Antenna For Rfid

RFID SYSTEM & ANTENNA AT 13.56MHZ Inderpal kaur* and Rajan Kumar** *Principal Design Engineer, CDAC-Mohali **Electronics and Product Design, PEC, Chandigarh, India

Abstract—RFID is an emerging AIDC technique which is finding more and more applications. The complete layer model approach for implementing a RFID system is explained in this paper. Antenna design parameters, especially at 13.56 MHz are discussed with simulation results. In this paper we present the implementation of RFID system as a single unit, discuss the hardware requirement and the various standards assigned for implementation RFID. Also, the advantages of 13.56MHz as compared to low frequency as well as the UHF range are defined. Keywords: RFID, TAG, Transponder, Reader, Interrogator, antenna, K, Q,ISO 15693.ISO14443, read range. INTRODUCTION Automatic Identification and Data Capture (AIDC) is a method of identification of physical objects without the use of manual intervention. The benefits of eliminating manual intervention are eliminating errors and Accelerate throughput. The RFID system consists of readers/interrogators, tags/transponders, and an information managing host computer. This paper focuses on the 13.56MHz inductive passive RFID systems, one of the mainstream RFID products, since this frequency is better able to penetrate nonmetal substrates than UHF and microwave bands due its longer wavelength. I.

RFID SYSTEMS As shown in the figure (1) below, the reader and transponders are in a masterslave relationship where the reader acts as a master and the transponders are II.

slaves. RFID readers themselves are in a slave position as well. A software application that performs the reading of data from the RFID reader acts as the master unit and sends commands to the reader. In a hierarchical system structure the application software represents the master while the reader is a slave.

Figure 1: Block Diagram of a RFID system, showing relationship between Reader, Tag and Application unit. Working of the RFID system can be explained with help of layer model as shown in the figure 2. As shown in the figure, the process start from the bottom layer 4 where RFID hardware interacts and generates required data. Layer 4 is discussed in detail in next section. In layer 3 (middleware), this data is picked up from the hardware; filtered; adapted and logged after which the data become usable for processing in the existing enterprise system. This layer acts as a link between the physical events occurring in the hardware layer and existing IT infrastructure on which the required business logic will be executed. Edge processing layer acts like a personal secretary bringing only the points of interest and filtering out operation details. It also provides tools to aggregate and manage EPC data. IT Infrastructure; It is the software (like SAP, Oracle) and hardware used in the enterprise which is responsible for storage, utilization, processing or

communication of the RFID event based data. Business Process Analytics is the layer where this data is utilized to meet the objective for which the RFID system has been deployed, that is, increased visibility.

Figure 2. Layer model displaying the working of the RFID system. RFID HARDWARE Communication between the RFID reader and tags occurs wirelessly and generally does not require a line of sight between the devices. These two important components of RFID system are discussed as follows. A. RFID readers are consisted of three main parts that allow them to function in RF and digital systems. These main three components are as shown in figure 3.
Control section
High frequency interface
Antenna Control unit consist of microprocessor/ microcontroller, memory block, analogto- digital converters and communication unit for software application. It performs the following functions:
It enables communication with the application software and the III.

execution of commands from the application software
Control of the communication with a RFID tag/ transponder. Signal coding and decoding

Figure 3. Block Diagram of RFID Reader/Interrogator. HF interface or RF interface, as shown in the diagram, transmits and receives the signal information from antenna. Its major components are oscillators, filters, envelope detector, directional coupler (at UHF applications), etc. It performs the following functions:
Generation of High Frequency Transmission Power to activate the tag and supply it with power.
Modulation of transmission signal to send data to tag.
Reception and demodulation of HF signals transmitted by tag. Antenna The design and configuration of antenna primarily depends on the frequency of operation, as in this paper we are concentrating around 13.56 Mhz which results in an Inductively coupled RFID systems. The basic requirements for designing the Reader antenna are:
Maximum current in the antenna coil, for the maximum magnetic flux.
Power matching so that the maximum available energy can be used for the generation of the magnetic flux.
Sufficient bandwidth for the undistorted transmission of the carrier signal modulated with the

data- this can be achieved by suitably selecting the Q of the circuit. B. TRANSPONDER / TAG: is located on the object to be identified. Tags can be categorized as:
Active tag, which has a battery that supplies power to all functions;
Semi-passive tag, which has a battery used only to power the tag IC, and not for communication;
Passive tag, which has no battery on it. The main components of a passive transponder are the antenna coil and the microchip. The necessary power required energizing and activating the tag’s microchip or low-power CMOS Integrated circuit is drawn from the localized oscillatory magnetic field created by the reader antenna. FREQUENCY RANGES Since RFID systems generate and radiate electromagnetic waves, they are legally classified as radio systems. The table given below various frequency ranges for RFID systems. Frequency Characteristi Application Band cs s Low (100- short to Access 500 KHz) medium read control, range; Animal Inexpensive; Identificatio low reading n, Inventory speed Control, Car Immobilizer Intermedia Short to Access te (10- medium read Control, 15MHz) range, smart cards Potentialy Inexpensive, medium reading speed High (850- Long read Railroad car 950 Mhz, range, High monitoring, 2.4-5.8 reading Toll GHz) Speed, LOS collection IV.

