taken for granted; mobile phones, lap tops, head-phones, radio and television. These are all products based on wireless technology. There are numerous different techniques and ways of communicating, the mentioned products above all use different techniques. Each product has different basic conditions and based on that the developer chooses the best suitable wireless communication technique.

RFID (Radio Frequency Identification) is yet another wireless technology and has got some specific benefits. RFID is mostly used for identification purposes. A RFID tag, also called transponder is a small chip which contains an

identification number and can be read wirelessly by a RFID reader. A tag doesn’t even necessarily need a power source of its own. It powers up using the transmission energy from the reader. Upon a successfully transmission of its identification number it goes back to sleep. What this means in practice is that a RFID tag can be implemented almost anywhere; on groceries, packages, inside animals and even inside humans. Battery life span is not an issue at all, only the reading distance is.

Many companies see new opportunities and advantages of investing in the RFID technology. Especially delivery firms and warehouses could draw huge benefits of RFID. All possibilities associated with RFID haven’t yet been discovered.

There are though some constraints associated with RFID. There are opinions such as a RFID tag can be used as a spy chip. The owner of the RFID tag never notice when it is read. The technology could in other words be abused because every single step an individual takes could be monitored. Despite opinions like this, if used in a proper way, RFID is a very useful aid and can save both time and money.

1.2

Background

Combiport AB is a company situated at Science Park in Jönköping. They have specialized knowledge in developing high tech equipment for various RFID applications. Their product line involves base stations, active tags and customized RFID solutions. What is missing in their line of products is a handheld reader for the UHF band.

A handheld UHF RFID reader could be used in many different fields. Because barcodes sooner or later will be replaced by a new technique i.e. RFID, there is a need for a new type of reader. Wherever barcodes are used, RFID technique can replace it. The perfect place for a handheld reader is in logistics, tracking packages. Nowadays all consignments being shipped are stamped with a barcode. When a truck driver delivers packages he or she reads the barcode on the package with a handheld device and then hands it over to the receiver. Now that RFID tags are as cheap as they are, a tag could replace the barcode with very low cost increment. The benefits are many. A RFID tag contains a unique identification number and can be read without aligning the package towards a reader in some special way. It is also possible to read many packages

concurrently. Another feature that can be added is when a package is lost and it is desired to search for it. The tracking id of the package is known but the package is lost in a big pile of other packages. The tracking id can be typed into the handheld reader and a reverse read is initiated. When close enough to the package a voice signal notices the user. The stronger signal, the closer the reader is to the package.

Other applications where the handheld reader could be used is doing inventory at any storage company. If all products are equipped with tags it is very easy to do inventories. Imagine how much time and money could be saved if manual inventory is avoided.

The fields where the reader can improve things are almost infinite; one of the more obvious is at grocery stores. Shop express which nowadays use barcode readers could be replaced by RFID readers. The reader must be very intelligent to make this possible. This is due to that, what if a reading is initiated inside a store, everything within reading distance will be read.

The proposed RFID reader improves the reading in additional ways. It can be equipped with either a Bluetooth- or WLAN- module. The Bluetooth device makes it possible to communicate with a Bluetooth compatible mobile phone. This means that it is possible to be connected to the internet without the

requirement of an additional account for the reader – a GPRS/3G mobile phone can be used. If a WLAN is close it is possible to constantly be connected to the internet. The idea with the internet connection is that when a tag is read it is possible to transmit the information to some database and register a successful reading.

1.3

Problem description

Based on a RFID-chip from Microchip-IDS a small handheld RFID reader is to be designed and implemented. The reader shall be able to read passive tags according to the EPC™ Radio-Frequency Identity Protocols Class-1 Generation-2 UHF RFID Protocol for Communications at 860 MHz – 960 MHz version 1.0.9 on limited distance. The reader shall also implement a search RFID function. The reader shall have the ability to communicate with a server on Internet. A Bluetooth compatible mobile phone will be used to pass the information further on to this server using GPRS. The project will involve designing both hardware and software. Below the whole system is described in a block diagram.

Figure 1-1– MobiReader block diagram

The RFID-chip (IDS-R900G) is the foundation of the design. Suitable components, such as micro controller, Bluetooth device, antenna, a user interface and peripheral components are to be chosen. The PCB must be designed in such a way that it fits into a specific casing. Hardware will be chosen with focus on low power consumption. The IDS-R900G chip handles all analogue parts when communicating with a tag. An interface to communicate with the RFID chip digitally is to be set up with a micro controller. The project involves software design for the micro controller and for the mobile phone. The software code in the mobile phone will be written in Java and the code for the micro controller will be written in C/C++. A good user interface is to be considered, perhaps using a display. The display in the mobile phone might be used for displaying messages to the user. Reading distance between the tag and the antenna must also be investigated to know the limitations of the designed UHF RFID reader.

1.4

Purpose and goals

The goal is to, either by connecting modules together or even better; make both the designed hardware and software work together. A working prototype must be able to read RFID-tags, send the information via Bluetooth to a mobile phone. The mobile phone will send the information using GPRS to a server on the Internet. When a read tag can travel this whole chain the goal is fulfilled. In order to reach the final goal the following smaller goals needs to be fulfilled. -Create a PCB layout of the RFID-reader using Eagle CAD software. The PCB will contain a newly developed RFID chip that can handle the whole analogue- and parts of the digital part. There will also be a micro controller, a Bluetooth chip, some buttons, a buzzer, LEDs and as an option a small display.

-Connect suitable evaluation boards to develop and test the program for the microcontroller in parallel to designing the PCBs.

-Program a Java module for a mobile phone to receive data on its Bluetooth radio and transmit the read data via GPRS to a software database.

-Program the micro controller and set up the Bluetooth communication with a mobile phone.

-Measure the maximum reading distance at different angles between the tag and the antenna.

-If time allows, the mobile phone shall be able to buffer read data if a GPRS connection is not available.

The final goal is when a tag is read by mobireader the tag number is to be transferred to a server on the internet.

