Transcript Inductive Coupling

Overview of RFID System
Characteristics
•
•
•
•
•
•
•
Operating Frequency
Method of Coupling
Transmission Range
Data Storage Capacity
Power Supply (Active, Passive)
Read Only / Read-Write
Transmission Types (HDX, FDX, SEQ)
Method of Coupling
• All RFID systems have two basic ways of
exchanging information:
(1) inductive coupling 
or
(2) electromagnetic backscatter
Inductive Coupling
• Operating Frequency: 13,56 MHz (HF)
– Also 135 kHz
• Transmission Range: <1 meter
• Data Storage Capacity: up to 100 KB
– EEPROM memory
• Power Supply: Passive
• Both Read Only and Read-Write
• Transmission Types: HDX, FDX, SEQ
Inductive Coupling
Inductive coupling means that the transponder and the
antenna are coupled by the magnetic flux through both
coils, much like a transformer. All the energy used in the
tag is drawn from the primary coil of the antenna.
Inductance
• Electromagnetic induction is the production of
voltage across a conductor situated in a
changing magnetic flux.
• Faraday found that the voltage produced around
a closed path conductor is proportional to the
rate of change of the magnetic flux through any
surface bounded by that path.
Faraday's law:
Oscillator
• A capacitor and an inductor connected together
form an oscillator. Both capacitors and inductors
store energy.
A capacitor
stores energy
in the form of
an electrostatic field
an inductor
stores energy
in a magnetic field
Once the capacitor is charged:
•The capacitor will start to discharge through the inductor. As it does,
the inductor will create a magnetic field.
•Once the capacitor discharges, the inductor will try to keep the current
in the circuit moving, so it will charge up the other plate of the capacitor.
•Once the inductor's field collapses, the capacitor has been recharged
(but with the opposite polarity), so it discharges again through the inductor.
Resonators
• The antenna coil of the transponder and the
capacitor form a resonant circuit tuned to the
transmission frequency of the reader. The voltage U
at the transponder coil reaches a maximum due to
resonance in the circuit.
– This is way a radio receiver works
• The efficiency of power transfer between the antenna
coil of the reader and the transponder is proportional
to:
– the operating frequency f
– the number of windings n
– the area A enclosed by the transponder coil
– the angle of the two coils relative to each other
– the distance between the two coils.
The rest of the picture
• The energy in the coil can is harvested
using diodes and capacitors (C2) to rectify
the current.
LC circuit
oscillator
Method of Coupling
• All RFID systems have two basic ways of
exchanging information:
(1) inductive coupling
or
(2) electromagnetic backscatter 
Backscatter Coupling
• Operating Frequency: 915MHz (UHF), 2.5 GHz
& 5.8 GHz (Microwave)
– Also 868MHz (Europe)
•
•
•
•
•
Transmission Range: >1 meter
Data Storage Capacity: high bandwidth
Power Supply: passive, semi-passive, active
Both read only and read-write, etc
Transmission Types: HDX, FDX, SEQ
What is backscatter
• Electromagnetic backscatter is quite similar to
radars.
• Depending on its characteristics, an antenna
reflects part of an incoming electromagnetic
wave back to the sender.
• Electromagnetic wave are reflected by most
objects that are larger than half the wavelength.
• The efficiency of reflection is particularly large
for antennas that are in resonance with the
incoming waves.
– The short wavelengths of UHF facilitate the
construction of antennas with smaller dimensions and
greater efficiency.
Far field not near field
• Opposite to inductive coupling, electromagnetic
backscatter works beyond the near field.
• The energy available at the transponder is found
by calculating the free space path loss aF
between the reader and the transponder.
aF = −147.6 + 20 log(r) + 20 log(f ) − 10 log(GT) − 10 log(GR)
How it works
• The power reflected from the
transponder is radiated into
free space. A small proportion
of this (free space attenuation)
is picked up by the reader’s
antenna.
• The reflected signal travels
into the antenna connection of
the reader in the backwards
direction and can be
decoupled using a directional
coupler.