1. RF Basics of Near Field Communications
Somnath Mukherjee
Thin Film Electronics Inc., San Jose, CA, USA

2. 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

3. Attendee Background Fundamental circuit theory
Complex number notation
Fundamental linear system theory
Fundamental electromagnetic fields

4. Disclaimer Cannot divulge proprietary information
Not responsible for design using this information

5. 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

6. Introduction

7. Readers
13.56 MHz
Few centimeter range

8. Tags Reader (e.g. Smart Phone) can behave like (emulate) a Tag
We still call that Tag during this discussion

9. Chip generates talk-back signals once powered up
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

10. 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

11. 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 applicable

12. Criteria for defining near field
l/2p
2D2/l
How ‘flat’ are wavefronts
Valid for propagating waves. Not applicable here

13. Radiation Resistance of a Circular Loop
N turn circular loop with radius a:
Radiation Resistance
6 turns, a = 25mm => Rr = 18 mW << few ohms dissipative resistance

14. Self Quiz
Which of the following uses propagating electromagnetic waves
Satellite links
WiFi
Cell Phone
Smart Card
Bluetooth

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

16. Background Material

17. Fields

18. 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

19. Vector Calculus - review
Stokes’ theorem
Curl is line integral per unit area over an infinitesimal loop da
Component of curl normal to the infinitesimal surface

20. Self Quiz
What is the curl at the center? Away from the center?

21. Electric <>Magnetic Field

22. Electric <>Magnetic
Magnetic field is generated by current or changing electric field
Second term is negligible in the present discussion
Biot and Savart’s (Ampere’s) Law
Electric field (voltage) is generated by changing magnetic field
Faraday’s Law

23. Magnetic Coupling
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
Reader ~
Tag
Circuit representation is often adequate
Z1’ ~
Z2’ . + V

24. Magnetic Field from Currents

25. Magnetic Field from a Circular Coil
I= 1 A H
Small coils produce stronger field at close range, but die down faster
Field is calculated along the axis – not necessarily the most important region

26. 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

27. Magnetic Field from some common Readers
Excitation current ?
Measured using single turn 12.5mm diameter loop
Hmin ISO 14443: 1.5 A/m Hmin ISO 15693: 0.15 A/m

28. Magnetic Flux and Relatives
B, H B n E V
Induced EMF E=
V.s
Flux
[1] B
Flux Density
V.s.m-2 = Tesla
Magnetic Field
A.m-1
[2]
1. Multiply by N if multi-turn
2. Not always valid
In air:
m0 = 4p H/m

29. 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

30. Self Quiz
Top View
All in one plane
Where is the flux is larger?

31. EMF from Magnetic Field

32. => B = 12p. 10-7 V.s.m-2 (or Tesla)
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 = m2
Flux is varying sinusoidally with a frequency MHz => w = 2p rad/s
Consider H = 3 A/m (2X minimum field from Reader per ISO 14443)
=> B = 12p V.s.m-2 (or Tesla)
=> Flux = B. Area = 12p ( ) V.s = V.s
=> Induced EMF = w. Flux = (2p ). ( ) V = 1.08 V

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

34. E1 E1 E2 E2 Multi-turn loops + + E = E1+E2 If
Turns are close to each other
Loop dimension << wavelength (22 m for MHz)
=> E ~ N.E1 N = number of turns

35. 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?

36. Self Inductance Depends on geometry and intervening medium
=>
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

37. Mutual Inductance
=>
M21=M12
Depends on geometry, relative disposition and intervening medium

38. Calculation of Mutual Inductance
Neumann formula
Calculates mutual inductance between two closed loops
Difficult to find closed form expression except for simple cases C1 C2

39. Example: Two circular coils with same axis
Closed form expression using Neumann’s formula available* r1 r2 h
h= 0.3r1
h= r1
h= 3r1
r1= 10mm
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)

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

41. Circuit Representation - Dot Convention

42. 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!

43. ~ 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

44. Skin Effect

45. Skin Effect
Cause:
Electromagnetic Induction
E/I H I
Conductor

46. Current tends to concentrate on surface
Effect
Current tends to concentrate on surface
Skin Depth
Skin depth ↓ (more pronounced effect)
permeability ↑ (induced EMF ↑)
frequency↑ (induced EMF ↑)
resistivity ↓ (induced current ↑)
Current density reduces exponentially. Beyond 5.ds not much current exists

47. Skin Depth at 13.56 MHz Sheet of paper ~ 40 mm thick Material
Conductivity S/m at 20◦C
Permeability
Skin Depth mm
Silver
6.1 x 107 1
17.2
Copper
5.96 x 107
17.7
Aluminum
3.5 x 107
22.9
Iron
1 x 107
4000
0.7
Solder
7 x 106
51.3
Printed Silver
4 x 106
68.6
Sheet of paper ~ 40 mm thick

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

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

50. Sheet resistance DC
Sheet resistance RF
If thickness << skin depth, DC and RF sheet resistances are close

51. Sheet resistance mW/square
Material
Skin Depth mm
Sheet resistance mW/square
t= 10 mm
t= 20 mm
t= 30 mm
t= 40 mm
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
1.2 Al
22.9
3.5
2.8
0.9
1.5
0.7 Fe
146
10.0
5.0
3.3
2.5
Solder
51.3
15.5
14.1
8.5
7.0
6.2
4.7
5.1
Printed Silver
68.6
27.1
25.2
14.5
12.6
10.4
8.4
8.3
6.3

52. Self Quiz
6 turns 40mm X 40mm
30 mm thick Al => 1.7 mW/square at 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

53. Quality Factor

54. Q (Quality) Factor jX jX R Storage Storage R Dissipation Dissipation L

55. Unloaded Q : Q of the two-terminal device itself
Loaded Q: Dissipative element (resistor) added externally
Loaded Q < Unloaded Q
Rext L R

56. Q and Bandwidth
for resonant circuits
3 dB bandwidth

57. 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)

58. 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 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 MHz. What is the Q of the capacitor by itself and with a 4W resistance in series?

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