1.5

Limitations

Because the extent of this thesis it is very important to limit it in such a way that not everything is considered. Components and solutions already existing will be chosen primarily.

A very important, though complicated and complex area is antennas. The most suitable antenna for the project would have been a Patch antenna. The problem with patch antennas at this specific frequency is that they become too large, in this study the size is a problem with the proposed casing. A possibility would have been to design a custom micro strip patch antenna. The problem is that all antennas need to be tuned to a specific frequency. This tuning procedure is not simple and can take months. Therefore, a standard Omni-directional chip antenna will be put in the designed PCB. Using this type of antenna would result in a non desirable radiation pattern. For the reading distance measure part of this study an already designed patch antenna is available and will be used. The thesis will be limited even further and a search tag function will not be implemented in this project.

A display will not be implemented. Connections will be prepared to make it easy to connect a display in the future.

In addition the thesis will be limited so that only a proposal of hardware is described. The hardware will not be manufactured; instead evaluation boards will be used and connected, to prove functionality of the chosen hardware.

1.6

Disposition

In the first section the basic theories underlining the master thesis are

described. It is important to understand these fundamental theories to be able to make good choices in the design.

In the next section called “design decisions” components and the design choices made are described. All calculations are presented here.

In the implementation section an example of a poor PCB design is described and compared with a much improved example. Also included in the

implementation section is a description of the software implemented in the project.

In the next section results are presented. The calculated reading distance is compared with the simulation results.

Finally, a discussion regarding the results and what can be improved are presented.

2

Data Communication

In the same way communication between humans is a fundamental function it is also essential for computer systems to communicate. Data communication is nowadays fully integrated into the society and we are totally dependent up on it. There are many different techniques used to transfer data from one source to another. Which technique is used is dependent on the purpose. Security, speed, reliability and cost are all important factors when choosing the type of

communication.

As technology constantly improves wireless data communication tends to take over fields where wired communication was the only alternative just a couple of years ago. The benefits of wireless technology are many. Why use a cord when it is not necessary?

2.1

Wireless communication

Wireless communication doesn’t only just replace cords but also opens up new doors and opportunities.

2.1.1 Radio frequency basics (RF)

Radio frequency refers to electromagnetic waves propagating in air within a certain spectra in frequency. Electromagnetic waves occur when an alternating current is input to an antenna. This electromagnetic field can be used for broadcasting or communication between around 3 Hz to about 300 Gigahertz [3]. The spectrum is referred to as the RF spectrum. As the frequency increase beyond the RF spectrum electromagnetic energy takes the form of infrared, visible light, ultraviolet, x rays and gamma rays.

2.1.1.1 Carrier wave

The carrier wave is the information carrier. Together with a baseband signal (the information being transferred) the two signals are modulated into one signal [3]. The frequency of the carrier wave is much higher than the information itself. The reason to use a carrier wave is that the frequency spectrum in free space is limited and therefore must be shared with millions of others.

2.1.1.2 Modulation

Modulation is when the baseband signal is mixed together with the carrier wave into one signal. This can be done in a couple of different ways. Some common analogue modulation techniques are AM, FM, PM.

AM – Amplitude modulation – The amplitude of the carrier wave is varied

with respect to the baseband signal. The frequency is kept constant. This is the simplest modulation technique. The drawback of this modulation is among others bad noise susceptibility. Early radio transmissions used this modulation.

FM – Frequency modulation - The Frequency of the carrier wave is varied

with respect to the baseband signal. Amplitude is kept constant. This

modulation technique is slightly more complex than AM but has got a many benefits. Noise susceptibility is one of them.

PM – Phase modulation – The phase of the carrier wave is varied with respect

to the baseband signal. This is the most complex modulation technique. When demodulating a PM signal it requires complex techniques and is therefore not used as much as AM or FM.

There are also a couple of digital modulation techniques used. Some of the most important are described below.

OOK – On off keying – The presence or absence of a carrier for a specific

duration represents a binary 1 or a 0.

FSK – Frequency shift keying – Digital information is transmitted through

discrete frequency changes of the carrier wave.

ASK – Amplitude shift keying – Digital information is transmitted through

variations in the amplitude of the carrier wave.

PSK – Phase shift keying – Digital information is transmitted through

variations in the phase of the carrier wave.

QAM – Quadrature amplitude modulation – Digital information is

transmitted by modulating the amplitude of two carrier waves. These two waves (sinusoids) are 90 degrees out of phase.

There exist numerous modulation techniques where combinations of the above are used together. Some of them are APSK, MSK, CPM, PPM, TCM, OFDM. Finally there is a modulation technique based on something called spread spectrum. Spread spectrum spreads the information over some bandwidth using a hopping scheme where the frequency changes within some given time.

FHSS – Frequency hopping spread spectrum – Is a method of transmitting

radio signals by switching a carrier between many frequency channels using a random sequence known by both the transmitter and the receiver.

The benefits of spread spectrum are.

- Difficult to intercept.

- Spread spectrum can share a frequency band with other conventional transmissions with minimal interference.

- Bandwidth can be used more efficiently.

AFH –Adaptive frequency hopping spread spectrum – Bluetooth use this

type of modulation. It improves FHSS by avoiding crowded frequency channels.

2.1.1.3 Bandwidth

Bandwidth is the width in frequency that the channel occupies [3]. Many types of wireless devices share the RF spectrum, such as radio, television, cordless phones, cell phones and satellite communication systems. Some wireless

devices, such as remote controls and cordless mice operate at infrared or visible light frequencies.

