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RF Basics of Near Field Communications
Somnath Mukherjee
Thin Film Electronics Inc., San Jose, CA, USA
1
What it covers
RF Power and Signal Interface
• Mechanism behind Reader powering tag chip
• Modulation used to convey Tag information to Reader
• Theoretical background related to above
• Measurement of various parameters related to above
What it does not cover
• Protocol details and standards
• Higher layer description above PHY
• Software, middleware
• Security
• Applications of NFC
• Chip design
2
Attendee Background
• Fundamental circuit theory – Complex number notation
• Fundamental linear system theory
• Fundamental electromagnetic fields
3
Disclaimer
• Cannot divulge proprietary information
• Not responsible for design using this information
4
Topics
• Introduction• Background Material• Powering up the RFID chip - Remotely• Chip talks back
– Load Modulation and related topics• Miscellaneous topics
– Tag antenna design considerations– Effect of metal nearby
• Introduction to NFC Forum Measurements
5
Introduction
6
Readers
13.56 MHzFew centimeter range
7
Tags
Reader (e.g. Smart Phone) can behave like (emulate) a Tag
We still call that Tag during this discussion
Reader (e.g. Smart Phone) can behave like (emulate) a Tag
We still call that Tag during this discussion
8
• Energy from Reader activates the chip inside the Tag (tens of w to few mW) – Tag and Reader are a few centimeters apart
• Chip generates talk-back signals once powered up
• Tag communicates above signals back to Reader
chip
9
Propagating Waves used in most Wireless Communication
Bluetooth (m) to Deep Space Communication (hundreds of thousands km)
• Not in NFC– No intentional radiation
• Simpler to analyze => quasi-static analysis
10
Far Field Near Field
Energy transfer Propagating waves to infinity
Confined (Very small amount propagates)
Load connected or not Source transfers energy irrespective
Source transfers energy only when it sees a load
Dimensions of antennas Comparable to wavelength
Much smaller than wavelength
Fields Electric (E) and Magnetic (H)
Magnetic (H)
Phase between E and H Zero ≠ Zero
Analysis Tool Wave theory Quasi-static Field and Circuit Theory
Antenna gain/directivity Applicable Not applicable11
Criteria for defining near field
D
• How ‘flat’ are wavefronts• Valid for propagating waves. Not applicable
here
12
Radiation Resistance of a Circular Loop
N turn circular loop with radius a:N turn circular loop with radius a:
24
2 N.a.2
..20Rr
Radiation ResistanceRadiation Resistance
6 turns, a = 25mm => Rr = 18 few ohms dissipative resistance 6 turns, a = 25mm => Rr = 18 few ohms dissipative resistance
13
Self Quiz
• Which of the following uses propagating electromagnetic waves– Satellite links– WiFi– Cell Phone– Smart Card– Bluetooth
14
Self Quiz
• Which of the following uses propagating electromagnetic waves– Satellite links– WiFi– Cell Phone– Smart Card– Bluetooth
How about UHF RFID?How about UHF RFID?
15
Background Material
16
Fields
17
Scalar and Vector Fields
Scalar Field example:
A pan on the stove being heated. Temperature at different points of the pan is a scalar field
Vector Field example:
Water flowing through a canal. Velocity highest at middle, zero at the edges
Scalar Field example:
A pan on the stove being heated. Temperature at different points of the pan is a scalar field
Vector Field example:
Water flowing through a canal. Velocity highest at middle, zero at the edges
18
Vector Calculus - review
C S
d.curld. aAlA Stokes’ theoremStokes’ theorem
Curl is line integral per unit area over an infinitesimal loopCurl is line integral per unit area over an infinitesimal loop
Component of curl normal to the infinitesimal surface
Component of curl normal to the infinitesimal surface
dada
19
Self Quiz
What is the curl at the center? Away from the center?What is the curl at the center? Away from the center?
20
Electric <>Magnetic Field
21
Electric <>Magnetic
tcurl
D
JH
3
d.
