Wireless Power in Passive RFID Systems
Contents
1. so I could not know the absolute value of the carrier and sidebands In order to know the power received by the tag the power scale of the analyzer should be calibrated and the measurement should be made by using an antenna which is similar to the antenna in the tag 47 Figure 34 The carrier Figure 35 The upper sideband 48 4 3 4 The results of the experiment This experiment showed that the UHF RFID reader generated signals modulated by ASK When the tag picked up the signal it needed to demodulate the signal at first I also measured the power of the carrier and sidebands which were separately 70 dB and 10 dB above the noise level The system worked at 865 MHz which was the carrier s frequency The original signal was at 6 MHz 5 CONCLUSION AND FUTURE DEVLOPMENT 5 1 Conclusion These three experiments revealed the working principles of three kinds of RFID systems They have similarities as well as differences Especially I concentrated on the part of wireless power transfer Basically the wireless power transfers in these three types of systems all work based on the Faraday s law of induction All of them transmit the energy in the form of electromagnetic field All the tags have coil antennas in order to pick up the energy When the antenna enters an electromagnetic field the field can produce a current in the antenna which is the power source of the tag In the UHF tags also dipole antennas are possible2. Finkenzeller 2003 163 2 5 RFID standards Standards play a significant role in the RFID industry because they control the process of communication There are existing and proposed RFID standards that deal with the air interface protocol the way tags and readers communicate data content the way data is organized or formatted conformance ways to test that products meet the standard and applications how standards are used on shipping labels for example A Summary of RFID Standards referred 2 4 2010 The International Organization for Standardization and the Electronic Product Code are the two organizations in the world working on the RFID Standards 2 5 1 ISO standards The International Organization for Standardization ISO has addressed many standards concerning with RFID Perhaps the most important one is ISO 18000 series which is for item management and automatic identification This set of standards defines the parameter of the air interface including the bit rate and the way of modulation and coding as well as the collision arbitration For example ISO 18006 10 type A sets that we shall use pulse interval encoding at 33 kbps for the forward link and bi phase space FMO encoding at 40 or 160 kbps for the return link Aloha based mechanism shall be used for collision arbitration Totally there are seven parts in ISO 18000 each of which defines the air interface for a certain range of frequency e 18000 1 Gene
3. been dramatic as well The global sales of the RFID system were approximately 900 million USD in the year 2000 Finkenzeller 2003 1 This is quite little compared to what it is now A new market data report released by ABI Research predicts the overall RFID market to reach 5 35 billion USD this year a glimmer of optimism after the economic slide required the firms to adjust their RFID forecasts downward for 2009 and 2010 Sara Pearson Specter referred 26 3 2010 Compared to a traditional Auto ID system the RFID technology has many advantages Firstly the identification of tags is more accurate and the distance is more flexible Secondly the maximum memory capacity of RFID tags can be several megabytes which is much more than that of the traditional Auto ID s Thirdly RFID has better performance in anti pollution because the data is saved in a chip wrapped by the tag Finally RFID tag is rewritable We can add modify and delete the data stored in the tag repeatedly There are three types of RFID tags passive tag semi passive tag and active tag Since there are no batteries in the passive tag 1t has to get power from the reader to drive the chip inside How is the power transported from readers to tags How does the tag turn the high frequency AC to DC How does the tag use this power to send signals back These are the questions I concentrated on in this thesis Finding out the answers to these questions can help users to imp
4. nothing 1 0 1 0 1 1 0 0 1 0 NRZ L NRZ M NRZ S RZ Biphase L Biphase M Biphase S Differential Manchester Bipolar Figure 19 The waveforms of common encoding methods Communication method line codes referred 24 4 2010 3 6 2 Pulse interval encoding Pulse interval encoding is used in EPCglobal Class 1 Generation 2 standards It can 31 solve the power problem caused by a long string of 0 s PIE is usually implemented before the modulation In PIE a binary 1 is coded as a short power off pulse following a long full power pulse and a binary 0 is coded as a shorter full power interval with the same power of pulse Dobkin 2008 60 The symbols are illustrated in Figure 20 By encoding the data in this way we can make sure that at least 50 of the maximum power is transferred to the tag even though there is a long string of 0 s The durations of off time and on time are restricted by the standards The durations set by EPCglobal Class 1 Generation 2 are shown in Figure 21 Please notice that the Tari which equals to the duration of data 0 is the reference time interval of reader to tag communication As we can see from the figure the duration of binary O is much shorter than that of binary 1 Thus the data rate is related to the data a stream of 0 s is transmitted more rapidly than a stream of 1 s high power low power Figure 20 Pulse interval coding symbols Dobkin 2008
5. object identified e Transmission of relevant information from the interrogator to the transponder Since then scientists have focused on making the tag cheaper and more functional In order to achieve these they removed the transmitter and battery from the tag After the invention of integrated logic circuitry in the 1960s the RFID technology came to its spring The tag became smaller and smaller but its capability became more and more powerful Around the beginning of the 1970s tags based on resonant circuit were used in one of the first major commercial implementations of RFID technology by the Schlage Lock Company Dobkin 2008 13 Nowadays we can find RFID tags in many places including libraries hospitals shopping malls and highway toll stations 2 3 The components of RFID system Usually a RFID system contains three components tags a reader and middleware as shown in Figure 1 Reader Tags are attached to each objects EE or cable Wireless or mM cable NO sent Antenna _ Energy Host Figure 1 The components of RFID system Sorour referred 29 3 2010 2 3 1 The tag A RFID tag is a microchip combined with an antenna in a compact package Sorour referred 29 3 2010 The tag is designed to be easily attached to objects It is usually very small even as small as a rice grain The microchip is the heart of the tag which stores the identification data The antenna is used for picking up the
6. reader signal and sends back the data in the microchip There are three types of the RFID tag passive tag semi passive tag and active tag The options for tag power transmit configuration of these three types of tags are shown in Figure 2 power for tag and power for tag gt backscattered signal power for radio backscattered signal MES transmitted signal battery from tag le power for tag and radio Figure 2 Options for Tag Power Transmit Configuration Dobkin 2008 35 Passive tags contain neither electrical power nor a radio transmitter The power to drive the circuitry and microchip is received from rectification of the received power from the reader and the tag sends the signal back through a backscattered signal Because of functioning without batteries tags of this type are cheap small and have a long life time Though they have a limited range of communication typically a few feet they are the most popular RFID tags in applications Semi passive tags have their own battery to power the circuitry but no radio transmitters They still use backscattered communication for tag to reader communication Active tags are equipped with batteries and antennas Therefore they can initiate communication and work in a large range up to hundreds of feet They can even have sensors inside However these types of tag are expensive and large Thus they don t have as many appli
