Classification Of Rfid Systems

Classification of RFID systems Prof. Robert Morelos-Zaragoza Department of Electrical Engineering San Jose State University Fall 2007

Characteristics of RFID systems • Operating frequencies – Inductive coupling: 100 KHz to 30 MHz – Antenna coupling: 2.45 GHz to 5.8 GHz

• Range – Fundamental factors affecting range are: • Spatial accuracy of the reader • Minimum distance between readers • Speed of reader in interrogation zone

• Modulation type • Security – Industrial (closed) applications – Public applications Fall 2007

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A classification of RFID systems [1] Radio frequency I. 1-bit (EAS)

Microwaves Frequency divider

RFID systems

Electromagnetic Inductive coupling Full- and half-duplex

Backscatter Close coupling

II. n-bit (memory)

Electrical coupling

Sequential

Inductive coupling SAW

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I. 1-bit (EAS) RFID systems • A tag stores one bit of information: – “Tag in interrogation zone” (1) – “Tag NOT in interrogation zone” (0) • Application: Electronic Anti-theft Surveillance (EAS) in shops • Components – Reader and (optional) detector – Tag (or security element) – Deactivation device (optional) Fall 2007

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I.1 EAS using Radio Frequency • Based on LC resonant circuits at frequency fR • Tag is an LC circuit with a foil capacitor (which can be destroyed with strong magnetic field) • The reader generates an alternating magnetic field with (sweeping) frequency fG • Proximity of the tag causes a sympathetic oscillation that reduces the voltage (or impedance) across the generator • Relative magnitude of this “dip” depends on distance and Q factor Fall 2007

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Operating principle of EAS-RF Alternating Magnetic Field

fG

Generator coil

Sensor coil

Transmitter

EAS tag Feedback

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UHF

Receiver (optional)

Feedback

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I.2 EAS using microwaves • Exploit the harmonics produced by nonlinear devices (e.g., diodes) • The n-th harmonic (typically n=2) is detected • To avoid false alarms, transmitter sends a modulated signal • The tag uses capacitance diode to produce and regenerate the n-th harmonic • Tags cannot be destroyed

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Example of EAS-µWave system

Alarm

Demodulator

Receiver

Modulator

4.90 GHz (2nd harmonic)

2.45 GHz

Oscillator (1 KHz)

Transmitter

Antenna

Capacitance diode

1-bit tag

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I.3 EAS using frequency divider • Operating band: 100 KHz to 135.5 KHZ • The reader sends a magnetic field at frequency fG • The tag uses a frequency divider to produce a magnetic field at a frequency fG/2 • Tags cannot be destroyed

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Example of EAS - frequency divider Power, clock fG Tag Reader

fG

C1

C2

Divide by 2 fG/2

fG/2 detector Clock fG/2

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I.3 EAS – Electromagnetic type • Operating band: 10 KHz to 20KHz • Idea: Use strong magnetic fields in the near field (NF) range • Hysteresis curve (relation between magnetic field strength H and magnetic flux B) of soft (low Br) magnetic amorphous metal strip used • Harmonics at the base frequency are generated by the nonlinear relation between B and H and detected by the reader • Tags are available as self-adhesive strips of lengths from 2 cm to 20 cm. • Tags can be activated and reactivated any number of times. Main application: Libraries Fall 2007

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Typical hysteresis curve B Br Saturation Virgin curve Hc

H

Br: Remanence (flux density at null field strength) Hc: Coercive field strength

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Harmonics in electromagnetic type EAS systems: An example • Given a main signal of frequency f0=20 Hz and two signals at f1=3.5 KHz and f2=5.3 KHz, signals at the following frequencies are generated: q f1+f2=8.8 KHz q f1-f2 =1.8 KHz q f0+f1=3.52 KHz … and so on. The reader is designed to detect the signal at frequency f1+f2 only.

