Transcript Presentation 1

RF Basics of Near Field Communications
Somnath Mukherjee
Thin Film Electronics Inc., San Jose, CA, USA
[email protected]
[email protected]
1
What it covers
What it does not cover
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
• 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 MHz
Few centimeter range
7
Tags
Reader (e.g. Smart Phone) can behave like (emulate) a Tag
We still call that Tag during this discussion
8
chip
• Energy from Reader activates the chip inside the Tag (tens of mw 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
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
Energy transfer
Load connected or not
Far Field
Near Field
Propagating waves to
infinity
Confined
Source transfers
energy irrespective
Source transfers
energy only when it
sees a load
(Very small amount
propagates)
Dimensions of antennas Comparable to
wavelength
Much smaller than
wavelength
Fields
Magnetic (H)
Electric (E) and
Magnetic (H)
Phase between E and H Zero
≠ Zero
Analysis Tool
Wave theory
Quasi-static Field and
Circuit Theory
Antenna gain/directivity
Applicable
Not applicable
11
Criteria for defining near field
 l/2p
 2D2/l
• 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:
4
Radiation Resistance
 2 p .a 
2
Rr  20 .p .
 .N
 l 
2
6 turns, a = 25mm => Rr = 18 mW << 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?
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
18
Vector Calculus - review
 A .d l   curl A .d a
C
Stokes’ theorem
S
Curl is line integral per unit area over an infinitesimal loop
da
Component of
curl normal to
the
infinitesimal
surface
19
Self Quiz
What is the curl at the center? Away from the center?
20
Electric <>Magnetic Field
21
Electric <>Magnetic
Magnetic field is generated by current or changing electric field
D
curl H  J 
d B  I.
m
4p
.
Second term is negligible in the present discussion
t
dl  r
r
Biot and Savart’s (Ampere’s) Law
3
Electric field (voltage) is generated by changing magnetic field
curl E  
EMF  
B
t

t 
B .d a  

t
Faraday’s Law
S
22
Magnetic Coupling
Interaction between Reader and Tag
is due to magnetic coupling
Reader
Field generated by Reader (Cause)
Biot and Savart’s (Ampere’s) Law
Tag
Induced EMF in Tag (Effect)
Faraday’s Law
Z1’
+
V
~
. .
Z2’
Circuit representation is often
adequate
23
Magnetic Field from Currents
24
Magnetic Field from a Circular Coil
Parameter: Radius in mm
N=1
I= 1 A
40
15mm
25mm
45mm
H A /m
30
H
20
10
0
0
20
60
40
80
100
z mm
Small coils produce stronger field at close range, but die down faster
Field is calculated along the axis – not necessarily the most important region
25
Field generated by Reader Coil
Tag Antenna
49mm X 42mm
2 turns
Reader Antenna
Magnetic field curling
around current
Field is strongest here
Field outside the loop is
in opposite direction to
that inside
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Magnetic Field from some common Readers
10.00
8.00
Kovera
Inside
H A/m
6.00
Nokia
minimum@14443
4.00
springcard
LG Nexus
2.00
0.00
0
5
10
15
20
25
30
35
Distance mm
Excitation
current ?
Measured using single turn 12.5mm diameter loop
Hmin ISO 14443: 1.5 A/m
Hmin ISO 15693: 0.15 A/m
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Magnetic Flux and Relatives
B, H
n
B
  B .d s
E
Induced EMF E=
 E .d l  
C
   B .d s
Flux

