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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 2p.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.dl curlA.da 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 curlH J dB I. D t m dl r . 3 4p r Second term is negligible in the present discussion Biot and Savart’s (Ampere’s) Law Electric field (voltage) is generated by changing magnetic field curlE EMF B t B.da t 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 26 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 27 Magnetic Flux and Relatives B, H n B B.ds E Induced EMF E= E.dl t C B.ds Flux [1] V V.s S Flux Density Magnetic Field In air: H B m0 B B H [2] m r .m 0 m0 = 4p. 10-7 H/m V.s.m-2 = Tesla A.m-1 1. Multiply by N if multi-turn 2. Not always valid 28 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 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 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 32 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 d L di => di E L. 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 d 21 M 21 di1 => di1 E 2 M 21 . 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 m0 M . 4p C1 C 2 dl1.dl 2 r 2 r1 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 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 40 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! 42 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 43 Skin Effect 44 Skin Effect • Cause: – Electromagnetic Induction E/I H I Conductor 45 Effect – Current tends to concentrate on surface Skin Depth s 2. w.m0.mr 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 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 49 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 51 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 52 Quality Factor 53 Q (Quality) Factor Peak energy stored Q 2p Energy dissipated in a cycle jX Storage jX R Storage R Dissipation Dissipation L C R 1 .L.I0 2 wL Q 2p 2 2 R I .R.T L Q R R R wL Q 1 wCR C R Q wCR 54 Unloaded Q : Q of the two-terminal device itself Loaded Q: Dissipative element (resistor) added externally Loaded Q < Unloaded Q Rext L R R || R ext Q wL 55 Q and Bandwidth ω0 Q Δω for resonant circuits 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