RF Basics of Near Field Communications
Somnath Mukherjee
Thin Film Electronics Inc., San Jose, CA, USA somnath.mukherjee@thinfilm.no
somnath3@sbcglobal.net
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

• Fundamental circuit theory
– Complex number notation
• Fundamental linear system theory
• Fundamental electromagnetic fields
3

• 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

6

13.56 MHz
Few centimeter range
7

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 m 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
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
Far Field Near Field
Propagating waves to infinity
Confined
(Very small amount propagates)
Load connected or not Source transfers energy irrespective
Dimensions of antennas Comparable to wavelength
Fields Electric ( E ) and
Magnetic ( H )
Phase between E and H Zero
Analysis Tool Wave theory
Antenna gain/directivity Applicable
Source transfers energy only when it sees a load
Much smaller than wavelength
Magnetic ( H )
≠ Zero
Quasi-static Field and
Circuit Theory
Not applicable
11

 l/2p
 2
D
2 /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:
Radiation Resistance
Rr

20 .
p 2
.


2 p
.
a l


4
.
N
2
6 turns, a = 25mm => Rr = 18 mW << few ohms dissipative resistance
13

• Which of the following uses propagating electromagnetic waves
– Satellite links
– WiFi
– Cell Phone
– Smart Card
– Bluetooth
14

• Which of the following uses propagating electromagnetic waves
– Satellite links
– WiFi
– Cell Phone
– Smart Card
– Bluetooth
How about UHF RFID?
15

16

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
C

A .
d l

S
 curl A .
d a Stokes’ theorem
Curl is line integral per unit area over an infinitesimal loop d a
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 curl H

J


D
 t d B
 m
I .
4 p
.
d l
 r r
3
Second term is negligible in the present discussion
Biot and Savart’s (Ampere’s) Law
Electric field (voltage) is generated by changing magnetic field curl E
 

B
 t
EMF  

 t
S

B .
d a
 
 
 t
Faraday’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
V
+
~
Z 1
’
. .
Z 2
’
Circuit representation is often adequate
23

24

Magnetic Field from a Circular Coil
Parameter: Radius in mm
N=1
I= 1 A
40
30
15mm
25mm
45mm
20
H
10
0
0 20 40 60 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
Magnetic field curling around current
Field is strongest here
Tag Antenna
49mm X 42mm
2 turns
Reader Antenna
Field outside the loop is in opposite direction to that inside
26

Magnetic Field from some common Readers
10.00
8.00
6.00
4.00
Kovera
Inside
Nokia minimum@14443 springcard
LG Nexus
2.00
0.00
0 5 10 15 20
Distance mm
25 30 35
Measured using single turn 12.5mm diameter loop
H min
ISO 14443: 1.5 A/m H min
ISO 15693: 0.15 A/m
Excitation current ?
27

B, H
Magnetic Flux and Relatives n
B
 
B .
d s
E
Induced EMF
E=
Flux
 E .
d l
C
 
 
 t
 
S
 B .
d s [1]
Flux Density B
Magnetic Field
H

B m r
.
m
0
[2]
In air: H

B m
0 m
0
= 4 p . 10
-7 H/m
V.s
V
V.s.m
-2 = Tesla
A.m
-1
1. Multiply by N if multi-turn
2. Not always valid
28

H
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

31

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 m 2
Flux is varying sinusoidally with a frequency 13.56 MHz => w
= 2 p
.13.56.10
6 rad/s
Consider H = 3 A/m (2X minimum field from Reader per ISO 14443)
=> B = 12 p
. 10
-7
V.s.m
-2 ( or Tesla )
=> Flux = B. Area = 12 p
. 10
-7
. (3.375. 10
-3 ) V.s = 1.27.10
-8 V.s
=> Induced EMF = w
. Flux = ( 2 p
.13.56.10
6 ). (1.27.10
-8 ) V = 1.08 V
32

B at an angle to loop q n
Flux (and therefore induced EMF) reduced by cos( q
)
33

E
1
+
+
E
2
E
+
1
+
E
2
E = E 1 + E
2
Multi-turn loops
If
1. Turns are close to each other
2. Loop dimension << wavelength (22 m for 13.56 MHz)
=> E ~ N.
E 1 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
• ~ N 2 [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 di 1
=> E 2
 
M 21 .
di 1 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
 m
0
4 p
.
C 1

C 2
 d l 1 .
d l 2 r 2
 r 1
C1
C2
38

Example: Two circular coils with same axis
Closed form expression using Neumann’s formula available* r1= 10mm
15 h= 0.3r1
r2
10 h r1
5 h= r1
0
0 1 2 h= 3r1
3 4 5 6 r2/r1
Maximum occurs for r2 ~ r1
7 8 9 10
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