7. 60 1 5 Tari lt data 1 lt 2 0 Tari Er Tari 0 5 Tari lt x lt Tari Figure 21 The duration restrictions of PIE data 0 and data 1 EPC Classi Gen 2 32 PIE consumes more bandwidth compared to OOK for the same data rate It also produces a strong narrow emission far from the carrier as well as a higher average signal power far from the carrier Dobkin 2008 65 This is illustrated in Figure 22 sk AL AN l Imn OT m 400k symbols L PIE symbols diffuse power strong signal far far from carrier from carrier z 1 1 _1 5 data rate power relative to carrier dB frequency Figure 22 The spectrum of PIE and OOK Doobkin 2008 65 4 EXPERIMENTS 4 1 One bit tag experiment I firstly did an experiment with a one bit tag 4 1 1 The aim of the experiment One bit tags are widely used in shops and supermarkets to protect goods from being stolen This kind of tag is usually attached to the merchandise If somebody wants to 33 bring it outside the shop without paying the antennas at the gate will detect the tag and trigger the alarm In this experiment we firstly opened a one bit tag to check its physical structure Then we simulated the situation that a tag passed through the antennas in order to find out the working principle of this type of tag and what happens in the physical layer 4 1 2 The equipment of the experiment The equipment of the experiment included the following O Aone bi
8. Wu Shen Wireless Power in Passive RFID Systems Bachelor s Thesis Information Technology May 2010 ee MIKKELIN AMMATTIKORKEAKOULU Mikkeli University of Applied Sciences DESCRIPTION Date of the bachelor s thesis ASF im MIKKELIN AMMATTIKORKEAKOULU 10th May 2010 Mikkeli University of Applied Sciences Author s Degree programme and option Wu Shen Information Technology Name of the bachelor s thesis Wireless Power in Passive RFID Systems Abstract Radio frequency Identification RFID technology has many applications in various fields Compared to a traditional Auto ID system the RFID technology has many advantages Firstly the identification of tags is more accurate and the distance is more flexible Secondly the maximum memory capacity of RFID tags can be several megabytes which is much more than that of the traditional Auto ID s Thirdly RFID has better performance in anti pollution because the data is saved in a chip wrapped by the tag Finally RFID tag is rewritable We can add modify and delete the data stored in the tag repeatedly There are three types of RFID tags passive tag semi passive tag and active tag Since there are no batteries in the passive tag it has to get power from the reader to drive the chip inside How is the power transported from readers to tags How does the tag turn the high frequency AC to DC How does the tag use this power to send signals back These are the questi
9. a e NS 2 E Whats AAA i E a ERRES 2 DD a EIO AAEE E E E oaiealp ban oenaunoeauesperaenes 3 2 3 The components of RFID system sss sese eee eee 3 PA A E E vid E E E ca ah E N lens 4 2 3 2 The readers nada nel last 6 2 3 3 Rs e AAA A ren 6 2A Operating UT 7 Ns A RE Gee A A A RNa 9 2 5 1 ISO Bandas ida 9 2 5 2 EPC standards 2 ii lied 10 3 WORKING PRINCIPLE OP RIB UMD ig ii 12 3 1 Propagation of electromagnetic Wave sees ee eee eee ee eee eee eee 12 32 a STH 16 2 2 1 Backscatter COI IM tasa 16 7 2 ZANGUCLIVE POP ia 17 3 23 CApaciliyvo COUP LING parceria a nd S 19 2 3 R SONANEEN LEY T rs ee nn es ee en ne 19 SAAC to DC ui ie aay dinar agen teen tarte 20 3 5 Modu lati n method a4 04 2 SR E 23 3 5 Amplit de shift RENNES SNA nr ne de E 24 3 5 2 Frequency shift Ke yim viario ee eee eee 26 30 37 Phase shift keying iS in Te Mon et A tee nn ees 21 O EA O O de 28 AOL ES A O res et Rai 29 3 6 2 Pulse intery al encoding srt ao aa ES ais 30 A EXPERIMENTS cuisine daa 32 Al Ojn pittas Exentos 32 4 1 1 The ain of the experiments aa 32 4 1 2 The equipment of the experiment 33 4 1 3 The process of the experiment sees eee eee eee 34 4 1 4 The results of the experiment sese minimes tte 39 4 2 Low frequency RFID energy transfer sese ee eee eee eee 39 4 2 1 The aim of the experiment esse ee eee eee eee 39 4 2 2 The equipment Of the experiment haine 40 4 2 3 The process of the experiment A ele nn 40 4 2 4 The resul
10. are for example local database and Application Level Events ALE However it excludes the information management terminal software and device software Middleware should have the following functions Sorour referred 29 3 2010 O Device control configuration and monitoring for hundreds of devices Device operation Command the reader to read write tag s Tag data filtering and cleansing O Data routing and integration Which data is passed to which recipient Application services event management Respond to events generated by business apps O Data translation 2 4 Operating frequency The operating frequency of RFID can be ranged from 30 kHz to 30 GHZ covering four main frequency bands The characteristics of these frequencies are shown in Table 1 Table 1 RFID operating frequencies and associated characteristics IEE 2005 referred 30 3 2010 30 300kHz 3 30MHz 300 MHz 3GHz 2 30 GHz Frequency Typical RFID 125 134kHz 113 56 MHz 433 MHz or 2 45 Frequencies 865 956MHz 2 45 GHz 433 MHz up to 100 Approximate less than 0 5 Up to 1 5 metres metres read range metre 865 956 MHz 0 5 to 5 metres less than 1 Approximately 433 956 30 kbit s Up to 100 Typical data kilobit per 25 kbit s LF HF UHF Microwave Band Low High frequency Ultra high frequency frequency transfer rate second kbit s 2 45 100 kbit s Short range Higher ranges Long ranges high Long range Characteristics low data reasonable
11. because the wavelength is smaller The minimum sizes of efficient dipole antennas are from about 0 1 to 0 5 wavelengths At 865 MHz this means about 3 to 17 cm The differences among these three kinds of RFID systems are obvious Firstly the LF and UHF passive RFID tags both have microchips inside Thus DC power is needed to drive the chip so they must convert the high frequency AC power to DC and rectify circuitries are applied However the one bit RFID tag contains no microchips 49 It only has an inductor and a capacitor which are serially connected with each other Therefore it can use the AC power directly This simplifies its structure but limits the functions as well what it can send is only one bit so this kind of tag does not have diverse applications Secondly the signals generated by the LF RFID reader and the UHF one are different The LF RFID reader produces a simple sine wave which is not modulated while the UHF reader modulates the signal by ASK before sending it Thus when the UHF tag receives the signal 1t must firstly demodulate it and then extract the DC component to drive the microchip Finally the UHF reader can transmit the power to a very long distance even several meters away However the LF reader can only transmit the power as long as 10 centimeters The reason for this is the use of near field inductive coupling instead of far field communication in LF reader In LF the antennas are very small compared
