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 energyin 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

LC circuit oscillator

• The energy in the coil can is harvested using diodes and capacitors (C2) to rectify the current.

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.