V
t
[1]
V.s
S
Flux Density
Magnetic Field
In air:
H 
B
m0
V.s.m-2 = Tesla
B
H 
B
m r .m 0
[2]
m0 = 4p. 10-7 H/m
A.m-1
1. Multiply by N if multi-turn
2. Not always valid
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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
Top View
All in one plane
Where is the flux is larger?
30
EMF from Magnetic Field
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Example
B 90◦ to loop
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 => w = 2p.13.56.106 rad/s
Consider H = 3 A/m
(2X minimum field from Reader per ISO 14443)
=> B = 12p. 10-7 V.s.m-2 (or Tesla)
=> Flux = B. Area = 12p. 10-7. (3.375. 10-3) V.s = 1.27.10-8 V.s
=> Induced EMF = w. Flux = (2p.13.56.106). (1.27.10-8) V = 1.08 V
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B at an angle to loop
n
q
Flux (and therefore induced EMF) reduced by cos(q)
33
E1
E1
+
+
+ E2
+ E2
E = E1+E2
Multi-turn loops
If
1. Turns are close to each other
2. Loop dimension << wavelength
=> E ~ N.E1
(22 m for 13.56 MHz)
N = number of turns
34
Self Quiz
Two identical loops are immersed in uniform timevarying magnetic field. What is the induced EMF
between the terminals in the two cases?
35
Self Inductance
L 
d
di
=>
E   L.
di
dt
• 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
36
Mutual Inductance
M 21 
d  21
di1
=>
E 2   M 21 .
di1
dt
M21=M12
Depends 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
M 
m0
4p
.
d l1 .d l 2
  r 2  r1
C1 C 2
C1
C2
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Example: Two circular coils with same axis
Closed form expression using Neumann’s formula available*
r1= 10mm
15
h= 0.3r1
r2
M nH
10
h
5
r1
h= r1
h= 3r1
0
0
1
2
3
4
5
6
7
8
9
r2/r1
Maximum occurs for r2 ~ r1
M is small when relative dimensions are significantly different e.g. Portal and EAS Tag
* Equivalent Circuit and Calculation of Its Parameters of Magnetic-Coupled-Resonant Wireless Power Transfer by Hiroshi Hirayama (In Tech)
39
10
Circular coils with same axis - continued
r1= 20mm
30
M nH
20
r1=15mm
10
r1=30.5mm
r1=5mm
0
0
10
20
30
40
50
h mm
Larger loop maintains higher mutual inductance at farther distances
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Circuit Representation - Dot Convention
41
Dot Convention
I2
Magnetic fluxes add up if current
flows in same direction WRT dot
I1
Both I1 and I2 flow away from dot
 Fluxes add up
Realistic situation – source in loop 1,
resistive load in loop 2
I2
I1
+
~
+
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!
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I2
I1
+
jwM.I2
+
jwM.I1
+
Vi
~
Loop 1: Vi +jwM.I2-Z1.I1 = 0
Loop 2: jwM.I1-Z2.I2 = 0
General Expression
Z1, Z2: Self Impedances
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Skin Effect
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Skin Effect
• Cause:
– Electromagnetic Induction
E/I
H
I
Conductor
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Effect
– Current tends to concentrate on surface
Skin Depth
s 
2 .
w .m 0 .m r
Skin depth ↓ (more pronounced effect)
permeability ↑ (induced EMF ↑)
frequency↑ (induced EMF ↑)
resistivity ↓ (induced current ↑)
Current density reduces exponentially. Beyond 5.s not
much current exists
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Skin Depth at 13.56 MHz
Material
Conductivity S/m at
20◦C
Permeability
Skin Depth
mm
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 mm thick
47
Sheet Resistance
l2
l1
l1
l2
t
R sh   .
l1
l1 .t


t
Both have same resistance – Sheet resistance
Expressed as ohms/square
Depends on material conductivity and thickness only
48
w
Tape of
• Length = l
• Width = w
• Thickness = t
t
Each square of length w and width w
Resistance of the tape = Rsh. Number of squares
= Rsh. l/w
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Rsh 

t
Sheet resistance DC
Rsh 

t


 s .1  e  s






Sheet resistance RF
If thickness << skin depth, DC and RF sheet
resistances are close
50
Sheet Resistance
Material
Skin
Depth
mm
Sheet resistance mW/square
t= 10 mm
t= 20 mm
t= 30 mm
t= 40 mm
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
6.3
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Self Quiz
•
•
•
•
6 turns 40mm X 40mm
30 mm thick Al => 1.7 mW/square at 13.56 MHz
Width = 300 mm
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 mW = 4.6 W
DC Resistance = 0.9X 2700 mW = 2.4 W
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Quality Factor
53
Q (Quality) Factor
Peak energ
Q  2p
Energy dis
y stored
sipated in
a cycle
jX
jX
Storage
R
Storage
R
Dissipation
Dissipation
L
C
R
1
.L .I 0
2
wL
Q  2p 2

2
R
I .R .T
L
Q 
R
wL
C
R
R
Q 
1
w CR
R
Q  w CR
54
Unloaded Q : Q of the two-terminal device itself
Loaded Q: Dissipative element (resistor) added externally
Loaded Q < Unloaded Q
Rext
L
R
Q 
R || R ext
wL
55
Q and Bandwidth
Q 
ω0
for resonant circuits
Δω
3 dB bandwidth
56
Effective Volume
Tag
Consider small Tag passing through a large Portal
=> Field is uniform through the area of the Tag
Portal
How much magnetic energy stored in the
Portal gets dissipated per cycle in the Tag?
Peak energy stored in a volume Veff
= ½.mo. (√2.H)2.Veff = mo.H2.Veff
energy dissipated per cycle in Tag (at resonance)
= (w.mo2.H2.N2.area2/R).2p
=> Veff = (w.mo.N2.area2/R).2p
Unit: m3
Now, L = mo. N2.area. scale_factor
Ability to extract energy
=> Veff = Q.area.2p /(scale factor)
57
Self Quiz
• Planar coil with DC resistance 6W and RF resistance
6.001W. Is the thickness of metal > skin depth?
• By increasing thickness, the DC resistance of the above
coil becomes 2W and RF resistance 4W. The inductive
reactance at 13.56 MHz is 200W. What is the unloaded
Q?
• A chip resistor of 16W 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 4W resistance in series?
58
•
•
•
•
•
•
•
Introduction
Fields
Electric <> Magnetic
Magnetic field from current
EMF from Magnetic field
Circuit Representation
Losses – Skin Effect, Q Factor
59