12. cations as passive tags do 2 3 2 The reader A RFID reader is at heart a radio transceiver a transmitter and receiver that work together to communicate with the tags Dobkin 2008 103 The reader has its own antenna and power supply and usually it is connected to a computer or microcomputer which can manage the data read from the tag Communication between readers and tags can be with or without line of sight propagation We use special protocols to control the communication One reader can support one or multiple protocols RFID readers can be divided into two classes read only and read write The read only reader usually operates with passive tags It can do nothing except read out the data in the tag Read write reader can modify the information in the tag in the case that the tag 1s equipped with re writable memory The reader can either be fixed in a place or handheld even embedded in PDA or a mobile phone Some handheld readers resemble a bar code scanner to make them have more functions Sometimes handheld readers support 802 11 protocols so that they are able to send the information to a database wirelessly 2 3 3 The middleware Evens P H gt Middleware gt Recio Figure 3 RFID Middleware Middleware refers to a set of software between the reader and information management application Figure 3 which is used to deal with the data flow and control the operation of the reader It includes all the softw
13. ccording to which kind of properties is altered we divide the digital modulation into three basic types amplitude shift keying ASK frequency shift keying FSK and phase shift keying The waveforms of these modulation methods are shown in Figure 14 24 0 0 1 1 O 1 0 0 O 1 0 a ASK b BFSK c BPSK Figure 14 Digital modulation method Juutilainen 2010 3 5 1 Amplitude shift keying ASK alters carriers amplitude in accordance with the data sequence This kind of modulation is widely used in the RFID tags because it is easy to implement The simplest and most common form of ASK operates as a switch using the presence of a carrier wave to indicate a binary one and its absence to indicate a binary zero This type of modulation is called on off keying OOK and it is used at radio frequencies to transmit Morse code Amplitude shift keying referred 24 4 2010 For example we can use amplitude Ac stands for bit 1 and 0 for bit 0 We assume that the frequency of the carrier is fc Then we can figure out the formula of ASK A sint 2nf t for bit 1 0 for bit 0 Formula 3 5 1 1 s t The time domain of ASK is illustrated in Figure 15 25 Modulating frequency 3 Carrier frequency 0 2 3 4 5 6 7 8 9 10 11 12 O my RT BR D Es A DE Output frequency Figure 15 Amplitude shift keying Juutilainen 2010 Amplitude modulation results in a shift of the spectrum in freq
14. ce this current will produce a voltage over the circuit The reader can keep sweeping a range of frequencies including the resonant frequency When encountering with it the reader can detect the drop in voltage In this way one bit is sent to the reader Since the tag can carry multiple resonances of various frequencies the system can have a large ID space 3 4 AC to DC The energy an antenna picks up from the electromagnetic field is high frequency altering current AC power which cannot directly drive the microchip in the tag Therefore we have to turn it to direct current power DC This kind of power should have magnitude from 1 to 3 V and both the magnitude and the direction of the current should seldom change with time In order to achieve this we use a rectifier circuit 21 antenna resistor Figure 10 Simple rectifier circuit Dobkin 2008 19 Figure 10 shows a simple rectifier circuit Two basic components of this circuit are the diode and capacitor A diode can only pass currents in one direction It has a very high impedance in the opposite direction We can see its current voltage characteristics in Figure 11 In the forward direction current increases rapidly after it reaches a certain turn on voltage In the reverse direction only small leakage current is allowed till its reaches the breakdown voltage which can damage the diode seriously Based on these features we can turn the sine wave into sinusoidal pul
15. data data transfer rate high data transfer rate rate similar to concurrent read of transfer rate penetrates GSM phone lt 100 items cannot cannot water but not penetrates water penetrate water or penetrate metal but not metal metals water or metal Animal ID Smart Labels Specialist animal Moving Typical use Car Contact less tracking vehicle toll immobiliser travel cards Logistics Security As we can see from the table the high frequency wave has a higher data rate and larger read range than the low frequency one However there is one thing which is not shown in the table the low frequency wave can penetrate walls but the high frequency wave cannot So if we are going to design an indoor RFID system this factor must be taken into consideration RFID systems based on LF and HF frequencies make use of near field communication and the physical property of inductive coupling from a magnetic field On the other hand RFID systems based on UHF and higher frequencies use far field communication and the physical property of backscattering or reflected power Ward amp Kranenburg referred 30 3 2010 Another important issue is that different countries have different restrictions in the frequency band For example in order to prevent collisions the future Licensing Act for Inductive Radio System in Europe 220 ZV 122 will define a protected zone of between 70 and 119 kHz which will no longer be allocated to RFID system
16. e spectrum analyzer oooocnnoccnnnnnnoncnns 45 The spectrum of the UHF signal ss sese esse eee 46 TICA 47 THE Upper Side band nn ti TNT errad 47 1 INTRODUCTION Radio frequency Identification RFID technology has many applications in various fields Mario Cardullo s U S Patent 3 713 148 in 1973 was the first true ancestor of modern RFID a passive radio transponder with memory Radio frequency Identification referred 26 3 2010 Since then the application of RFID developed rapidly Animal identification is one of the oldest uses of the RFID technology By injecting an IC chip which is about the size of a large grain of rice into animals skin we can easily identify it This is very useful in the return of lost pets RFID also contributes a lot to the supply chain management With its help we can improve the efficiency of inventory tracking and management In an academic study performed at Wal Mart RFID reduced Out of Stocks by 30 percent for products selling between 0 1 and 15 units a day Radio frequency Identification referred 26 3 2010 So now many big companies including Wal Mart require their suppliers to attach RFID labels to all shipments Recently the uses of RFID have become more and more diverse For example it can be used in a public transportation tolling system in libraries to track books even in mobile phones to make payments Similarly with the development of technology the growth of the RFID market has
17. enerated by the reader was not modulated when there were no tags nearby The power that the reader sent to the tag depended on the distance between them The maximum power the antenna picking up was 172 mV This was very small because the antenna was a single turn coil antenna In real tags there are more turns in the antenna so the magnitude can be many times of the measuring value 44 4 3 The ultra high frequency RFID energy transfer This experiment is done with a high frequency RFID reader 4 3 1 The aim of the experiment The ultra high frequency UHF RFID system works at frequencies from 300 MHz to 3 GHz This type of system is widely used in supply chain management Its antennas structure 1s different from that of LF RFID system so the energy transition is different as well This experiment was aimed to illustrate the spectrum of the UHF signal sent by the reader and measure how much power the antenna can pick up I also expected to find the modulation type of the signal 4 3 2 The equipment of the experiment The equipments of the experiment included the following O A Vilant UHF RFID reader and its antenna Appendices 3 4 A UHF receive antenna A spectrum analyzer A computer installed with RFID software According to the user manual the Vilant UHF RFID reader works at 865 MHz so I selected it as the object The RFID software can control the reader The UHF antenna is used for picking up the signal and transm
