classification of rfid systems
TRANSCRIPT
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
Fall 2007 RFID - SJSU 3
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
Fall 2007 RFID - SJSU 4
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 RFID - SJSU 5
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 RFID - SJSU 6
Operating principle of EAS-RF
fG
Transmitter EAS tag
UHF
Generatorcoil
Sensorcoil
Receiver (optional)
Alternating Magnetic Field
Feedback Feedback
Fall 2007 RFID - SJSU 7
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
Fall 2007 RFID - SJSU 8
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
Fall 2007 RFID - SJSU 9
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
Fall 2007 RFID - SJSU 10
Example of EAS - frequency divider
Divideby 2C1 C2
fG
fG/2
fG/2detector
Reader
Tag
Power, clock fG
Clock fG/2
Fall 2007 RFID - SJSU 11
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 RFID - SJSU 12
Typical hysteresis curve
B
H
Virgincurve
Saturation
Br
Hc
Br: Remanence (flux density at null field strength)Hc: Coercive field strength
Fall 2007 RFID - SJSU 13
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.
Fall 2007 RFID - SJSU 14
Typical reader antenna and tag in an electromagnetic EAS system
Coil
Column
EAS – ElectromagneticAntenna
EAS – ElectromagneticTag
Fall 2007 RFID - SJSU 15
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 RFID - SJSU 16
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:
Fall 2007 RFID - SJSU 17
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
Fall 2007 RFID - SJSU 18
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
Fall 2007 RFID - SJSU 19
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:
Fall 2007 RFID - SJSU 20
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
Fall 2007 RFID - SJSU 21
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
Fall 2007 RFID - SJSU 22
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−
==
Fall 2007 RFID - SJSU 23
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
Fall 2007 RFID - SJSU 24
Typical capacitor voltage signal in an SEQ RFID tag
t
Chargingphase
Readingphase
Dischargingphase
Vmax
Vmin
Fall 2007 RFID - SJSU 25
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
Fall 2007 RFID - SJSU 26
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
Fall 2007 RFID - SJSU 27
Reference
1. Finkenzeller, RFID Handbook: Fundamentals and Applications, 2nd ed., Wiley 2003. (Chapter 3)