1. RFID II Inductive and Microwave Systems

2. Outline Inductive Systems Microwave Systems Magnetic Field
Tag-Reader Coupling
Load Modulation
Microwave Systems
Electromagnetic Waves
Antennas
Electromagnetic Coupling and Backscatter Modulation
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3. Fundamental Operating Principles
Inductive coupling
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4. 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
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5. 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)
Radius R
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6. 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
with and
we obtain
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7. Optimal Radius of Coil at a Given Distance d
H strength versus distance d and coil radius R
Optimal coil radius for given distance d: d R
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8. Outline Inductive Systems Microwave Systems Magnetic Field
Tag-Reader Coupling
Load Modulation
Microwave Systems
Electromagnetic Waves
Antennas
Electromagnetic Coupling and Backscatter Modulation
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9. Mutual Inductance Inductance L and mutual inductance M
It describes the coupling of two circuits via the medium of a magnetic field
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10. 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
If we consider two coupled coils with different number of windings, the mutual inductance can exceed the inductance of the coil with less windings (as psi ~ N). Let a1 = psii21/psi1 and a2=psi12/psii2 (i.e. the ratio of the total flux induced in the coupled coil and the total flux induced in the source coil). The coupling coefficient is the geometric mean of both values: k=sqrt(a1*a2). It is not affected if we change the number of windings without modifying the geometry of the coils.
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11. Coupling Coefficient (2)
Example
1) Available RF power rapidly falls off with distance
even 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
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12. Faraday's Law of Induction
For the depicted two-port we have
note: R2 represents the ohmic losses in coil 2
Note change of sign
reversed direction of
i2 reference in 2-port
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13. Resonant Tag: Capacitive Matching (Simple)
L1-M
L2-M R2
Resonance U2 M Cp
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14. Resonant Tag: Optimum Matching Network
L1-M
L2-M I1 L2 C2 R2 R2 M Cp Cp Lp
equvalent voltage source
resonance: M
L2-M R2 R2
RL =R2
maximizes
power extracted
from reader field Cp
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15. Effective Field Strength at Tag
Faraday's law
Voltage at tag load resistor RL with capacitive matching
Heff
area A
uQ2
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.
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16. 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)
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17. 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)
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18. Orientation of Coil Interrogation zone of readers
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19. Outline Inductive Systems Microwave Systems Magnetic Field
Tag-Reader Coupling
Load Modulation
Microwave Systems
Electromagnetic Waves
Antennas
Electromagnetic Coupling and Backscatter Modulation
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20. Tag-Reader System Equivalent circuit for a reader
Load modulation at tag I1
L1-M
L2-M R2
detects voltage fluctuation
due to load modulation at
tag U2 M C2
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21. Inductive Systems - challenges
For LF/HF systems the most challenging part is the tuning and positioning of the antennas
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22. Outline Inductive Systems Microwave Systems Magnetic Field
Tag-Reader Coupling
Load Modulation
Microwave Systems
Electromagnetic Waves
Antennas
Electromagnetic Coupling and Backscatter Modulation
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23. Fundamental Operating Principles
Backscatter coupling
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24. 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
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25. 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
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26. 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
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27. 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
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28. Outline Inductive Systems Microwave Systems Magnetic Field
Tag-Reader Coupling
Load Modulation
Microwave Systems
Electromagnetic Waves
Antennas
Electromagnetic Coupling and Backscatter Modulation
Communication Technology Laboratory
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29. Antenna Gain Antenna gain Gi and directional effect
radiation density in look direction
of antenna:
equivalent isotropically
radiated power:
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30. 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
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31. Power Supply to Tag Passive tags
Effective aperture Ae of antenna determines
available receive power Pe
Passive tags
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32. Outline Inductive Systems Microwave Systems Magnetic Field
Tag-Reader Coupling
Load Modulation
Microwave Systems
Electromagnetic Waves
Antennas
Electromagnetic Coupling and Backscatter Modulation
Communication Technology Laboratory
Wireless Communication Group

33. 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)
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34. Friis's Law Received power density Aperture of dipole
Available power at the receiver
Rw ; available powerPRX
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35. Equivalent Two-Port (Reciprocity)Z1 Z2 I1 I2 U1 U2
reader
tag Z3
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)
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36. Backscatter ModulationRW RW
reader U1
tag Z3
Switch closed
Switch open
with
Available voltage swing at reader
Note: available power
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37. 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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