Classification of RFID Systems

Classification of RFID systems

Prof. Robert Morelos-ZaragozaDepartment of Electrical Engineering

San Jose State University

Fall 2007

Fall 2007 RFID - SJSU 2

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


A classification of RFID systems [1]

RFIDsystems

I. 1-bit(EAS)

II. n-bit(memory)

Radio frequency

Microwaves

Frequency divider

Electromagnetic

Full- andhalf-duplex

Inductive coupling

Backscatter

Close coupling

Electrical coupling

SequentialInductive coupling

SAW


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)


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


Operating principle of EAS-RF

fG

Transmitter EAS tag

UHF

Generatorcoil

Sensorcoil

Receiver (optional)

Alternating Magnetic Field

Feedback Feedback


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


Example of EAS-µWave system

Modulator Oscillator(1 KHz)

2.45 GHz

Capacitance diode

Antenna

Transmitter

1-bit tag

4.90 GHz(2nd harmonic)

Demodulator

Receiver

Alarm


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


Example of EAS - frequency divider

Divideby 2C1 C2

fG

fG/2

fG/2detector

Reader

Tag

Power, clock fG

Clock fG/2


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


Typical hysteresis curve

B

H

Virgincurve

Saturation

Br

Hc

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


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.


Typical reader antenna and tag in an electromagnetic EAS system

Coil

Column

EAS – ElectromagneticAntenna

EAS – ElectromagneticTag


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


II.1 Inductive coupling RFID

ChipC1 C2fG

Reader Tag

Ri

Cr

• Power supply to an inductively coupled tag from the energy of the magnetic field generated by the reader:


Load modulation with subcarrier

ChipC1 C2fG

Reader Tag

Ri

Cr

BPF

DEMOD

Binary signal

FET

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

Subcarrier


Spectrum of load modulation

0 dB

-80 dB

Carrier signal of the readermeasured at the antenna coil

Intermodulation products produced by load modulation with subcarrier

fT=13.56 MHz 13.772 MHz13.348 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


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 pathloss αF is needed (in dB):

Distance, r (m) 868 MHz 915 MHz 2.45 GHz

0.3 18.6 19.0 27.61.0 29.0 29.5 38.03.0 38.6 39.0 47.6

10.0 49.0 49.5 58.0

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:


Modulated reflection cross-section

ChipC1 C2

Reader Tag

RLTx

Rx

Transceiver

Directionalcoupler

P1 P’1= P1/ 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 withthe antenna

Data

P’2= P2/ αF


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


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 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

,minmax VV

ItVQ

C−

==


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

Tag loadimpedance

Power

FDX/HDXpower matching

SEQvoltage matching

LOW MEDIUM HIGH

Voltage


Typical capacitor voltage signal in an SEQ RFID tag

t

Chargingphase

Readingphase

Dischargingphase

Vmax

Vmin


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


An example of a reader using a direct-conversion architecture

LNA×

×

A/D

A/D

D/A

D/A

D/A

D/A

I

Q

×

×Σ

1.2 GHzsynthesizer

PA

PA

I

Q

×

×Σ

I

Q

FPGA

card

Host

computer

Transmit path

Compensate path

Receive path

Compmax +7dBm

Tx+25dBm

Rx+7dBm leak &-80dBm tag

2x

2x

2x

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

Circulator

Toantenna


Reference

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

Documents

Classification of RFID Systems