RFID Tags

By: Zeina Muallem

1-Bit Transponder1-bit Transponder- has 2 states : 1 and 0- Corresponds to: “transponder in interrogation zone” and “no transponder in interrogation zone”- Applied in EAS: electronic article surveillance (electronic anti-theft devices in shops)

EAS system made up of:-antenna of reader (interrogator), security element (tag), optional deactivation device

Main Performance CharacteristicRecognition or detection rate(maximum distance between transponder and interrogator antenna)

RFID Systems

Radio FrequencyRF procedure- LC circuits adjusted to Fr (resonant frequency)- Reader generates magnetic alternating (MAF) field in RF range- LC resonant circuit moved close to MAF If f of MAF corresponds with Fr of LC circuit (in the transponder) transponder oscillates sweep signal

produces a dip in voltages at generator and sensor coils

EAS

Microwaves

EAS systems in microwave range:- Generation of harmonics at components with nonlinear characteristic lines (diodes)- 1-bit transponder: a capacitance diode connected to base of dipoleadjusted to carrier wave (frequencies 915 MHz, 2.45 GHz, 5.6 GHz)If transponder located within transmitter’s range diode generates current re-emits harmonics of carrier signals at 2 or 3 times the carrier wave obtained e.g. second harmonic re-transmitted detected by receiver

Microwave Tag

Frequency DividerFrequency divider- Operates in 100-135.5kHz- Security tag (transponder): microchip-resonant circuit

Resonant circuit resonates at operating frequency of EAS systemMicrochip- Receives power supply from MF of security device

freq. of self-inductive coil divided by two by microchip sent back to security device signal fed back to resonant circuit

Frequency Divider (EAS)

Full and Half DuplexHalf Duplex- Data transfer from transponder to reader alternates with data transfer from reader to transponder- Load modulation procedure: below 30 MHz-with or without sub-carrierFull Duplex- Data transfer from transponder to reader at same time with data transfer from reader to transponder- Data transmission from transponder at sub-harmonic or an-harmonic freq. of reader Transfer of energy from reader to transponder is continuous ≠ SEQ (sequential systems)

Full and Half Duplex

Inductive couplingInductively coupled transponders- contains a single microchip and large area coil (antenna)- microchip operates passively energy supplied by reader- Reader antenna coil generates high freq. EF penetrates cross-section and area of coil voltage generated in transponder antenna coil by inductance voltage rectified serves as power supply to microchip- Antenna coil of transponder and capacitor form a resonant circuit

Tuned to transmission freq. of readerTwo coils form a transformer

Load ModulationData transfer transponder reader- Transformer type coupling (primary coil: reader-secondary coil :transponder)- Transponder in near-field of readerIf transponder placed close to MAF reader

Transponder draws energy from the MF Load resistor switched on/off (transponder)

change in reader voltage Amplitude Modulationdata transfer when: timing where load resistor switched on/off is controlled by data

Load Modulation

With Sub-carrierLoad modulation with sub-carrier- ISM freq. ranges: 6.78 MHz, 13.56MHz, 27.125MHz- voltage at antenna of reader (useful signal)<<output voltage of readere.g. 100V 10mV useful signalRequires modulation sidebands created by AM:- Two modulation sidebands at the reader antenna - At a distance of the sub-carrier frequency around freq. Freader- Separated from stronger signal of reader by BP filtering on freader±fsamplified demodulation

Example CircuitLoad modulation with sub-carrier- Operating freq. 13.56MHz generates sub-carrier of 212kHz- voltage induced at coil L1 by MAF (reader) is rectified using bridge rectifier (D1-D4) - C1 does smoothing available as supply voltage- regulator (ZD 5V6) regulates the supply voltage prevents uncontrolled increase- high freq. antenna volt. (13.56MHz) travels to CLK via R1 internal clocking signal for transponder- Division by 64 a sub-carrier of 212kHz at Q7 controlled by DATApassed to switch T1 load resistor R2 switched on/off with sub-carrier freq.