Frequency

range Bandwidth description Used for

0 to 3 kHz

Extremely Low Frequency

(ELF)

3 kHz to 30 kHz

Very Low Frequency (VLF)

Maritime / Aeronautical mobile (9kHz - 540kHz)

30kHz to 300 kHz Low Frequency (LF)

Maritime / Aeronautical mobile (9kHz - 540kHz)

300 kHz to 3000

kHz Medium Frequency (MF)

Maritime / Aeronautical mobile (9kHz - 540kHz)

AM Radio Broadcast (540 Hz to 1630 kHz) Travellers Information Service 1610 kHz

3 MHz to 30 MHz High Frequency (HF)

Shortwave Broadcast Radio 5.95 MHz to 26.1 MHz

30 MHz to 300 MHz

Very High Frequency

(VHF) Low Band: TV Band 54 MHz to 88 MHz

Mid Band: FM Radio Broadcast 88 MHz to 174 MHz

High Band: TV Band 174 MHz to 216 MHz

Super Band (mobile/fixed radio and TV) 216 MHz to 600 MHz 300 MHz to 3000 MHz Ultra-High Frequency (UHF)

Super Band (mobile/fixed radio and TV) 216 MHz

to 600 MHz

TV Band 470 MHz to 806 MHz

L-band 500 to 1500 MHz

Personal Communication Services (PCS) 1850 MHz

to 1990 MHz

Unlicensed PCS Devices 1910 to 1930 MHz

RFID 888 MHz to 889 MHz RFID 902 MHz to 928 Mhz RFID 2400 MHz to 2485 Mhz 3 GHz to 30 GHz Super-High Frequency (SHF) C-band 3600 MHz to 7025 MHz X-band 7.25 MHz to 8.4 MHz Ku-band 10.7 GHz to 14.5 GHz Ka-band 17.3 GHz to 31 GHz RFID 5725 MHz to 5875 MHz 30 GHz to 300 GHz Extremely High Frequency (EHF)

Additional Fixed Satellite 38.6 GHz to 275 GHz 300 GHz to 430 THz Infrared Radiation 430 THz to 750 THz Visible Light 1.62 PHz to 30 PHz Ultraviolet Radiation 30 PHz to 30 Ehz X-rays 30 Ehz to 3000

Ehz Gamma Rays

Table 2-1– Allocation of the RF spectrum.

2.1.2 Radio frequency identification (RFID)

The most common radio frequency identification RFID frequency bands and the standards associated with their usage, according to [4], are the following:

• 25 kHZ LF − Near field, all passive −ISO 18000-2

• 13.56 MHZ HF − Near field, mostly passive −ISO 18000-3 Mode 1, Mode 2

−ISO 14443 TypeA, TypeB −ISO 15693

−EPCglobal Class-1 HF

• 900 MHZ UHF − Far field, some active −EPCglobal Gen2

−EPCglobal Class-0, Class-1 (Not standardized) −ISO 18000-6 Type A, Type B

• 2.45 GHz UHF − Far field, some active −ISO 18000-4 Mode 1, Mode 2

Table 2-2– Frequecy ranges used for RFID-systems (August 2006) [10]

2.1.2.1 LF Tags (<135KHz)

Since these tags operate in the KHz spectrum, they are the most adaptive to metal environment. Tags can be placed on metal and embedded into metal with some loss of performance. These tags are inductive coupled and inductively coupled tags are almost always operated passively, meaning that all the energy needed for the operation of the microchip has to be provided by the reader. How an inductive coupled tag communicates with a reader is described in chapter 2.1.2.2.

2.1.2.2 HF Tags (13.56 MHz )

As read from [1], HF tags are also inductive coupled. The connection between the reader and the tag can be seen as a transformer with very weak coupling. This is true when the distance between the coils does not exceed 0.16λ, so that the transponder is located in the near field of the transmitter antenna. The required inductance of the transponder coil and the number of windings decreases as the frequency increases (135 kHz: typical 100–1000 windings, 13.56MHz: typical 3–10 windings) [1]. Since the voltage induced in the transponder is still proportional to the frequency, the reduced number of windings barely affects the efficiency of power transfers at higher frequencies.

Figure 2-1– Power supply to a passive tag with inductive coupling.

In most of the operating conditions only a small part of the emitted field reaches the antenna coil of the tag. When a tag is within the alternating magnetic field of the readers’ antenna, energy is collected to the tag from the magnetic field. The resulting feedback can be represented as transformed impedance in the reader’s antenna coil. The tag switches a load on and off in it’s antenna, to have a change in the impedance of the reader’s antenna

resulting in a voltage change at the reader’s antenna. The result is an amplitude modulation on the reader’s antenna.

2.1.2.3 UHF Tags (900 MHz and 2.45GHz)

RFID systems where the distance between the reader and the transponder is larger than 1m are called long-range systems. These systems operate in the UHF frequencies of 860-960MHZ and at the microwave frequencies 2.5 GHz and also at 5.8GHZ. The short wavelengths of these frequencies results in smaller antennas with greater efficiency than would be possible using frequencies below 30MHz. These tags use electromagnetic backscattering coupling. As read from [1], “We know from the field of radar technology that electromagnetic waves are reflected by objects with dimensions greater than around half the wavelength of the wave. The efficiency with which an object reflects electromagnetic waves is described by its reflection cross-section. Objects that are in resonance with the wave front that hits them, as is the case for antennas at the appropriate frequency, for example, have a particularly large reflection cross-section.”

To transmit data from a tag to the reader, a load resistor is connected parallel to the antenna on the tag. If this load is altered, the reflection characteristic of the tag antenna is influenced resulting in amplitude modulation. The resistor is switched on and off in time with the data stream to create a modulated backscatter on the carrier wave.

2.1.2.4 Close coupling tags

Close coupling RFID systems are designed for a range of 0.1cm to 1m. Tags are therefore inserted into a reader or placed on the reader. Close coupling system can either be magnetically coupled or capacitive coupled.

The functional layout of magnetically coupled system corresponds with that of a transformer, the primary winding is represented by the reader and the

secondary winding is represented by the tag.

Figure 2-2– Close coupling transponder in an insertion reader with magnetic coupling coils.

