RFID Tags By: Zeina Muallem
1-Bit Transponder 1-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 Characteristic Recognition or detection rate (maximum distance between transponder and interrogator antenna)
RFID Systems
Radio Frequency RF 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 dipole adjusted 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 Divider Frequency divider - Operates in 100-135.5kHz - Security tag (transponder): microchip-resonant circuit Resonant circuit resonates at operating frequency of EAS system Microchip - 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 Duplex Half 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-carrier Full 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 coupling Inductively 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 reader Two coils form a transformer
Load Modulation Data transfer transponder reader - Transformer type coupling (primary coil: reader-secondary coil :transponder) - Transponder in near-field of reader If transponder placed close to MAF reader Transponder draws energy from the MF Load resistor switched on/off (transponder) change in reader voltage Amplitude Modulation data transfer when: timing where load resistor switched on/off is controlled by data
Load Modulation
With Sub-carrier Load modulation with sub-carrier - ISM freq. ranges: 6.78 MHz, 13.56MHz, 27.125MHz - voltage at antenna of reader (useful signal)<<output voltage of reader e.g. 100V 10mV useful signal Requires 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±fs amplified demodulation
Example Circuit Load 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 DATA passed to switch T1 load resistor R2 switched on/off with subcarrier freq.
With Subcarrier
EM backscatter coupling Power 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 dimensions Backscatter 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 only Data transmission: power of EMF emitted by reader
Close Coupling Power supply to transponder - for ranges of 0.1cm-1cm transponder inserted or placed into the reader Functional 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 Coupling Data transfer transponder reader Magnetic coupling - load modulation with sub-carrier used - used in close coupling chip cards Capacitive coupling - plate capacitors arranged in transponder and reader // when transponder inserted in reader - used in close coupling smart cards
Capacitive Coupling
Electrical coupling Power 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 coupling Data transfer reader transponder Three basic procedures: - ASK (most preferable due to simplicity in demodulation) -FSK - PSK
Sequential procedures SEQ: transmission of data and power from reader data carrier alternates with data transfer from transponder reader Comparison 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 30MHz Magnetic 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 low In 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 services Frequency range 6.78MHz - short wave frequencies - permit short ranges up to few 100km in daytime Frequency range 13.56MHz - permits transcontinental connections Frequency range 27.125MHz - 27MHz RFID systems for industrial applications (like hospitals) Frequency range 40.680MHz - No RFID systems operating in this range Frequency range 433.920MHz - applied in Backscatter (RFID) systems
Frequency Ranges Frequency range 915MHz - Backscatter (RFID) systems Frequency range 2.45GHz - Backscatter systems Frequency range 5.8GHz - backscatter systems Frequency range 24.125GHz - no RFID systems operating
Frequency Ranges <135kHz - large ranges - low cost transponders - high power to transponder - transponder has low power consumption 6.78MHz - low cost transponders - medium speed transponders 13.56MHz - high speed/high end applications - medium speed/low end applications 27.125MHz - low ranges - large bandwidth very fast data transmission