18. ese 22 Figure 13 The result of sinusoidal pulses passing by a capacitor esse ee ee eee eee 23 Figure 14 Digital modulation method 24 Figure 15 Amplitude shift keying nn A A NE ue A 25 Pisure 16 The spectrum of ASR snoot 26 Figure 17 Frequency shift keying sienne nes ee nt en tdi ee 27 Figure 18 Phas shift Keyin iii dci eres 28 Table 2 Common encoding ME RE RS 29 Figure 19 The waveforms of common encoding methods 30 Figure 20 Pulse interval coding symbols sss sees eee 31 Figure 21 The duration restrictions of PIE data 0 and data 1 sss 31 Figure 22 The spectrum of PIE and OOK ocio as 32 Fig r 23 N antennas 4 225 Mae ites TS ae ea ete eed 34 Figure 24 The components inside the one bit tag oo sese eee eee 35 Figure 25 The measurement of the tag cigs sees eee eee eee 36 Table 3 The measurement result of the capacitance and inductance of the tag 36 Figure 26 The network analyzer connected antennas sse sese ee seer ee eee eee 38 Figure 27 Figure 28 Figure 29 Figure 30 Figure 31 Figure 32 Figure 33 Figure 34 Figure 35 The spectrum of the receiving antenna without the tag 38 The spectrum of the receiving antenna with the tag 39 The antenna approaching the reader sss sese seer eee eree eee 41 The waveform of the LF RFID signal when the antenna was near the reader A a a iS 42 The waveform of the signal when the antenna was far from the reader 43 The UHF antenna connected with th
19. he transmitter port of the network analyzer and the other one with the receiver port Figure 26 I set the start frequency of the spectrum to 7 2 MHz and terminal frequency to 9 2 MHz so as to put 8 39 MHz around the middle of the spectrum We can see the spectrum when there is no tag between the antennas in Figure 27 Then I put the tag between two antennas which caused a big change in the spectrum As we can see from Figure 28 there is a sharp burst about 4 dB above the normal level between 8 2 MHz and 8 4MHz This can be considered as the signal that the tag sent to the reader When the reader detected this signal it triggered the alarm Figure 26 The network analyzer connected antennas Pl Transmission h Ref 53 00 dB seme ba Gee Log Mag 0 5 dB CE r ve A RME Figure 27 The spectrum of the receiving antenna without the tag 39 Log Mag 0 5 dB Ref 53 00 dE Meas1i Mkr1 8 201 MHz 52 110dB Scale Diy Figure 28 The spectrum of the receiving antenna with the tag 4 1 4 The results of the experiment This experiment verified that one bit tags used in the shop work based on resonant energy transfer Their resonant frequency was between 8 2 MHz and 8 4 MHz When a tag passed through the antennas it would produce a burst at its resonant frequency which could trigger the alarm 4 2 Low frequency RFID energy transfer This experiment is done with a low frequency RFID reader 4 2 1 The aim of the e
20. hown in Table 3 36 Figure 25 The measurement of the tag Table 3 The measurement result of the capacitance and inductance of the tag Capacitance Inductance Frequency Hz C pF 0 L U H 0 1k 120 87 89 97 2 9514 13 70 10k 120 78 89 98 2 9543 67 58 100k 121 08 89 93 2 9347 87 18 1M 125 20 89 95 2 8668 89 46 3M 127 42 90 37 2 7644 89 82 4 5M 137 20 90 18 2 6235 90 05 The 0 in the table indicates the property of the component An ideal capacitor is supposed to have a 0 of 90 an ideal inductor of 90 and wire of 0 We can see from the table that the inductor s 0 was only 13 70 at 1 kHz which meant that it almost 37 worked like a wire rather than an inductor Thus I inferred that the tag could not work at a low frequency band So I used the values measured at 4 5 MHz to calculate the resonant frequency The calculation was based on Formula 3 3 1 and the process was shown as follows 1 1 eS aaa 28 2nvLC 2 x 3 14 x V137 20 x 10 x 2 6235 x 10 6 The resonant frequency was 8 39 MHz so I should have measured the capacitance and inductance at that frequency but the maximum frequency of the HiTester was only 4 5 MHz Thus I decided to use the measurements at 4 5 MHz Finally I used the network analyzer to check what would happen in the spectrum at 8 39 MHz when the tag passed through the antennas One of the antennas was connected with t
21. ically works based on inductive coupling and can partially solve the problems I just mentioned It still has some drawbacks like high cost and so on and it is still not able to be implemented at home However as more and more scientists and engineers concentrate on this field we are confident about the future of wireless power 51 REFERENCES Books Carlson A Bruce 1986 Communication Systems An Introduction to Signals and Noise in Electrical Communication Third Edition Printed in Singapore McGraw Hill Dobkin Daniel M 2008 The RF in RFID Passive UHF RFID in Practice the United States of America Newsnes Finkenzeller Klause 2003 RFID Handbook Fundamentals and Applications in Contactless Smart Cards and Identification Second Edition Wiley microID 125kHz RFID System Design Guide 1998 Microchip Technology Inc Unpublished source Juutilainen Matti 2010 Digital Modulation Course Slides Mikkeli University of Applied Sciences Electronic sources Amplitude shift keying referred 24 4 2010 Available in www format lt URL http en wikipedia org wiki Amplitude shift_keying gt A Summary of RFID Standards RFID jounal referred 2 4 2010 Available in www format lt URL http www rfidjournal com article articleview 1335 1 129 gt Backscatter referred 18 4 2010 Available in www format lt URL http en wikipedia org wiki Backscatter gt Capacitive coupling referred 19 4 2010 A
22. its it to the spectrum analyzer Because of the frequency band limitation I could not use the oscilloscope to observe the signal in time domain Thus I chose the spectrum analyzer to observe the signal in frequency domain which could also be used for measuring the power 4 3 3 The process of the experiment At first I connected the RFID reader to the computer through serial port Then I used 45 the software to make the reader begin to detect the tags After that I used the antenna to pick up the signal Figure 32 The antenna was connected to the spectrum analyzer so that I could see the spectrum of the signal which is shown in Figure 33 Figure 32 The UHF antenna connected with the spectrum analyzer 46 soi ul lb SEN si acer ul Ws oe PANA M don Figure 33 The spectrum of the UHF signal As we can see from the figure the spectrum was quite clear There was a carrier in the middle and two sidebands symmetrically around it This was a typical spectrum of ASK Therefore I inferred that the reader modulated the signal by ASK The carrier was located at 865 MHz The frequency difference between the carrier and sidebands was 6 MHz which indicated that the modulating signal s frequency was 6 MHz The carrier was about 70 dB above the noise level Figure 34 It was not very stable because of interference and noise The sidebands were 10 dB above the noise level Figure 35 The spectrum analyzer can only show the decibel