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Typical reader antenna and tag in an electromagnetic EAS system

Coil

Column

EAS – Electromagnetic Antenna

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EAS – Electromagnetic Tag

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II. n-bit RFID systems • Tags use an electronic microchip as a data-carrying device • Transfer of data (communication) between reader and tag is thus needed • Data transfer procedures – Full (FDX) and half (HDX) duplex procedures • Transfer of energy from reader to tag is continuous and independent of data flow – Sequential (SEQ) procedures • Transfer of energy from reader to tag takes place for a limited period of time (power-supply pulses) • Data transfer from tag to reader takes place between power-supply pulses Fall 2007

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II.1 Inductive coupling RFID • Power supply to an inductively coupled tag from the energy of the magnetic field generated by the reader:

Reader

Tag

fG

Cr

C1

C2

Chip

Ri

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Load modulation with subcarrier • Generation of load modulation in the tag by switching drain-source resistance of an FET

Reader

Tag

fG

Cr

C1

Chip

C2 FET

Ri

Binary signal

BPF Subcarrier

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DEMOD

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Spectrum of load modulation 0 dB

Carrier signal of the reader measured at the antenna coil Intermodulation products produced by load modulation with subcarrier

-80 dB

13.348 MHz

fT=13.56 MHz

13.772 MHz

fS=212 KHz

• Applicable to ISM bands: 6.78 MHz, 13.56 MHz and 27.125 MHz

More on this when we discuss modulation techniques Fall 2007

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II.2 Backscatter coupling RFID • Refers to RFID systems in which the spacing between reader and tag of at least 1 m (long range systems) • To estimate power supply at tag, the free space path loss αF is needed (in dB): αF = -147.6 + 20 log(r) + 20 log(f) – 10 log(GT) – 10 log(GR) Free path loss at different frequencies with GT=1.64 (dipole) and GR=1: Distance, r (m) 0.3 1.0 3.0 10.0 Fall 2007

868 MHz 915 MHz 2.45 GHz 18.6 29.0 38.6 49.0 RFID - SJSU

19.0 29.5 39.0 49.5

27.6 38.0 47.6 58.0 19

Modulated reflection cross-section Reader

P1

P’1= P1/ αF

Tag

Directional coupler Rx

C1

Tx

C2

Chip

RL

Transceiver

Data P’2= P2/ αF

P2

P1: Power emitted by reader P2: Power reflected by tag

Data transmission from tag to reader is achieved by switching on and off in time a load resistor RL connected in parallel with the antenna Fall 2007

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Reader-to-tag data transfer • All known digital modulation procedures can be used • There are three basic modulation formats used in RFID – Amplitude-shift keying (ASK) – Frequency-shift keying (FSK) – Phase-shift keying (PSK) • Due to its simplicity in demodulation (at the tag), most systems use ASK modulation

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II.3 Inductive coupling SEQ RFID • Operating frequencies: Up to 135 KHz • Tag frequency must be matched to that of the reader – Tags contain a so-called trimming capacitor • Reader does not transmit continuously • Energy transferred to tag is stored in a charging capacitor of value

Q It , C= = V Vmax − Vmin Q=It is the charge, Vmin and Vmax are limit operating voltages of the chip in the tag, I is the consumption current (note typo in Ref. [1]) and t is the time required for transmission of data from tag to reader Fall 2007

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A comparison between FDX/HDX and SEQ RFID systems • FDX/HDX – Power matching is needed, as power is both harvested and consumed by the tag • SEQ – Voltage matching needed by tag capacitor SEQ voltage matching Voltage

FDX/HDX power matching Power

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HIGH

Tag load impedance

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Typical capacitor voltage signal in an SEQ RFID tag Charging phase Reading phase Discharging phase

Vmax Vmin

t

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RFID reader architectures • Irrespective of the type of RFID system, readers typically use a quadrature demodulator • Four possible architectures: – Super-heterodyne – Direct-conversion (Homodyne) – Low-IF – Bandpass sampling More on this later in the course

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An example of a reader using a direct-conversion architecture Rx +7dBm leak & -80dBm tag

Receive path

I ×

A/D

LNA

To antenna

×

Comp max +7dBm

PA

I

D/A

×

D/A

Q Transmit path

I

2x

D/A

×

Σ Q

FPGA card

Host computer

×

Σ

Tx +25dBm

PA

A/D

2x

Compensate path

Circulator

Q

D/A

×

2x

1.2 GHz synthesizer

Ref: MIMOSA project, STMicroelectronics . See also www.mimosa-fp6.com Fall 2007

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Reference 1. Finkenzeller, RFID Handbook: Fundamentals and Applications, 2nd ed., Wiley 2003. (Chapter 3)

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