The tags power supply is generated by a high frequency magnetic field in the core and air gap of the arrangement, caused by a high frequency current in the primary windings. In contrary to inductively coupled systems, the efficiency of power transfer is very good, and therefore the controlling chip can have higher power consumption. This includes microprocessors which require some 10mW power.

In a capacitive coupled system, the reader and the tag have capacitive plates which are parallel when the tag is inserted in a reader. Contact–less close coupling chip cards are defined in their own standard, ISO 10536.

2.1.2.5 Passive, active and semi-passive tags.

The simplest version of an RFID tag is a passive identification (ID) tag. It does not contain its own power source but instead harvests the power it needs from the readers RF emissions. It holds a Tag identifier (TID), an electronic code (EPC) identifier and has a “kill” function that permanently disables the tag. Optionally it can have password-protected access control and user memory. Because no battery is needed in passive tags they are very suitable for

numerous applications. The integrated CMOS circuit is powered by the antenna and receives just enough energy to power up and transmit a response to the reader [1]. The reading distance of a passive tag can vary from 10 cm up to a couple of meters, depending on the radio frequency and the design and size of the antenna [1].

In 2006 Hitachi developed a tag which is only 0.15x0.15mm and thinner than a piece of paper (7.5 micrometers) [9]. The tag contains a 128-bit unique ID number which is hard coded into the chip. The reading range of the chip is around 30 cm. In February 2007 Hitachi unveiled an even smaller RFID device measuring 0.05×0.05 mm, and thin enough to be embedded in a sheet of paper [9]. The new chips can store as much data as the older chips, and the data contained on them can be extracted from as far away as a few hundred meters. Unlike passive RFID tags, active RFID tags have their own internal power source, which is used to power the integrated circuits and to broadcast the response signal to the reader. Communications from active tags to readers is typically much more reliable (i.e. fewer errors) than from passive tags due to the ability for active tags to conduct a "session" with a reader. Active tags, due to their on board power supply, also may transmit at higher power levels than passive tags, allowing them to be more robust in "RF challenged" environment with humidity and spray or with dampening targets, reflective targets from metal, or at longer distances. In turn, active tags are generally bigger, caused by battery volume, and more expensive to manufacture, caused by battery price.

Semi-passive tags, also called semi-active tags, are similar to active tags in that they have their own power source, but the battery only powers the microchip and does not power the broadcasting of a signal. The response is usually powered by means of backscattering the RF energy from the reader, where energy is reflected back to the reader as with passive tags. An additional

application for the battery is to power data storage. Whereas in passive tags the power level to power up the circuitry must be stronger than with active or semi-active tags, also the time consumption for collecting the energy is omitted and the response comes with shorter latency time. The battery-assisted reception circuitry of semi-passive tags leads to greater sensitivity than passive tags. The enhanced sensitivity can be leveraged as increased range (by one magnitude) and/or as enhanced read reliability (by reducing bit error rate at least one magnitude). Semi-passive tags have three main advantages 1) Greater sensitivity than passive tags 2) Longer battery powered life cycle than active tags. 3) Can perform active functions (such as temperature logging) under its own power, even when no reader is present for powering the circuitry. 2.1.2.6 UHF EPC tag behavior

The tags of interest to this thesis are passive tags that comply with the EPC Class 1 generation 2 UHF protocol [8]. Such a tag has flags used for distinction among a population of tags during interrogator and multiple tag

communications. These flags are called selected flag (SL), inventoried flag (its values are denoted either A or B) and four sessions (S0, S1, S2 and S3). A tag shall maintain independent inventoried flag for each session and shall only participate in one and only one session at an inventory round. The SL flag is independent of the session a tag is participating in. The tag also implements a slot counter that is affected by a parameter Q of range 0 to 15 which is used to regulate the probability that a tag replies. This slot counter is 15 bits wide and assumes a random number between 0 and 2Q – 1, and when the slot counter reaches or assumes zero the tag will reply. Found in the protocol is the tag state diagram, shown in the figure below.

In order to read the tag ID stored in the EPC membank, various commands needs to be sent to the tag in order to move it from the ready state to acknowledged state where it replies selected data from its memory. The commands needed are Select command, Query command and ACK

(acknowledge) command. The Select command specifies which of the tags that shall participate in an inventory round, by user-selected criteria, and how the flags of the tags are modified or not. The Query command comes in some different variations to modify the slot value differently. The Query command starts a new inventory round among the selected tags with the same flags as given in the Query command and results in a new random slot value with a given Q value. Query adjust command can adjust a previous given Q value up, down and no change to Q, and can change the range of the slot counters random value before a new random slot value is generated. QueryRep

command will decrease the slot counter by one, so that tags can move to reply state when their slot counter reaches zero. The ACK command will make a tag backscatter its selected memory contents when it is in the reply state and move it to the acknowledged state. If a new Query command is issued with matching flags and session, a tag in the acknowledged state will invert is inventoried flag (A→B or B→A) and therefore not participate in the remainder of the inventory round.

2.1.2.7 Summary

The 900 MHz UHF band seems to be the preferred RF band for supply-chain applications, primarily for reasons of read speed and range. According to [4], passive 900 MHz UHF tags can be read at rates from 100 to 1,000 tags per second and at a range measured in meters compared to passive 13.56 MHz tags which can be read at rates from 10 to 100 tags per second and at a range

measured in centimeters. The 900 MHz RFID has far greater problems with signal fading due to multipath effects (interfering reflections of the signal that cause adjacent regions to vary dramatically in reception) and signal attenuation by liquids and metals than does 13.56 MHz RFID. Comparing tags in the 900MHz UHF band to tags in the 2.45GHz microwave band, the 900MHz tags has longer range but the 2.45GHz tags has higher data rate but both of these’s signals are easily reflected or absorbed. The LF tags have short reading

distances and slow data rates, but they can be read through most materials and be read round corners.

The protocol, EPC Class 1 generation 2 UHF protocol [8], describes physical and logical requirements for a passive-backscatter, Interrogator-talks-first (ITF), radio-frequency identification (RFID) system operating in the 860 MHz – 960 MHz frequency range.