23. ke the image on the screen clear and stable Then I continued approaching the reader till the last few millimeters before the antenna could touch the reader At this time I could get the maximum power that the reader was sending to the tag The waveform of the signal received from the antenna is shown in Figure 30 C Figure 29 The antenna approaching the reader 41 42 2010 04 14 Figure 30 The waveform of the LF RFID signal when the antenna was near the reader As we can see from the figure the signal that the antenna received was a sine wave without modulation Its frequency was 125 007 kHz which verified my supposition The magnitude from peak to peak was 172 mV This voltage though fairly small was supposed to be the largest the antenna could get The magnitude was related to the distance between the antenna and the reader When I moved the antenna a little further the peak to peak magnitude was 168 mV smaller than the former one Figure 31 43 2010 04 14 Figure 31 The waveform of the signal when the antenna was far from the reader In the end I tried to find what would happen to the waveform when the reader was transmitting signals but failed because the voltage difference was too tiny to be seen from the oscilloscope 4 2 4 The results of the experiment The experiment verified that the RFID access control on the door was a LF system Its working frequency was 125 007 kHz The signal g
24. lement a RFID system The structure of the thesis is as follows In Chapter 2 I will give an overview about the RFID technology describing its history and components In Chapter 3 I will introduce the working principle of passive RFID tags and RFID communication process In Chapter 4 I will show how I did the experiments and what results I received eventually as well as the analysis of the results In Chapter 5 I will draw the conclusions based on my work and discuss the problems and further research as well 2 OVERVIEW 2 1 What is RFID Radio frequency identification RFID is the use of an object typically referred to as an RFID tag applied to or incorporated into a product animal or person for the purpose of identification and tracking using radio waves Radio frequency Identification referred 26 3 2010 2 2 History The history of using radio frequency to identify objects can be dated back to World War II a transponder attached on the aircraft to show whether it is friendly or hostile Though this seems to have nothing to do with modern RFID tags it does have some features that are related to RFID Dobkin 2008 9 O Identification of an object using a radio signal without visual contact or clear line of sight radio frequency identification e An ID space big enough to allow unique identification of the object O Linkage to a sensor to provide information about the state of the object identified e Location of each
25. ll as drawbacks and its own suitable application The most common encoding methods are shown in Table 2 and their waveforms are illustrated in Figure 19 Table 2 Common encoding methods Communication method line codes referred 24 4 2010 NRZ L Non return to zero level This is the standard positive logic signal format used in digital circuits 1 forces a high level O forces a low level NRZ M Non return to zero mark 1 forces a transition O does nothing Non return to zero space 1 does nothing O forces a transition Return to zero 1 goes high for half the bit period O does nothing Biphase L Manchester Two consecutive bits of the same type force a transition at the beginning of a bit period 1 forces a negative transition in the middle of the bit O forces a positive transition in the middle of the bit Biphase M There is always a transition at the beginning of a bit period 1 forces a transition in the middle of the bit O does nothing 30 There is always a transition at the beginning of a bit period 1 does Biphase S nothing O forces a transition in the middle of the bit There is always a transition in the middle of a bit period Differential 1 does nothing Manchester O forces a transition at the beginning of the bit The positive and negative pulses alternate Bipolar 1 forces a positive or negative pulse for half the bit period O does
26. nite transmission line which is shown in Figure 4 14 Zo characteristic impedance Zs Zo of transmission line Zo V inc r uw iy E Vier O all the incident power is absorbed in the load Figure 4 Transmission line terminated with Zo Network Analyzer Basics referred 16 4 2010 When a transmission line is terminated with open or short circuit no energy will be absorbed and all the power will be reflected back The reflected wave and the incident wave will be identical in amplitude but out of phase 180 for short circuit and in phase 0 for open circuit In both case the reflected and incident waves will travel in opposite direction and a standing wave pattern will be set up on the transmission line The valleys will be zero and peaks twice the magnitude of an incident wave This is shown in Figure 5 15 Zs Zo x E gt TATA TATA Vre phase 0 for open out of phase 180 for short Figure 5 Transmission line terminated with short open Network Analyzer Basics referred 16 4 2010 When a transmission line is terminated with a resistance other than the characteristic impedance some power will be absorbed by the load while the rest will be reflected back The reflected wave will also travel in the opposite direction but its magnitude will be no longer the same with the incident wave and its phase will alter to the distance along the transmission line from the load A standing wave
27. nsformer The tags and readers have coil antennas The reader coil can generate a strong electromagnetic field Once a tag enters this field it can produce a voltage in the tag s coil just like the transformer s primary winding generating a voltage in the secondary winding Figure 8 This voltage serves as the power supply for the IC in the tag Then the IC controls the transistor to shunt the tag coil The RF link behaves essentially as a transformer when the secondary winding is momentarily shunted the primary winding experiments a momentary voltage drop By repeatedly shunting the tag coil through the transistor the tag can cause slight fluctuations in the reader s RF carrier amplitude microID 125kHz RFID System Design Guide 1998 2 Though the fluctuations seem tiny 100mV riding on a 100V sine wave for example they are already huge enough for the reader to detect them In this way we are able to send meaningful signals back to the reader Magnetic field H Transponder iiad madudalor http RFID handbook com Figure 8 Inductive coupling power supply Finkenzeller 2003 42 19 3 2 3 Capacitive coupling In electronics capacitive couplingis the transfer of energy within an electrical network by means of the capacitance between circuit nodes Capacitive coupling referred 19 4 2010 Capacitive coupling tags are cheaper than the inductive coupling ones because instead of the coil they use a small quantity of
28. ons I concentrated on in this thesis Finding out the ansewers to these questions can help users to implement a RFID system Subject headings keywords RFID wireless power passive tag radio frequency Pages Language URN URN NBN fi mamk opinn2010A 53 pages English 8586 Remarks notes on appendices Employer of the bachelor s thesis Osmo Ojamies ACKNOWLEDGEMENT At first of all I would like to express my greatest appreciation to my parents You always support me no matter what happens I could get nothing without you I will love you forever Then I would like to thank my tutor Mr Osmo Ojamies Thank you for conducting me to complete the final thesis and helping me do the experiments I did learn a lot from you It s my honor to meet you Thirdly I want to thank my mental tutor Mr Matti Koivisto and all the other teachers who have taught me in the Mikkeli University of Applied Sciences Thanks for your help in my study and life in Finland I can see the kindness of Finnish people from you Fourthly I want to thank my teachers in the Beijing University of Technology Thank you for helping me when I was in the campus and giving me the chance to study in Finland Last but not least I would like to give my appreciation to my friend Miss ZengZhen Thank you for helping me search documents and most importantly encouraging me whenever I feel frustrated and lonely CONTENTS IIS INTRODUCHON T 1 2 OVERVIEW
