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

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