RFID II Inductive and Microwave Systems. 2 Communication Technology Laboratory Wireless...

RFID II

Inductive and Microwave Systems

2Communication Technology LaboratoryWireless Communication Group

Outline

• Inductive Systems– Magnetic Field– Tag-Reader Coupling– Load Modulation

• Microwave Systems– Electromagnetic Waves– Antennas– Electromagnetic Coupling and Backscatter Modulation


– Inductive coupling

Fundamental Operating Principles


Biot-Savart Law

• We consider a current I flowing in an infinitesimally thin conducting loop of arbitrary shape• Solving the line integral along the loop, it is possible to compute the static magnetic field H

at any point in the space• There is no closed form solution for many configurations


Example: Circular Coil

– Magnetic field at center of circular coil– The magnetic field is perpendicular to the surface spanned by

the coil (i.e. HY = HX = 0)

3

2 2

4

2

4 4 2

Z Z Z

Z

I dl RdH dH e e

R

I I R IH dl

R R R

Radius R


Example: Circular Coil

– Magnetic field along radial axis of coil (z-axis)

– For symmetry reasons the dHX and dHY components cancel, when we evaluate the line integral and again only HZ is nonzero

3

sin

4Z

I dl rdH

r

– with and

we obtain

sin /R r 1/22 2r R z

z z


Optimal Radius of Coil at a Given Distance d

– H strength versus distance d and coil radius R

dR

2optR d

Optimal coil radius for given distance d:


Outline




Mutual Inductance

• Inductance L and mutual inductance M– It describes the coupling of two circuits via the medium of a

magnetic field

1

1 11 1 1 1

1 1

1

A

IL B I dA

I I

2

21 121 2 1 2

1 1

1

A

IM B I dA

I I

: magnetic flux density

: permeability

: magnetic flux

: number of turns of coilN

N

B H


Coupling Coefficient (1)

• Coupling coefficient k– It is a qualitative measure about the coupling of loops

independent of their geometric dimensions

– k = 0 : no coupling– k = 1 : total coupling– In practice, inductively coupled tag systems operate with

coupling coefficients that may be as low as 0.01 – An analytical calculation is only possible for very simple antenna

configurations


Coupling Coefficient (2)

– Example

1) Available RF power rapidly falls off with distanceeven when in a range corresponding to antenna diameter

2) For randomly orientated objects, field "shaping" is essential, e.g. by multiplexing reader coils with different orientations

5 , 1.5

0R Tr cm r cm


Faraday's Law of Induction

Note change of signreversed direction ofi2 reference in 2-port

2 2 /with Li u R

2Qu

( )i i

d tu

dt

E ds

For the depicted two-port we have • note: R2 represents the ohmic losses in coil 2

2 1 22 2 2 2 2 2

( )d t di diu i R M L i R

dt dt dt


Resonant Tag: Capacitive Matching (Simple)

• ResonanceL1-M L2-M R2

M Cp

I1

U2


Resonant Tag: Optimum Matching Network

L1-M L2-M R2

M Cp

I1

M L2-M R2

Cp

1j MI

L2 R2

Cp

I1

1j MILp

R2

1j MI

equvalent voltage source resonance: 2 2p pL C L C

C2

RL =R2

maximizespower extractedfrom reader field


Effective Field Strength at Tag

• Faraday's law

• Voltage at tag load resistor RL with capacitive matching

212 0

0

Q eff

eff

d du H N A

dt dtj H N A

uQ2area A

Heff

N turns

Note: Heff is the field component, which is perpendicular to the blue area. We assume that the coil is so small that the magnetic field is approximately constant in the blue area.


Minimum Field Strength at Tag for Given Minimum Load Voltage

– Solving this equation for Heff we obtain the minimum effective field strength as a function of the minimum load voltage u2,min

– It can be shown that Hmin is at its minimum value if the transmission frequency of the reader corresponds to resonance frequency of the tag, i.e. (capacitive matching)

2,minu

22 2res L C


Energy Range

• The energy range of a tag is the maximum distance from the reader antenna at which there is enough energy to operate the tag

• If the minimum interrogation field strength Hmin is known, then we can also assess the energy range associated with a certain reader

• For a round coil with N1 turns , we have (s.f. slides 6 and 7)