This protocol specifies:

• Physical interactions (the signaling layer of the communication link) between Interrogators and Tags,

• Interrogator and Tag operating procedures and commands, and

• The collision arbitration scheme used to identify a specific Tag in a multiple-Tag environment.

2.1.3 Antennas

An antenna can be any conductive structure that can carry an electrical current. All structures carrying time varying signals will radiate, maybe not efficiently or in a desirable manner but they will radiate. Antennas consist of a structure designed to radiate electromagnetic radiation efficiently with some specific characteristics. If the antenna part is not carefully designed other things may radiate as well, such as the transmission line or the power supply line. If a PCB is placed close to the antenna, components may also radiate leading to

undesirable behavior. In this section the focus will be on the antenna itself. In order to make an antenna transfer power as efficiently as possible the impedance of the antenna must be matched with the transmission line impedance [2]. The transmission line shall transfer all of the power to the antenna and radiate as little as possible itself. The mode of the transmission line shall match the mode of the antenna. A common mode is 50Ω transmission line and antenna. Depending on the application different antennas and radiation patterns are available.

2.1.3.1 Antenna Impedance

The simplest antenna is a thin, center fed, very short dipole called a Hertizian dipole. The equivalent circuit is a RLC circuit shown in Figure 2-4 and Figure

2-5 where XL represents the inductance of the conductors, XC the capacitance

between the conductors and R represents the energy lost to radiation.

Figure 2-4–Equivalent circuit of a one half wavelength dipole.

Figure 2-5–Equivalent circuit of a one wavelength dipole.

When the antenna (dipole) is very short the circuit is dominated by a large capacitive reactance and a small radiation resistance. As the dipole is made longer, R increases as well as XL, XC becomes smaller. When the antenna is approximately one half wavelength long XL and XC cancel each other out and the antenna is in resonance. This is the ideal operating condition. When the dipole is made even longer and reaches one wavelength the antenna is in resonance again and the equivalent circuit is shown in Figure 2-6. 2.1.3.2 Transmission lines

In most cases the length of the wires connecting components can be ignored. When a wire carries a time varying signal with corresponding wavelength comparable to or less than the length of the wire the length becomes important. The length of the wire should be treated as a transmission line if the length is greater than 1/10 of the wavelength [2]. If a transmission line is not designed properly it can lead to unpredictable behavior in the system due to phase delay and reflections.

A transmission line is the path connecting one place to another directing the transmission of energy. A transmission line can consist of coaxial cables, dielectric slabs, optical fibers, electric power lines and waveguides or the copper on a PCB. It can be any structure or material.

Wires connecting antennas to some circuitry should be treated as transmission lines. The most important parameters to be considered are impedance,

propagation velocity, loss and mode. All transmission lines have a

characteristic impedance which is determined mainly by the geometry of the conductors and the dielectric constant of the material supporting them. If a surface mounted antenna is mounted on a PCB the dielectric is the material on the PCB and the geometry is the copper and the surrounding components. All components close to the transmission line affects the impedance. If the

impedance of the antenna doesn’t match the transmission line impedance there will be reflections at the discontinuity and the power transfer will not be perfect. Transmission lines also have loss mainly from conductive loss, dielectric loss and radiation. Open wires have a tendency to radiate and this is dependent mostly upon the spacing of the conductors.

Figure 2-6– 4 port Transmission line

In the case above the network is assumed to be linear, which means the complex voltage at any of the ports are proportional the complex current flowing into it when there are no reflections. When a transmission line is uniform across its whole length the behavior can be described by a single parameter called the characteristic impedance Z0. Z0 is the ratio of the complex voltage to the complex current of a given wave at any point of the line. Typical values of Z0 are 50, 75, 100 or 300 ohms [2].

Transmission lines have currents flowing in two directions. The vector sum of these currents should be zero. If the sum is not zero a so called common mode current exists. The common mode current radiates and distorts the pattern of the antenna. Common mode current is introduced in two ways, one by coupling to the energy radiating from the antenna and secondly from a mismatch of modes which occurs when a balanced antenna is connected to an unbalanced transmission line or vice versa.

When sending power via a transmission line it is desirable that as much power as possible is absorbed by ZL and as little as possible is reflected back to the source. This can be ensured by making the source and load impedances equal to Z0.

Some of the power inserted into a transmission line is lost because of

resistance. This is only resistive loss. At high frequencies, another effect called dielectric loss becomes significant and adds up to the resistive losses. Dielectric loss occurs when the insulating material around the transmission line absorbs energy from the alternating electric field and converts it to heat.

2.1.3.3 Polarization

Polarization is a property of a traveling wave in a direction perpendicular to the direction in which the oscillations that produce the wave are moving. If a wave is moving in the positive x-direction its up and down oscillations are in up and down directions that lie in the yz-plane. All electromagnetic waves traveling in free space have an electric field component E and a magnetic field component H which are usually perpendicular to each other. Both these components are perpendicular to the direction of propagation.

Figure 2-7– Linear, Circular and Elliptical polarization.

In the leftmost figure above, the two perpendicular components are in phase. In this case the ratio of the strengths of the two components is constant, so the vector sum of these two components is constant. When the E field is oriented vertically the wave is said to be vertically polarized. This is also called linear polarization.

In the middle figure, the two orthogonal components have exactly the same amplitude and are exactly ninety degrees out of phase. In this case the E component is zero when the H component is at maximum or minimum

amplitude. In this special case the electric vector traces out a circle in the plane, so this special case is called circular polarization. The direction the field rotates in depends on which of the two phase relationships exists. These cases are called right-hand circular polarization and left-hand circular polarization, depending on which way the electric vector rotates.

All other cases, that is where the two components are not in phase and either do not have the same amplitude and/or are not ninety degrees out of phase are called elliptical polarization because the electric vector traces out an ellipse in the plane (the polarization ellipse).