required, Expensive

systems

The use of 13.56 MHz frequency is proven to be very advantageous over these other bands:
Frequency band available worldwide as an ISM frequency
Excellent Immunity to environmental noise and electrical interference
Minimal shielding effects from adjacent objects and the human body
Freedom from environmental reflections that can plague UHF systems
Good data transfer rate
On-chip capacitors for tuning transponder coil can be easily realized
Cheap ICs, disposable tags
Cost effective antenna coil manufacturing
Low RF power transmission so EM regulation compliance cause no problems ANTENNA PARAMETERS FOR RFID SYSTEMS AT HF The link between tag and reader, in UHF range, is based on Far field or radiated power mechanisms. In UHF range the physical length of the antenna is equal to the quarter or half of the wavelength which results in small form factor. Antenna size at 13.56MHz:At 13.56 MHz we have wavelength of nearly 22.12 meters and hence the physical length (quarter of wavelength) is 5.53 meters. Antenna with such a dimension is not possible. Solution to this problem is “Magnetic coupling”. Here the tag and reader antenna coils act as a loosely coupled transformer. In order to maximize the communication range with the tag, it is necessary to create the strongest possible magnetic field so that V.

the tag will be able to pick up enough power in order to energize itself. Since the magnetic field from the loop is proportional to the current flowing through the conductor that actually constitutes the loop, this current has to be maximized. Secondly, the tag wants to be able to collect in as much energy as possible from the ambient magnetic field generated by the base station loop antenna. This energy gathering capacity must, therefore, be maximized as well. Q-factor of antenna: Higher values of Q affect the data transmission or bandwidth. A tuned circuit acts as a bandpass filter. Enough bandwidth must be left for the subcarrier and its modulation sidebands. ISO15693 standard there is a subcarrier at 423 KHz. Minimum Bandwidth requirements: The graph below represents the tag frequency response for a Q value of 12 and the ISO 15693 single subcarrier/ASK power spectrum for a pseudo random bit sequence.

Figure 5 Tag frequency response for a Q value of 12 and the ISO15693 standard single subcarrier. A set of rules for the minimum bandwidth requirements, as seen from the tag point of view: B = FTOL + FSUB + data rate (for single subcarrier/ASK mode) BW = FTOL + FSUB + data rate (for double subcarrier FSK or BPSK mode) Where: BW is the minimum bandwidth, that is fc/2*Q, fc = 13.56 MHz.

FTOL is the frequency tolerance of the tuned circuit.FSUB is the subcarrier frequency.

The base station antenna point of view is exactly the same as the tag antenna point of view. In fact, the base station antenna must have enough bandwidth to recover the tag modulation. The base station sends commands to the tag by direct modulation of the 13.56 MHz carrier. The protocol does not use a subcarrier for the base station to tag communications. For both ISO standards, the data rates and modulation techniques used yield spectrums that have much smaller bandwidths than the tag. Therefore, for all practical purposes, the minimum system bandwidth requirements are set by the tag modulation spectrum. The coupling Factor (K): The RFID system can be assumed as the circuit given below, a loosely coupled transformer. The minimum bandwidth requirements are valid only if the coupling factor between the tag and the base station antenna is kept low. Because we are dealing with a magnetic coupling problem and know that the magnetic field induced by a coil is proportional to the current flowing through it, we shall visualize this current. Both the antenna and tag are tuned to 13.56 MHz. In the middle, the linear coupling factor k=0.1 is introduced. The Q factor of both devices is equal to 9.

Figure 6 an equivalent tag reader circuit, depicting a loosely coupled transformer

Figure7 below shows the multiple run simulations where coupling factor varies from 1% to 20% in logarithmic scale. This graph shows that if coupling factor is too low (curve with highest pesk) no energy transfer will be possible and the system will not work. Also higher values of coupling factor (curve with two peaks and deepest) creates a communication problem as it represents two different resonant frequencies. The coupling factor can be expressed as

Figure 7Multiple run simulations on varing the coupling factor K from 1% to 20%

From above equation, it is obvious that coupling factor will be maximum when r1=r2. Thus careful selection of coupling factor is must for proper operation of RFID systems at HF range. VI.

CONCLUSION

RFID covers a vast area of applications, from animal identification to rail- road monitoring. All major players of semiconductor industry deal in RFID techniques and provide technical support, in the form of datasheets, application notes and technical forums. Currently, Ford motor company uses RFID in engine assembly plant; Walmart uses tamper proof RFID tags; Nokia, Xerox, Pfizer and many more, implementing this technique and getting good results. Also design considerations of RFID antenna at different frequency ranges is a vast area of research. Highly

efficient and low power consumption RFID antennas are big in demand. REFERENCES

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