Circular coils with same axis - continued r1= 20mm
30
20
10 r1=15mm r1=30.5mm
0
0 r1=5mm
10 20 h mm
30 40 50
Larger loop maintains higher mutual inductance at farther distances
40

Circuit Representation - Dot Convention
41

I 1
I 1
+
~
Dot Convention
I 2
I 2
+
Magnetic fluxes add up if current flows in same direction WRT dot
Both I 1 and I 2 flow away from dot

Fluxes add up
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

I 2
I 1 j w
M.I2
+
Vi
+
~
+ j w
M.I1
Loop 1 : Vi +j w
M.
I 2Z 1.
I 1 = 0 Loop 2 : j w
M.
I 1Z 2.
I 2 = 0
General Expression
Z 1, Z 2 : Self Impedances
43

44

• Cause:
– Electromagnetic Induction
I
H
E/I
Conductor
45

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
46

Material
Silver
Copper
Aluminum
Iron
Solder
Printed Silver
Skin Depth at 13.56 MHz
Conductivity S/m at
20
◦
C
6.1 x 10 7
Permeability Skin Depth m m
1 17.2
5.96 x 10 7
3.5 x 10 7
1 x 10 7
7 x 10 6
1
1
4000
1
17.7
22.9
0.7
51.3
4 x 10 6 1 68.6
Sheet of paper ~ 40 m m thick
47

Sheet Resistance l2 l1 l1 t
R sh
 
.
l 1 l 1 .
t

 t
Both have same resistance – Sheet resistance
Expressed as ohms/square
Depends on material conductivity and thickness only l2
48

w
Tape of
• Length = l
• Width = w
• Thickness = t
Each square of length w and width w t
Resistance of the tape = R sh
. Number of squares
= Rsh. l/w
49

Rsh

 t
Rsh


 s .




1
 e
 t
 s




Sheet resistance RF Sheet resistance DC
If thickness << skin depth, DC and RF sheet resistances are close
50

Material Skin
Depth m m
Ag
Cu
Al
Fe
Solder
Printed
Silver
17.2
17.7
22.9
0.7
51.3
68.6
Sheet Resistance
Sheet resistance m
W
/square
3.5
146
15.5
27.1
t= 10 m m
13.56
MHz
2.1
DC
1.6
2.2
1.7
2.8
10.0
14.1
25.2
t= 20 m m
13.56
MHz
1.3
DC
0.8
1.4
0.8
2.1
146
8.5
14.5
1.4
5.0
7.0
12.6
t= 30 m m
13.56
MHz
1.1
DC
0.5
1.2
0.5
1.7
146
6.2
10.4
0.9
3.3
4.7
8.4
1.5
146
5.1
8.3
t= 40 m m
13.56
MHz
1.0
DC
0.4
1.1
0.4
0.7
2.5
3.5
6.3
51

• 6 turns 40mm X 40mm
• 30 m m thick Al => 1.7 m
W
/square at 13.56 MHz
• Width = 300 m 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
W
= 4.6
W
DC Resistance = 0.9X 2700 m
W
= 2.4
W
52

53

Q (Quality) Factor
Q

2 p
Energy dis sipated in a cycle
Storage jX
R
Dissipation
L
R
L
R jX
Storage
R
Dissipation
C
R C R
Q

2 p
1
2
.
L .
I 0
2
I
2
.
R .
T
 w
L
R
Q

R w
L
Q

1 w
CR
Q
 w
CR
54

Unloaded Q : Q of the two-terminal device itself
Loaded Q: Dissipative element (resistor) added externally
Loaded Q < Unloaded Q
R ext
L
R
Q

R || R w
L ext
55

Q and Bandwidth

for resonant circuits
3 dB bandwidth
56

Portal
Tag
Effective Volume
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?
Peak energy stored in a volume Veff
= ½.
m o. (√2.H) 2 .Veff = m o.H
2 .Veff
energy dissipated per cycle in Tag (at resonance)
= ( w
.
m o 2 .H
2 .N
2 .area
2 /R).2
p
Ability to extract energy
=> Veff = ( w
.
m o.N
2 .area
2 /R).2
p
Now, L = m o. N 2 .area. scale_factor
=> Veff = Q.area.2
p
/(scale factor)
Unit: m 3
57

• Planar coil with DC resistance 6 W and RF resistance
6.001
W
. Is the thickness of metal > skin depth?
• By increasing thickness, the DC resistance of the above coil becomes 2
W and RF resistance 4
W.
The inductive reactance at 13.56 MHz is 200
W.
What is the unloaded
Q?
• A chip resistor of 16 W 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 4
W resistance in series?
58

• Introduction
• Fields
• Electric <> Magnetic
• Magnetic field from current
• EMF from Magnetic field
• Circuit Representation
• Losses – Skin Effect, Q Factor
59