2.1.3.4 Power Gain

The power gain of a given antenna is a measurement of the directivity. An antenna with low gain radiates about the same in all directions. An antenna with high gain radiates more in a specific direction. The power gain of an antenna is defined as the ratio of the intensity radiated by the antenna in a given direction at a specific distance divided by the intensity radiated at the same distance by a hypothetical isotropic antenna. If the gain in an antenna is high it doesn’t mean the antenna adds power to the signal. The gain is a passive phenomenon; the power is only redistributed to radiate more in a certain direction. If an antenna radiates more in one direction it must also radiate less in another. High-gain antennas have the advantage of longer range and better signal quality, but must be aimed carefully in a particular direction. Low-gain antennas have shorter range, but the orientation of the antenna is not an issue.

2.1.3.5 Radiation pattern

Depending on the application a suitable radiation pattern must be chosen. Laptops communicating wirelessly use an antenna with low gain and therefore have an Omni directional radiation pattern. An example where a radiation pattern with high gain is desired might be when using a parabolic dish, a remote control or like in this case; a handheld RFID reader which aims in the direction to read.

Radiation Patterns

Power Gain Directivity Polarization

Dipole Broadside Low Low Linear

Multi Element Dipole

Broadside Low/Medium Low Linear

Flat Panel Antenna

Broadside Medium Medium/

High

Linear/Circular

Parabolic Dish Antenna

Yagi Antenna Endfire Medium/High Medium/ High

Linear

Slotted Antenna

Broadside Low/Medium Low/ Medium

Linear

Microstrip Antenna

Endfire Medium Medium Linear

Table 2-3– Comparison different antennas

Below the radiation pattern of a patch antenna is shown. This is the desired radiation pattern for the purpose of directivity.

2.1.4 Circulator

When communicating with passive tags, an antenna is used to both send and receive data due to the fact that tags modulate backscatter on the carrier wave from the antenna. The modulated backscatter is sensed on the antenna and is usually very weak. In order to be able to use the same antenna for transmitting and receiving a ferrite control device called a circulator must be used. A

circulator is a device with very low insertion loss in one direction and very high insertion loss in the reverse direction. Low loss is normally less than 0.5dB and high loss more than 20dB [2]. A circulator can consist of multiple ports; a three port circulator with power flow from port 1 to port 2 and from port 2 to port 3 can be seen below.

Figure 2-9− 3-port clockwise Circulator.

2.1.5 Bluetooth

Bluetooth is a very useful wireless standard between components who wish to communicate over short distance with each other. The Bluetooth specification specifies the radio system and the complete protocol stack to ensure that different devices will be compatible with each other.

Figure 2-10– The Bluetooth protocol stack

The Bluetooth protocol stack is made up of many layers. The HCI layer is usually the layer that separates the hardware from the software and can be partially implemented in hardware/firmware and software. The layers below HCI layer are usually implemented in hardware and the layers above HCI are usually implemented in software. The Bluetooth radio is the lowest layer of Bluetooth communication.

The Industrial, Scientific and Medical (ISM) band at 2.4 GHz is used for radio communication in Bluetooth technology and in Wi-Fi technology like IEEE 802.11. The Bluetooth radio utilizes a technique, Frequency Hopping Spread Spectrum (FHSS), to switch between 79 sub channels of the radio band. FHSS is used to reduce interference caused from other devices using the same

frequency band. The Bluetooth radio switches band every 625 microseconds, making 1600 frequency hops every second. In the Bluetooth 1.2 specification AFH (Adaptive Frequency Hopping) is introduced, AFH is useful if a device communicates through both Bluetooth and Wi-Fi simultaneously (e.g. a laptop computer with both Bluetooth and WLAN). The adaptive frequency hopping algorithm can then avoid using Bluetooth channels overlapping the Wi-Fi channel in use, hence avoiding interference between a laptops own radio communications.

2.1.5.1 Bluetooth communication

There are some roles every device using Bluetooth can assume when connecting to other Bluetooth devices. A client, an initiator of connections, may search for other devices and for services provided by those devices and connect to one service of interest. A server, an acceptor of connections, usually provides a service and accepts a client who tries to connect to this service.

Another role that devices must choose is to be either master or slave between connected devices. The master slave role is decided between devices when they establish a connection. A master can setup connection to other devices, perform searches and accept connections from other devices, while a slave cannot connect to another device, cannot perform searches and cannot accept connections from other devices. Connected devices can switch master slave role at any time. In a piconet of up to 8 devices, one unit will act as a master for synchronization purposes and the others as slaves for the duration of the

piconet connection. One slave or master in a piconet may also be a slave in another piconet and thus be a communication link between two piconets to combine them to a scatternet consisting of many piconets.

Figure 2-11– A Bluetooth scatternet [5]

There are 13 "profiles" described in version 1.1 of the specification. These profiles are general behaviors through which Bluetooth units communicate with other units.

• Generic Access Profile (GAP)

• Service Discovery Application Profile (SDAP) • Cordless Telephony Profile

• Intercom Profile • Serial Port Profile • Headset Profile

• Dial-up Networking Profile • Fax Profile

• LAN Access Profile

• Generic Object Exchange Profile (GOEP) • Object Push Profile

• File Transfer Profile • Synchronisation Profile

2.1.5.2 Bluetooth security

A trusted relationship called paring can be established between pairs of devices by learning a passkey. The passkey is learned by user input, and must be identical in a pair of devices. A device communicating with a trusted device can cryptographically authenticate the identity of the other device, and may also encrypt the data the devices exchange. Pairing is preserved since the Bluetooth address is permanent. Devices generally require pairing or prompt the user before they allow a remote device to use any or most of their services.

2.1.6 General Packet Radio Service (GPRS)/ 3G GPRS is a mobile data service available for mobile phones using Global System for Mobile Communications (GSM). It is used for services such as Wireless Application Protocol (WAP) access, Short Message Service (SMS), and Multimedia Messaging Service (MMS) and for Internet communication services such as e-mail or World Wide Web (WWW) access.