4.Id
r
rlB
tcurl
B
E
Magnetic field is generated by current or changing electric fieldMagnetic field is generated by current or changing electric field
Electric field (voltage) is generated by changing magnetic fieldElectric field (voltage) is generated by changing magnetic field
Second term is negligible in the present discussionSecond term is negligible in the present discussion
Biot and Savart’s (Ampere’s) LawBiot and Savart’s (Ampere’s) Law
td.
tS
aBEMF Faraday’s LawFaraday’s Law
22
Magnetic Coupling
~
Reader
Tag
Interaction between Reader and Tag is due to magnetic coupling
Field generated by Reader (Cause)Biot and Savart’s (Ampere’s) Law
Induced EMF in Tag (Effect) Faraday’s Law
Circuit representation is often adequate
Z1’
~ Z2’
. .+
V
23
Magnetic Field from Currents
24
Magnetic Field from a Circular Coil
0 20 40 60 80 1000
10
20
30
40
15mm25mm45mm
Parameter: Radius in mm
z mm
H A
/m
N=1 I= 1 AN=1 I= 1 A
Small coils produce stronger field at close range, but die down fasterSmall coils produce stronger field at close range, but die down faster
Field is calculated along the axis – not necessarily the most important region Field is calculated along the axis – not necessarily the most important region
HH
25
Reader Antenna
Tag Antenna
Magnetic field curling around currentField is strongest here
49mm X 42mm2 turns
Field generated by Reader Coil
Field outside the loop is in opposite direction to that inside
26
0.00
2.00
4.00
6.00
8.00
10.00
0 5 10 15 20 25 30 35
Distance mm
H A
/m
Kovera
Inside
Nokia
minimum@14443
springcard
LG Nexus
Magnetic Field from some common ReadersMagnetic Field from some common Readers
Measured using single turn 12.5mm diameter loopMeasured using single turn 12.5mm diameter loop
Excitation current ?Excitation current ?
Hmin ISO 14443: 1.5 A/m Hmin ISO 15693: 0.15 A/mHmin ISO 14443: 1.5 A/m Hmin ISO 15693: 0.15 A/m27
B, HMagnetic Flux and Relatives
BFlux Density V.s.m-2 = Tesla
Magnetic Field A.m-1
0r .
BH [2]
1. Multiply by N if multi-turn2. Not always valid
td.
C
lE VInduced EMF E=
S
d. sB V.sFlux [1]
Bn
sB d.
E
In air:0
B
H 0 = 4. 10-7 H/m28
H or B
B determines• Force (e.g. in motor)• EMF (e.g. in alternator, transformer, RFID…)
curl H = J gives magnetic field from any current carrying structure irrespective of the medium. From that we can determine B
Describes the bending of B when going through media of different permeabilities
29
Self Quiz
Where is the flux is larger?Where is the flux is larger?
Top ViewAll in one plane
Top ViewAll in one plane
30
EMF from Magnetic Field
31
Consider H = 3 A/m (2X minimum field from Reader per ISO 14443)
=> B = 12. 10-7 V.s.m-2 (or Tesla)
=> Flux = B. Area = 12. 10-7. (3.375. 10-3) V.s = 1.27.10-8 V.s
=> Induced EMF = . Flux = (2.13.56.106). (1.27.10-8) V = 1.08 V
Assume field is uniform over a area of 75 mm X 45 mm (Credit Card size Tag) and normal to it. Area = 75X45 mm2 = 3.375. 10-3 m2
Flux is varying sinusoidally with a frequency 13.56 MHz => = 2.13.56.106 rad/s
B 90◦ to loop
Example
32
B at an angle to loop
n
Flux (and therefore induced EMF) reduced by cos()Flux (and therefore induced EMF) reduced by cos()
33
Multi-turn loops
If 1. Turns are close to each other2. Loop dimension << wavelength (22 m for 13.56 MHz)
=> E ~ N.E1 N = number of turns
E1
E2++
E1
E2++
E = E1+E2
34
Self Quiz
Two identical loops are immersed in uniform time-varying magnetic field. What is the induced EMF between the terminals in the two cases?