29. pattern will still be set up on the transmission line but its valley will be above zero and peaks below twice the magnitude of the incident wave This is shown in Figure 6 16 Zs Zo li 250 L VV PA KT a Figure 6 Transmission line terminated with a resistance other than the characteristic impedance Network Analyzer Basics referred 16 4 2010 3 2 Coupling 3 2 1 Backscatter coupling Passive RFID tags working on 868MHz and 915MHz use backscatter to send signals back to the reader In physics backscatter or backscattering is the reflection of waves particles or signals back to the direction they came Backscatter referred 17 4 2010 This kind of reflection is a little different from the mirror reflection It is diffuse reflection because the reflected lights or signals travel not only in the exact opposite direction but also other directions 17 tag antenna no IC current antenna current i no antenna current 7 antenna current IC current no IC current no IC voltage antenna current flows thru switch bypassing IC normal operation open circuit short circuit Figure 7 Elements of simple backscatter modulation Dobkin 2008 211 As we can see from Figure 7 there are three states of the tag antenna normal operation open circuit and short circuit In the normal operation the IC is perfectly matched to the antenna like a transmission line terminated with load ZO In this case some powe
30. r there are many problems of this application Firstly the power that we can transmit now is very low only about a few hundreds of milliwatts This is enough for a motor in an electric toothbrush or a microchip in a RFID tag but too low for a television set whose power is usually 100 watts Of course we can raise the voltage of the source but 1f so health problem will arise A very strong electromagnetic field can do damage to a human brain Therefore we need to find a way to send the power safely Secondly in most cases one wireless power source needs to supply several home appliances If there are many objects that need power from the source it is a big issue that the voltage will be unstable This may affect the performance of every appliance We need to keep the voltage stable even when there are lots of devices that need power The next issue is about the efficiency We don t like to waste power so we should figure out a way to make sure that little power goes to the unnecessary place The final problem is the authentication Your neighbors are never willing to know that they are actually paying your electricity fee every month How to control the power source so that it only supplies the power to the device allowed is a big issue Some scientists have already done a lot of research in this field The most successful one is eCoupled It is a kind of intelligent wireless power technology invented by Fulton Innovation This technology bas
31. r is backscattered In the open circuit because there is no current passing through the circuit nothing is backscattered In the short circuit since the current meets little impedance it reaches its maximum Consequently a large amount of power is scattered back In practice we use electronic switches transistors to control the circuitry The transistors gate pole is connected to the output of the IC in the tag Once the IC gets its power from the antenna it will control the transistor to switch on and off corresponding to the data in its memory and indirectly it can control the backscatter radiate power In this way we can amplitude modulate the signals sent back to the reader In the reader we use a directional coupler to transfer the signals to the receiver input of a reader 3 2 2 Inductive coupling In electrical engineering two conductors are referred to as inductively coupled or magnetically coupled when they are configured so that a change in the current flow through one wire induces a voltage across the ends of the other wire 18 through electromagnetic induction Inductive coupling referred 18 4 2010 Some RFID systems especially those using below 135kHz or 13 56MHz frequency band use inductive coupling to exchange information between the tags and readers antennas because their wavelength is much larger than the distance between the tags and readers The inductive coupling in the RFID system works like a tra
32. ric parameters for air interfaces for globally accepted frequencies 18000 2 Air interface for 135 KHz 18000 3 Air interface for 13 56 MHz 180004 Air interface for 2 45 GHz 18000 5 Air interface for 5 8 GHz 18000 6 Air interface for 860 MHz to 930 MHz 18000 7 Air interface at 433 92 MHz Besides this the ISO has created standards for tracking cattle with the RFID ISO 11784 defines how data is structured on the tag ISO 11785 defines the air interface protocol ISO has created a standard for the air interface protocol for RFID tags used in payment systems and contactless smart cards ISO 14443 and in vicinity cards ISO 15693 It also has established standards for testing the conformance of RFID tags and readers to a standard ISO 18047 and for testing the performance of RFID tags and readers ISO 18046 A Summary of RFID Standards referred 2 4 2010 2 5 2 EPC standards The Electronic Product Code originally divides RFID tags into 4 classes Tag Class Definitions referred 3 4 2010 O Class 1 Identity Tags Passive backscatter tags with the following minimum features lt gt An electronic product code EPC identifier A tag identifier Tag ID lt gt A function that renders a tag permanently non responsive lt gt Optional decommissioning or recommissioning of the Tag lt gt lt gt 11 Optional password protected access control Optional user memory Class 2 Higher Functionality Tags Passi
33. rier UHF Class 1 Gen 2 Standard v 1 2 0 referred 3 4 2010 Communication between the interrogators and the tags performs in half duplex way which means that readers and tags shall not talk simultaneously 3 WORKING PRINCIPLE OF RFID 3 1 Propagation of electromagnetic wave Electromagnetic wave carries RFID signals So first of all I will introduce some basic knowledge about it An electromagnetic field can be viewed as the combination of an electric field and amagnetic field whichoscillatesin phase perpendicular to each other and perpendicular to the direction of energy propagation The electric field is produced by stationary charges and the magnetic field is produced by moving charges currents These two are often described as the sources of the field Electromagnetic Field referred 7 4 2010 and also the electric field and the magnetic field interact with each other This kind of interaction is very complicated Simply speaking an altering electric field produces a magnetic field and so does an altering magnetic field An electromagnetic field propagates in wavelike manner at the speed of 3 X 10 m s in the vacuum of space so we usually call it an electromagnetic wave An electromagnetic wave will produce an electric current through any conductor which it passes This is where the passive RFID tags get their energy from Radio frequency wave is a type of electromagnetic wave whose frequency is below 300 GHz and abo
34. ses Figure 12 22 CURRENT FORWARD CURRENT BREAKDOWN VOLTAGE Ve LEAKAGE CURRENT VOLTAGE AVALANCHE CURRENT t REVERSE VOLTAGE Figure 11 Diode s current voltage characteristic Figure 12 The result of a sine wave passing through a diode The next step is to change these sinusoidal pulses into DC current All we need is a capacitor A capacitor can temporarily store electric charges and its charging speed is much faster than the discharging speed Thus when the sine wave is at its positive half the capacitor will charge itself When the sine wave reaches its negative half the capacitor begins to discharge Then the sine wave will reach its positive half again so the capacitor will charge again Because the speed of discharging is very slow the voltage only drops a little bit before it rises again In this way we can keep the voltage roughly constant Figure 13 23 Figure 13 The result of sinusoidal pulses passing by a capacitor 3 5 Modulation method Air can only transmit analog signals Therefore we need to modulate the signals before transmission Modulation is the process of varying one or more properties of a high frequency carrier signal corresponding to a meaningful low frequency signal modulating signal in order to make it suitable for the medium A digital signal can modulate the amplitude frequency or phase of a sinusoidal carrier wave Carlson 512 A