2.2

Wired Communication

2.2.1 UART and RS-232

A UART (Universal Asynchronous Receiver Transmitter) is a piece of hardware that translates data between parallel and serial forms. UARTs are commonly used in conjunction with other communication standards such as EIA RS-232. Microcontrollers usually implement UARTs and 9 or 25 pin serial ports on computers use UARTs. The UART usually does not directly generate or receive the external signals used between different items of equipment. Typically, separate interface devices are used to convert the logic level signals of the UART to and from the external signaling levels. External signals may be of many different forms. Voltage is by far the most common kind of signaling used. Examples of standards for voltage signaling are 232, 422 and RS-485 from the EIA. Some signaling schemes do not use electrical wires.

Examples of such are optical fiber, infrared, and Bluetooth in its Serial Port Profile.

2.2.2 Serial Peripheral Interface (SPI)

A protocol to communicate internally between the different devices on the PCB is needed. Using SPI will reduce the communication port pins needed

drastically compared to using a parallel port. There are different operating modes of SPI depending on what is most suitable for the configuration.

Individual slaves will be used. For every new device an additional pin must be added.

3

Design decisions

3.1

Hardware

The hardware is based on a newly developed RFID-chip from IDS-microchip. The RFID-chip is controlled by a microcontroller called ATmega 64(L) from Atmel. Both these chips contain sophisticated power saving functions. The communication can either be parallel or serial. SPI-bus is a serial protocol that will be used. The IDS-chip handles the analogue part when communicating with a tag. There is a built in power amplifier (100mW) for the RF transmission which will be used.

Because two 1.2V AA lithium batteries will be used there is need for higher voltage to supply the microcontroller and the IDS-chip. A dual step up device provides the reader with two individual voltages (3,3V and 5,3V).

The microcontroller communicates with a Bluetooth or WLAN module using AT commands. The Bluetooth device forwards the data to a Bluetooth

compatible mobile phone. The mobile phone must be connected to a server on Internet using some kind of protocol, preferably GPRS. A read tag will be transmitted to this server which can be a database containing tag-information. High switching speeds and large peak currents makes the PCB layout an important part of the design. Poor design can cause excessive EMI and ground-bounce which can cause instability or regulation error by corrupting the voltage and current-feedback signals. Components are placed as close together as possible. Traces are made as short and as wide as possible

3.1.1 Principal solution for MobiReader™

Figure 3-1– Principal solution for Mobireader

The MobiReader consists of two parts:

1. Peripheral board – Power Supply based on MAX863 dual step up device,

LEDs, Buzzer, Buttons, Tilt Sensor.

2. CPU board – ATmega 64(L) processor, Bluetooth module, UHF RFID reader

IDS 900.

All included in a handheld casing.

Note Regarding Potential products Questions/ Remarks

1 Internal amplifier, 100 mW

2 Circulator – clockwise – low return

losses needed. Miniature surface mount

circulator.

Will return losses be any problem

3 Chip Antenna _{For example:}

Fractus FR05-S1-R-0-105

Patch antenna

ATmega 64(L)

Microcontroller

UHF RFID reader

IDS 900 865 – 868 MHz Interface Bluetooth / WLAN 2) Power supply 2 x 1.2 V AA Lithium 4 x Buttons 5 x LEDs Display (Optional) Tilt Sensor Different signals Interface Mobile Phone GPRS 3) 1)

Dual step up device

MAX 863 5.3V, 3.3V

Table 3-1– Notes on the principal solution

3.1.2 Calculation of the reading distance

An important question to consider is the reading distance. Using the internal amplifier will provide a limited reading distance. The calculation of the reading distance is presented in this chapter. It is important to mention that the

calculations are based on perfect conditions. The transmission line on the PCB must be perfectly matched with the antenna. Reflections due to obstacles are not considered, neither is the alignment of antennas. The calculations just give a hint on how far the reading distance is.

The reading distance is calculated for 1000mW, 500mW and 100mW transmission power. A comparison between using a patch- and an omni-directional antenna is done when transmitting 100mW.

Calculation of dBm

When the dBm value is positive, i.e. +3dBm it means 3dBm more than 1mW. When the dBm value is negative, i.e. -3dBm it means 3dBm less than 1mW. A negative dBm value can be calculated similarly to a positive dBm. The procedure is, calculate the positive dBm X, and then calculate the inverse 1/X. When calculating the input sensitivity of some chip, of course the lower dBm the better handling of small signals. -66dBm is very sensitive (0.25 µW).

The input sensitivity of the tag used during the experiments has got a typical input sensitivity of -13 dBm or 50µW.

Calculation of reading distance using 1W transmission power

The antenna used is a patch antenna with 9dBic gain.

gain times G G 94 . 7 10 1 log 10 9 10 9 10 ⇒ = =      =

Compact antennas (such used in passive UHF tags) usually have gain less than that of a dipole — that is, less than 2 dBi — and can be regarded as isotropic in the plane perpendicular to their axis.

gain times G G 58 . 1 10 1 log 10 2 10 2 10 ⇒ = =      =

2 4      = R G G P P r t t r π λ where Pr = Power received Pt = Power transmitted

Gt =Gain transmission antenna Gr =Gain receiving antenna

λ = wavelength (velocity divided by frequency) R = distance in metres m 3468 . 0 10 865 10 3 6 8 = × × = λ 2 2 R 4 0.3468 58 . 1 94 . 7 1 4      × × =       = π π λ R G G P P_{r t t r}

The tag dissipates 1.5mW. Looking at the calculations, the limitation in the reading distance seems to be the power dissipation in the tag. When the transmitted power goes below 1.5mW the tag does not power up. The sensitivity of the IDS-R900G never limits the reading distance.

Using transmission power of 1W should give a maximum reading distance of 2.5m when operating at perfect conditions. Below the graph of transmitted power in mW can be observed. The x-axis is the distance in m.