35
Self Inductance
• Depends on geometry and intervening medium
• ~ N2 [H (flux) increases as N, back EMF increases as N times flux]
• Closed form expressions for various geometries available
dt
di.LE
di
dL
=>=>
36
Mutual Inductance
dt
1di.21M2E
1di
21d21M
=>=>
M21=M12M21=M12
Depends on geometry, relative disposition and intervening mediumDepends on geometry, relative disposition and intervening medium
37
Calculation of Mutual Inductance
• Neumann formula– Calculates mutual
inductance between two closed loops
– Difficult to find closed form expression except for simple cases
2C1C
12
2d.1d.
4
0M
rr
ll
C1C1
C2C2
38
Example: Two circular coils with same axis
Closed form expression using Neumann’s formula available*
* Equivalent Circuit and Calculation of Its Parameters of Magnetic-Coupled-Resonant Wireless Power Transfer by Hiroshi Hirayama (In Tech)
M is small when relative dimensions are significantly different e.g. Portal and EAS TagM is small when relative dimensions are significantly different e.g. Portal and EAS Tag
r1
r2
h
Maximum occurs for r2 ~ r1
0 1 2 3 4 5 6 7 8 9 100
5
10
15
r2/r1
M n
H
h= 0.3r1h= 0.3r1
h= r1h= r1
h= 3r1h= 3r1
r1= 10mmr1= 10mm
39
Circular coils with same axis - continued
0 10 20 30 40 500
10
20
30
h mm
M n
H
r1= 20mmr1= 20mm
r1=5mmr1=5mm
r1=15mmr1=15mm
r1=30.5mmr1=30.5mm
Larger loop maintains higher mutual inductance at farther distancesLarger loop maintains higher mutual inductance at farther distances
40
Circuit Representation - Dot Convention
41
Dot Convention
I1
I2 Magnetic fluxes add up if current flows in same direction WRT dot
Both I1 and I2 flow away from dot
Fluxes add up
I1
I2
~+
Realistic situation – source in loop 1, resistive load in loop 2
Direction of induced EMF in blue loop (secondary) such that tends to oppose the flux in primary (red) [Lenz’s Law]Dot becomes +ve polarity of induced EMF when current is flowing towards dot in excitation loop
Needs to be used with caution if load is not resistive!
++
42
I2
I1
+
+~Vi
jM.I1
+jM.I2
Loop 1: Vi +jM.I2-Z1.I1 = 0 Loop 2: jM.I1-Z2.I2 = 0
General Expression
43Z1, Z2: Self Impedances
Skin Effect
44
Skin Effect
• Cause:– Electromagnetic Induction
I
H
E/I
Conductor
45
Effect– Current tends to concentrate on surface
Skin DepthSkin Depthr.0.
.2s
Current density reduces exponentially. Beyond 5s not much current existsCurrent density reduces exponentially. Beyond 5s not much current exists
Skin depth ↓ (more pronounced effect)
permeability ↑ (induced EMF ↑)frequency↑ (induced EMF ↑)resistivity ↓ (induced current ↑)
Skin depth ↓ (more pronounced effect)
permeability ↑ (induced EMF ↑)frequency↑ (induced EMF ↑)resistivity ↓ (induced current ↑)
46
Skin Depth at 13.56 MHz
Material Conductivity S/m at 20◦C
Permeability Skin Depth
m
Silver 6.1 x 107 1 17.2
Copper 5.96 x 107 1 17.7
Aluminum 3.5 x 107 1 22.9
Iron 1 x 107 4000 0.7
Solder 7 x 106 1 51.3
Printed Silver 4 x 106 1 68.6
Sheet of paper ~ 40 m thick47
Sheet Resistance
tt.1l
1l.R sh
t
l1
l1
l2
l2
Both have same resistance – Sheet resistance
Expressed as ohms/square
Depends on material conductivity and thickness only 48
t
Tape of• Length = l• Width = w• Thickness = t
w
Each square of length w and width w
Resistance of the tape = Rsh. Number of squares
= Rsh. l/w
49
tRsh
s
t
e1.s
Rsh
Sheet resistance DCSheet resistance DC Sheet resistance RFSheet resistance RF
If thickness << skin depth, DC and RF sheet resistances are closeIf thickness << skin depth, DC and RF sheet resistances are close
50
Sheet Resistance
Material Skin Depth
m
Sheet resistance m/square
t= 10 m t= 20 m t= 30 m t= 40 m
13.56 MHz
DC 13.56 MHz
DC 13.56 MHz
DC 13.56 MHz
DC
Ag 17.2 2.1 1.6 1.3 0.8 1.1 0.5 1.0 0.4
Cu 17.7 2.2 1.7 1.4 0.8 1.2 0.5 1.1 0.4
Al 22.9 3.5 2.8 2.1 1.4 1.7 0.9 1.5 0.7
Fe 0.7 146 10.0 146 5.0 146 3.3 146 2.5
Solder 51.3 15.5 14.1 8.5 7.0 6.2 4.7 5.1 3.5
Printed Silver
68.6 27.1 25.2 14.5 12.6 10.4 8.4 8.3 6.351
Self Quiz
• 6 turns 40mm X 40mm• 30 m thick Al => 1.7 m/square at 13.56 MHz• Width = 300 m• RF Resistance?