35. silicon to accomplish the same function Capacitive coupling involves the reader emitting a propagating electromagnetic wave When this wave impinges on a tag the chip will modify the antenna radar cross section in such a way that the reflected signal containing the information on the chip can be detected by the reader This is the primary mode of operation at UHF and in the microwave region Physics of RFID referred 19 4 2010 3 3 Resonant energy transfer One bit passive RFID tags are working based on a resonant circuit The resonant circuit consists of an inductance and a capacitance series connected with each other The resonant circuit has its own resonant frequency only determined by the inductance and capacitance Formula 3 4 1 f 1 2nYLC Formula 3 3 1 When this frequency is reached the circuit s impedance will become minimum Thus a large current flows through transponder resulting in an abrupt drop in the voltage Figure 9 20 resonant circuit on tag capacitor detector Lo O O gt magnetic coupling f tag frequency Figure 9 Resonant circuit Dobkin 2008 12 A high frequency oscillating current flows through the reader producing a high frequency magnetic field According to Faraday s law of induction once the tag coil enters this field it can pick up most of the energy and generate an induced current Since both the coil and capacitance have impedan
36. t passive RFID tag Appendix 1 Aset of antennas ALCR Hifester A network analyzer The one bit passive tag was acquired from a shop The antennas Figure 23 were made by my tutor and me We used some wires wrapped over two cartons to make the antennas Although quite simple these could perfectly simulate the real antennas in the shop I used LCR HiTester to measure the capacitance and inductance of the tag and a Network analyzer to observe the spectrum of the signal between two antennas 34 Figure 23 The antennas 4 1 3 The process of the experiment At the first of all I opened the tag to check its physical structure Figure 24 shows the components inside the one bit tag As we can see from the picture the structure was quite simple There was no microchip in the tag The circuitry only consisted of an inductor coil serially connected with a capacitor This looked like the structure of resonant circuit as I mentioned in Chapter 3 4 so I supposed that the one bit tag worked based on resonant wireless power transfer 35 Figure 24 The components inside the one bit tag Secondly in order to verify my assumption I used LCR HiTester to measure the capacitance and inductance so that I could figure out the resonant frequency Figure 25 Since the frequency determines the value of capacitance and inductance I measured them at different frequencies from 1 kHz to 4 5 MHZ the maximum of the HiTester The result is s
37. to hit 5 35 billion referred 26 3 2010 Available in www format lt URL http www logisticsmgmt com article 453054 Logistics_technology Overall RFID market_to_hit_5_35_billion php gt 53 Tag Class Definitions 2007 EPCglobal referred 3 4 2010 Available in pdf format lt URL http www epcglobalinc org standards TagClassDefinitions_1_0 whitepaper 2007110 1 pdf gt UHF Class 1 Gen 2 Standard v 1 2 0 2008 EPC global referred 3 4 2010 Available in pdf format lt URL http www epcelobalinc org standards uhfc1g2 uhfc1g2_1_2 0 standard 20080511 p df gt Sorour Waleed 2009a RFID system components referred 29 3 2010 Available in www format lt URL http www rfidinregion com how rfid works 54 articles 8 1 rfid system components gt Sorour Waleed 2009b RFID tags referred 29 3 2010 Available in www format lt URL http www rfidinregion com how rfid works 54 articles 84 rfid tags gt Sorour Waleed 2009c RFID Software Middleware referred 29 3 2010 Available in www format lt URL http www rfidinregion com how rfid works 54 articles 86 rfid softwaremiddleware gt APPENDICES Appendix 1 One bit passive RFID tag 2010 04 14 Appendix 2 Indala LF RFID reader Appendix 3 Vilant UHF RFID reader antenna Appendix 4 Vilant UHF RFID reader
38. to the wavelength This makes them to be very inefficient far field radiators Another reason is security We all hope that a person can only open the door when he is near the door rather than when he is still 10 meters away from the door 5 2 My opinion and future development Through the experiments I learnt the working principle of the RFID systems and what happened in the physical layer both in time domain and frequency domain I also figured out how the readers transmit energy to the tags Ihave a thought that we can implement the working principle of wireless power in other applications Since tags can get power from the reader other electric stuff should also be able to get power from a wireless power source If we are able to install a receiver in a mobile phone it will charge by itself when entering a certain power supply area We can use the wireless power to replace the traditional wire power by installing a wireless power transmitter at home and a receiver to every electric product Actually wireless power has already been implemented in some fields of our life The electric toothbrush is a good example Because the head of the brush is often dipped 50 into water it is unsafe to use wire power Thus the engineers use two coils to transmit the power It works based on inductive coupling The motor in the head can power from the batteries at the end wirelessly like a RFID tag getting power from its reader Howeve
39. ts of the experiment sese eee eee eee eee 43 4 3 The ultra high frequency RFID energy transfer sese eee eee eee 44 Al Theam OF the Experiment 44 4 3 2 The equipment of the experiment sese NN ads 44 4 3 3 The process of the experiment 0 fon eee eee 44 4 3 4 The results of the experiment sese eee eee eee eee 48 5 CONCLUSION AND FUTURE DEVLOPMENT eee 48 S WG OVE STOW A a e E A A A AE etd a an RE ns 48 5 2 My opinion and future development sss sese sese eee eee 49 REPERENCE Sit ae a EE 51 LIST OF FIGURES AND TABLES Figure 1 The components of RFID system sss nn et ne nets 4 Figure 2 Options for Tag Power Transmit Configuration sss cee ee sese eee eee eee eee 5 Fig re 3 RFID Midd lew ates ee nee a 6 Table 1 RFID operating frequencies and associated characteristics sese ee ee eee e 8 Figure 4 Transmission line terminated With Zo sees sese eee 14 Figure 5 Transmission line terminated with short Open sse sees eee eee eee 15 Figure 6 Transmission line terminated with a resistance other than the characteristic o TTT 16 Figure 7 Elements of simple backscatter modulation 17 Figure 8 Inductive coupling power Supply sees sees eee ee ee eee eee 18 Figure 9 Resonant TSH AR AG th Paces ne SA PT A nt ee 20 Figure 10 Simple rectifier circuit sese eee diia 21 Figure 11 Diode s current voltage characteristic sese sese eee eee 22 Figure 12 The result of a sine wave passing through a diode oe s