With Subcarrier

EM backscatter couplingPower supply to transponder-Long range systems: distance between reader-transponder >1m- operates at UHF frequencies 868MHz (Europe), 915MHz (USA), 2.5GHz-5.8GHz

antennas with smaller dimensionsBackscatter transponders- long ranges up to 15m- greater power consumption- have a backup battery to supply power to transponder chip- transponder out of range of reader has a power saving “power down”- “stand-by” mode- battery supplies power for microchip onlyData transmission: power of EMF emitted by reader

Close CouplingPower supply to transponder- for ranges of 0.1cm-1cm transponder inserted or placed into the readerFunctional Layout- Transformer : reader represents primary winding and transponder coil secondary winding- high freq. alternating current generates high freq. magnetic field in transponder coil power rectified provides power supply to the chip- frequencies in range 1-10 MHz

Close Coupling

Close CouplingData transfer transponder readerMagnetic coupling- load modulation with sub-carrier used- used in close coupling chip cardsCapacitive coupling- plate capacitors arranged in transponder and reader // when transponder inserted in reader- used in close coupling smart cards

Capacitive Coupling

Electrical couplingPower supply of passive transponders- reader antenna made of electrode generates a high freq. electrical field- transponder made of 2 conductive surfaces- transponder placed within EF of reader electric voltage between 2 electrodes supply power to transponder chips

E.C. System

Electrical couplingData transfer reader transponderThree basic procedures:- ASK (most preferable due to simplicity in demodulation)- FSK- PSK

Sequential proceduresSEQ: transmission of data and power from reader data carrier alternates with data transfer from transponder readerComparison between SEQ and HDX/FDX- full source voltage of transponder coil is up to twice that of a HDX/FDX system- energy available to the chip is determined only by capacitance of charging capacitor and the charging period in SEQ- In HDX/FDX max power consumption fixed by coil geometry and field strength H

FDX/HDX and SEQ

Physical principles of RFID systems

Electromagnetic Fields- used in RFID systems- operate at above 30MHzMagnetic Field- conductor antennas generate MAF in the read/write devices of inductively coupled RFID systems- might be too low or high depending on radius of antenna transmitter (reader): If R very big H very lowIn Figure below:Inductively coupled RFID system- L1 transmitter antenna of reader- L2 antenna of transponder-R2 coil resistance- RL current consumption of data memory

Physical principles of RFID systems

H min (interrogation field strength of transponder)- is the minimum field strength (max distance between transponder and reader)- u2 is the high supply voltage for operation of data carrier- RL input resistance of data carrier- After rectification data carrier requires 3-5V constant voltage

Resonant circuit- C2 in // with transponder coil L2- Resonant frequency : f=1/2π√L2 C2- resonant freq. (transponder) matched to transmission freq. (reader)

Operating Principle

Frequency Ranges

RFID Systems- Frequency Ranges: 0-135kHz, ISM

frequencies 6.78MHz-13.56MHz-27.125MHz-40.68MHz-433.92MHz-869MHz-915MHz-2.45GHz-5.8GHz-24.125GHz

Frequency Ranges

Frequency range 9-135kHz- also used by other radio servicesFrequency range 6.78MHz- short wave frequencies- permit short ranges up to few 100km in daytimeFrequency range 13.56MHz- permits transcontinental connectionsFrequency range 27.125MHz- 27MHz RFID systems for industrial applications (like hospitals)Frequency range 40.680MHz- No RFID systems operating in this rangeFrequency range 433.920MHz- applied in Backscatter (RFID) systems

Frequency Ranges

Frequency range 915MHz- Backscatter (RFID) systemsFrequency range 2.45GHz- Backscatter systemsFrequency range 5.8GHz- backscatter systemsFrequency range 24.125GHz- no RFID systems operating

Frequency Ranges

<135kHz- large ranges - low cost transponders- high power to transponder- transponder has low power consumption6.78MHz- low cost transponders- medium speed transponders13.56MHz- high speed/high end applications- medium speed/low end applications27.125MHz- low ranges- large bandwidth very fast data transmission