¨ Figure 3-2– Transmitted power (mW) vs. distance (m)

Looking at Figure 3-3 it is obvious that beyond 2.5 metres the transmitted power goes below 1.5mW and the tag can therefore not power up properly.

Figure 3-3– Transmitted power (mW) vs. distance (1-3) m

Figure 3-4– Transmitted power from tag (mW) vs distance (1–3)m

At 2.5m the transmitted power from the tag is 47 x 10-6mW which is enough for the input on IDS-R900G.

Calculation of reading distance using a patch antenna and 501mW (27dBm) transmission power

These are the conditions for the experiments performed. The calculated reading distance is 1.75 m.

Figure 3-5– Transmitted power (mW) vs distance (0.5 - 1.8)m

Calculation of reading distance using a patch antenna and 100mW (20dBm) transmission power

This calculation is done in order to be able to tell how far the reading distance will be in the final product. The built in amplifier (100mW) will be used in the proposed PCB design.

The following calculation is based on a patch antenna like in the calculation above with 9dBic gain. The reading distance would be approximately 0.75m. At 0.75m the transmitted power from the tag is 0.0034 mW.

Calculation of reading distance using an omni-directional antenna and 100mW (20dBm) transmission power

If we instead of using a patch antenna we could use an omni-directional antenna. The following calculation is based on an antenna with 2dBic gain. The reading distance would be approximately 0.35m. At 0.35m the transmitted power from the tag is 0,0008 mW.

Figure 3-7–Transmitted power vs distance (0–0,35)m

Summary

Calculations are performed for a couple of different operating conditions. The reading distance is first calculated using the transmission power 1000mW. 1000mW is a very common transmission power used in RFID applications. 501mW is the operating condition for the evaluation boards. This reading distance is to be compared with the experimental results.

Finally the calculations are done with 100mW power. 100mW is the desired output power in the handheld reader. A patch antenna is compared with a standard omni-directional antenna.

Transmission Power Reading distance

1000 mW (patch antenna) 2.5m

501 mW (patch antenna) 1.75m

100 mW (patch antenna) 0.75m

3.1.3 ATmega64(L) – Micro controller

The simplest Micro controller available was first considered because complex functionality was not needed. The ATmega16 seemed to be the right choice. As more and more peripheral devices were added the Micro controller had too few I/O lines and too little memory. If a display shall be able to be added in the future quite much memory is needed. The ATmega64(L) has got all the desired functionality including very sophisticated power saving functions and a low price.

Key features of ATmega64(L)

• Low power 8-bit Microcontroller • 53 Programmable I/O Lines • 3.3V operating voltage

• 0 - 8MHz operating frequency

• 64K Bytes of in-System Reprogrammable Flash Program memory • 2K Bytes EEPROM

• 4K Bytes Internal SRAM

• SPI

• JTAG Interface

• Two 8-bit Timer/Counters with separate prescalers

• Two Expanded 16-bit Timer/Counters with separate prescalers • Two 8-bit PWM channels

• 6 PWM channels with programmable resolution from 1 to 16 bits • 8-channel, 10 bit ADC

• Dual programmable serial USARTs

• Programmable watchdog timer with On-chip oscillator • Internal Calibrated RC Oscillator

• External and Internal Interrupt Sources

• Six Sleep Modes: Idle, ADC Noise Reduction, save, Power-down, Stand by, Extended Stand by

Power consumption using an internal RC oscillator @ 8 MHz and 3.3V

Operating mode Operation condition Consumption

Normal Average consumption 9mA

Idle Average consumption 4.5mA

Sleep Average consumption 3µA

Table 3-3– Power consumption ATmega 64(L)

3.1.4 IDS_R900 – RFID chip

IDS_R900 is a newly developed small surface mounted RFID chip (9 x 9 mm) from IDS-Microchip. The chip takes care of the whole analogue part when communicating with a RFID-tag. A microcontroller is used to communicate with IDS_R900. Communication can be chosen to be either parallel or serial. When communicating with the chip different registers are read and written in. There is a built in amplifier for RF. Because the reader distance is not supposed to be long range the internal amplifier can be used. The power transmitted will be 100mW or 20dBm which will provide a reading distance of approximately 1m.

Key features of IDS_R900

• Parallel 8 bit or serial 4 pin SPI interface to MCU using 12 bytes FIFO

• Voltage range for communication with MCU between 1.8V and 5.5V • Selectable clock output for MCU

• Integrated supply voltage regulator (20mA) for supplying MCU and other external circuitry

• Integrated supply voltage regulator for the RF output stage, providing rejection to supply noise.

• Internal power amplifier (20dBm) for short range applications • Modulator using ASK or PR-ASK modulation

• Adjustable ASK modulation index

• AM and PM demodulation ensuring no “communication holes” with automatic I/Q selection

• Built in reception low-pass and high-pass filters having selectable corner frequencies

• Frequency hopping support

• On-board VCO and PLL covering complete RFID frequency range 860MHz to 960MHz

• Power down, stand by and active mode

Figure 3-8– Block diagram IDS-R900G

Looking at the block diagram makes it obvious that the communication with the IDS-R900G is digital. All the analogue parts are built into the chip.

Power consumption IDS-R900G

Operating mode Operating Condition Consumption

Normal/reading Average consumption 260mA

Idle Average consumption 3mA

Power down Average consumption 1uA

3.1.5 OEM-SPA310 – Bluetooth module

There are numerous available Bluetooth modules on the market. A module with all parts integrated, such as antenna and circuitry on a tiny PCB was chosen. Many different types and sizes of integrated modules exist on the market. The OEM-SPA310 is small in size, has low power consumption and a low price. An evaluation kit could also easily be acchieved. Communication between MCU will be using AT-commands on a standard USART. Communication between mobile phone is Bluetooth. As an option, this module can be mounted on a holder which a WLAN module is pin compatible with.

Key features of OEM-SPA310

• -Small in size • -Low profile

• -AT Command support

• -Configuration over Bluetooth • -Internal antenna