40. uency domain The spectrum of signals modulated by ASK consists of three parts the carrier the upper sideband and the lower sideband The carrier locates in the center of the spectrum and two sidebands locate symmetrically above and below the center The frequency difference between the center and the sidebands indicates the frequency of the modulating signal For example if the frequency of the carrier is f and that of the modulating signal is fo then the sidebands locate in f fp and f fo The spectrum is shown in Figure 16 26 Carrier Amplitude Lpper sideband x Lower sideband Frequency Figure 16 The spectrum of ASK 3 5 2 Frequency shift keying FSK changes the frequency of the carrier corresponding to the data sequence The carrier keeps its amplitude and phase constant but it has different frequency for different bits The formula of FSK is A sin 2zf t for bit 1 la sin 2rft for bit 0 Formula 0 241 Ac is the amplitude of the carrier and f and f 2 are two different frequencies The time domain of FSK is illustrated in Figure 17 27 l 1 0 l 0 Modulating frequency 3 2 1 0 l 2 3 Carrier frequency 0 l 2 3 4 5 6 7 8 9 10 11 12 Output frequency Figure 17 Frequency shift keying Juutilainen 2010 3 5 3 Phase shift keying PSK varies the phase of the carrier in accordance with the data sequence The difference between the phases can be 180 or an
41. vailable in www format lt URL http en wikipedia org wiki Capacitive_coupling gt 52 Communication method line codes referred 24 4 2010 Available in www format lt URL http en wikibooks org wiki Communication_Systems Line_Codes gt Electromagnetic field referred 7 4 2010 Available in www format lt URL http en wikipedia org wiki Electromagnetic_field gt IEE 2005 Radio Frequency Identification Device Technology RFID Factfile The Institution of Electrical Engineers Available in www format lt URL http www iee org Policy sectorpanels control rfid cfm gt Inductive coupling referred 18 4 2010 Available in www format lt URL http en wikipedia org wiki Inductive_coupling gt Ward Matt and Kranenburg Rob van 2006 RFID Frequency standards adoption and innovation Department of Design Goldsmiths College University of London referred 30 3 2010 Available in pdf format lt URL http www rfidconsultation eu docs ficheiros TS W0602 pdf gt Network Analyzer Basics 2005 Agilent Technologies referred 16 4 2010 Available in pdf format Physics of RFID referred 19 4 2010 Available in www format lt URL http www rfidmetaltag com physics of rfid gt Radio frequency identification referred 26 3 2010 Available in www format lt URL http en wikipedia org wiki Radio frequency_identification gt Specter Sara Pearson 2010 Logistics technology Overall RFID market
42. ve 3 kHz By systematically changing the amplitude frequency or 13 phase of the wave we can insert information into it When the wave hits an electric conductor it will induce a varying current which can be detected and transformed into an image sound or other meaningful signals When not propagating in free space electromagnetic waves behave like light Actually light is a kind of electromagnetic wave Reflection refraction and diffraction often occur during the propagation conductive medium is like a mirror When electromagnetic waves meet it it will reflect the waves back and change their phase as well Meanwhile some waves can pass through the mirror but propagate in another direction which is called refraction Diffraction occurs when waves bend around an obstacle Reflection is crucial to the RFID system The reflection in the RFID system works like in a transmission line A transmission line is a type of cable whose length is longer than or approximately equals to the wavelength of the RF signal passing through it Each transmission line has its own characteristic impedance Zo whose value depends only on the property of the line When a transmission line is terminated with load Zo all the incident power will be absorbed by the load Therefore there will be no energy reflected back so the envelope of the RF signals will remain constant For reflection a transmission line terminated in Zo behaves like an infi
43. ve tags with the following anticipated features above and beyond those of Class 1 tags An extended tag ID Extended user memory Authenticated access control Additional features TBD as will be defined in the Class 2 specification Class 3 Battery Assisted Passive Tags called Semi Passive Tags in UHF Gen2 Passive Tags with the following anticipated features above and beyond those of Class 2 Tags lt gt lt gt A power source that may supply power to the tag and or to its sensors Sensors with optional data logging O Class 4 Active Tags Active Tags with the following anticipated features An electronic product code EPC identifier An extended tag ID Authenticated access control A power source Communications via an autonomous transmitter Optional user memory Optional sensors with or without data logging Because I focus on the Class 1 tags in this thesis I will introduce the Class 1 Gen 2 UHF RFID protocols in detail This protocol is designed for communications at 860 MHz 960 MHz The bit rate shall be 26 7 kbps 128 kbps in both the reader to tag and tag to reader communication Interrogators shall use DSB ASK SSB ASK or PR ASK modulation 12 and tags shall be able to demodulate all these three modulation types Data shall be encoded by pulse interval encoding Tags communicate information by backscatter modulating the amplitude and or phase of the RF car
44. xperiment The low frequency LF RFID system refers to the RFID systems that operating below 300 kHz but above 30 kHz This type of system is mainly used in animal identification 40 car immobilizers and the access control on doors This experiment is targeted at measuring the operating frequency of a low frequency RFID system and how much energy the tag antennas can get from the reader 4 2 2 The equipment of the experiment The equipments of the experiment included the following O An Indala Low frequency RFID reader Appendix 2 O Atag antenna e A oscilloscope used the RFID reader on the door as the measuring object Though I didn t know its operating frequency at first I supposed it worked at LF band as most of the access control readers are LF systems It was very difficult to use the real tag antenna since it was too tiny so I used a cable to replace it I connected the cable s positive and negative ends together to make a sing turn coil antenna The real antenna is similar to this but has more turns The oscilloscope was used to show the waveform of the signal and measure its magnitude 4 2 3 The process of the experiment Firstly I connected the antenna to the CH1 of the oscilloscope After that I held the antenna and moved approaching the reader Figure 29 The antenna was able to pick up the signal when it was about 5 centimeters away from the reader I put the Autoset button on the oscilloscope to ma
45. y other degree The smaller the difference is the more symbol states we have which means higher efficiency but if the difference is too small it will be difficult to distinguish them from each other so we need to find a trade off The following formula and Figure 18 shows a very simple A sin 2xf t for bit 1 e la sin 2rf t n for bit 0 onu 3 38 28 Modulating frequency 3 Carrier frequency 0 1 2 3 4 5 6 7 8 9 10 11 12 a as R D D D a 1 Output frequency Figure 18 Phase shift keying Juutilainen 2010 We usually don t implement PSK solely but it is combined with other modulation methods For example the quadrature amplitude modulation is a combination of ASK and PSK It changes both the amplitude and phase of the carrier It is very widely used in wireless communication but not in the RFID tags because it is very complicated to implement 3 6 Encoding There is a drawback of OOK As I mentioned before ASK uses nothing to represent bit 0 This is unacceptable in the passive RFID system since the power of the passive tags comes from the reader If the reader is sending a signal containing a long string of 0 the tag will receive no power during that time which may cause the microchip in the tag to be power off One of the solutions to this problem is to encode the data 29 3 6 1 Types of encoding There are several ways to encode the data Each has its own merits as we
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