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

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

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

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

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

Introduction

Readers 13.56 MHz Few centimeter range

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

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

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

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

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

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

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

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

Background Material

Fields

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

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

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

Electric <>Magnetic Field

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

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

Magnetic Field from Currents

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

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

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

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. 10-7 H/m

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

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

EMF from Magnetic Field

=> 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 = 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

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

E1 E1 E2 E2 Multi-turn loops + + E = E1+E2 If Turns are close to each other Loop dimension << wavelength (22 m for 13.56 MHz) => E ~ N.E1 N = number of turns

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?

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

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

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

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)

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

Circuit Representation - Dot Convention

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!

~ 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

Skin Effect

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

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

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

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

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

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

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

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

Quality Factor

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

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

Q and Bandwidth for resonant circuits 3 dB bandwidth

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)

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?

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