Orientation of Coil

• Interrogation zone of readers


Outline




Tag-Reader System

• Equivalent circuit for a reader • Load modulation at tag

L1-M L2-M R2

M C2

I1

U2

detects voltage fluctuationdue to load modulation at tag


Inductive Systems - challenges

• For LF/HF systems the most challenging part is the tuning and positioning of the antennas


Outline




– Backscatter coupling

Fundamental Operating Principles


Radiation Density

• An electromagnetic wave propagates into space spherically (for isotropic source) from the point of its creation

• As the distance increases, the transported energy is divided over an increasing sphere surface area

• We talk of radiation power per unit area or radiation density S

• For an isotropic emitter with effective isotropic radiated power PEIRP, the radiation density at distance r is given by


Characteristic Wave Impedance and Field Strength

• The energy transported by the electromagnetic wave is stored in the electric and magnetic field of the wave

• In the far field we observe a transverse wave, i.e. E and H are perpendicular to each other and to the direction of wave propagation (i.e. the direction of the energy flux).

– The direction of the energy flux is given by the Poynting vector and we have (for nonlinear polarization we have to use the effective values)

• The relationship between E and H in the far field is defined by the permeability and the permittivity (in a vacuum and also in air)

where is termed the characteristic wave impedance • Furthermore, the following relationship holds

0

0

377FE H H Z H

S E H

0 0

S E H


Linear Polarization

• Polarization of electromagnetic waves– The polarization is determined by the orientation of the electric

field vector E of the wave– In general, we speak about elliptical polarization. The two

extreme cases are: linear polarization and circular polarization

Linear polarization


Circular Polarization

– The transmission of energy between two linear polarized antennas is optimal if the two antennas have the same polarization direction

– In RFID systems, there is no fixed relationship between the position of the tag and reader antennas.

– This can lead to fluctuations in the read range!– This problem is reduced by the use of circular polarization in the

reader antenna

Circular polarization


Outline




Antenna Gain

• Antenna gain Gi and directional effect

radiation density in look direction of antenna:

equivalent isotropicallyradiated power:


Effective Isotropically Radiated Power (EIRP)

• EIRP and ERP– ERP relates to a dipole antenna rather than a spherical emitter– ERP expresses the power at which a dipole antenna must be

supplied in order to generate a defined power at a given distance

– Since the gain of a dipole antenna Gi = 1.64 is known


Power Supply to Tag

• Passive tagsEffective aperture Ae of antenna determinesavailable receive power Pe


Outline




Backscattering

• Scattering of electromagnetic waves– An electromagnetic wave encounters various objects. Part of its

energy is either absorbed and converted into heat or backscattered (for simplicity we ignore other form of interaction such as reflection)

– In RFID systems the backscattering of electromagnetic waves is used for the transmission of data from the tag to the reader

– The tag’s antenna backscatters a power PS that is proportional to the radiation density S and the so-called radar cross-section

– At the reader, we have the following power density of the backscattered field (assuming that the tag acts like a point source)


Friis's Law

• Received power density

• Aperture of dipole

• Available power at the receiver

1 124

P GS

r

/ 22

1.644eA

21 1

2

2

2 1 1

1.644 4

4 Friis Law

RX eP A S

P G

r

G P Gr

73WR / 2

Rw ; available powerPRX


Equivalent Two-Port (Reciprocity)

• Far field approximations–

–

• Available power at port 2

• By equating P2,V and PRX we obtain the coupling impedance

– note that we ignore for simplicity the phase shift due to the propagation delay)

Z1 Z2

Z3

I1

U2

I2

U1

r

1 2 WZ Z R 3 WZ R

221

2, 34VW

IP Z

R

3 1 224WZ R GGr

reader tag


Backscatter Modulation

• Switch closed

• Switch open

– with

• Available voltage swing at reader

• Note: available power

RW RW

Z3

I1

tagU1

1,0 1 2 1W

aU R I b

a

1, 1WU R I b a

1 2

1

42 W

a GGr

b I R

2

1, 1,0

2

1 1 21

2

2 1

2 24W

a

aU U b

a

I R GGr

4/ r

reader


Active Tags

• The power supply of the chip is provided by a battery• The voltage supplied by the antenna is used to activate

the tag by means of a detection circuit• In absence of external activation, the tag is switched into

power saving mode• In general, a much lower received power is needed to

activate the tag• Thus the read range is greater compared to a passive

tag

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RFID II Inductive and Microwave Systems. 2 Communication Technology Laboratory Wireless...

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