– How it compares with DC resistance?
Length ~ 4X40X6 mm = 960 mm => 900 mm
No. of squares = 900/.3 = 2700
RF Resistance = 1.7X 2700 m = 4.6 DC Resistance = 0.9X 2700 m = 2.4
52
Quality Factor
53
Q (Quality) Factor
R
L
T.R.I
0I.L.2
1
2Q2
2
L
RQ
CR
1Q
RjXR
jX
StorageStorage StorageStorage
DissipationDissipation DissipationDissipation
LL
R RLL
CC
R CC R
CRQ
a cycle sipated inEnergy dis
y storedPeak energ2Q
54
Unloaded Q : Q of the two-terminal device itself
Loaded Q: Dissipative element (resistor) added externally
Loaded Q < Unloaded Q
Unloaded Q : Q of the two-terminal device itself
Loaded Q: Dissipative element (resistor) added externally
Loaded Q < Unloaded Q
RLLRext L
R||RQ ext
55
Q and Bandwidth
56
Δω
ω0Q for resonant circuits
3 dB bandwidth
Effective Volume
Consider small Tag passing through a large Portal=> Field is uniform through the area of the Tag Consider small Tag passing through a large Portal=> Field is uniform through the area of the Tag
How much magnetic energy stored in the Portal gets dissipated per cycle in the Tag?How much magnetic energy stored in the Portal gets dissipated per cycle in the Tag?
TagTag
PortalPortal
Peak energy stored in a volume Veff = ½.o. (√2.H)2.Veff = o.H2.VeffPeak energy stored in a volume Veff = ½.o. (√2.H)2.Veff = o.H2.Veff
= (.o2.H2.N2.area2/R).2= (.o2.H2.N2.area2/R).2
energy dissipated per cycle in Tag (at resonance) energy dissipated per cycle in Tag (at resonance)
=> Veff = (.o.N2.area2/R).2=> Veff = (.o.N2.area2/R).2
Now, L = o. N2.area. scale_factor Now, L = o. N2.area. scale_factor
=> Veff = Q.area.2 /(scale factor)=> Veff = Q.area.2 /(scale factor)
Unit: m3Unit: m3
Ability to extract energyAbility to extract energy57
Self Quiz
• Planar coil with DC resistance 6 and RF resistance 6.001. Is the thickness of metal > skin depth?
• By increasing thickness, the DC resistance of the above coil becomes 2 and RF resistance 4The inductive reactance at 13.56 MHz is 200What is the unloaded Q?
• A chip resistor of 16 is added between the terminals. What is the loaded Q?
• The chip resistor is taken out and replaced with a lossless capacitor such that the circuit resonates at 13.56 MHz. What is the Q of the capacitor by itself and with a 4resistance in series?
58
• Introduction
• Fields
• Electric <> Magnetic
• Magnetic field from current
• EMF from Magnetic field
• Circuit Representation
• Losses – Skin Effect, Q Factor
59