Chip Talks Back Tag sends signal back to Reader 1
Load Modulation Concepts I 1 C 1 + Vi I 2 R 1 ~ L 1 . . 2 L 2 1 C 2 R 2’ How does I 1 change when switching takes place in secondary (Tag) ? Let R 2’ < R 2 When switch moves from position 1 to 2: Current in secondary ↑ Current in primary ↓ 2
ISO 14443 Timing Frequency 13. 56 MHz 847 KHz 105. 9 KHz Carrier Sub-carrier =Carrier/16 Bit rate = Sub-carrier/8 Period 74 ns 1. 18 ms 9. 44 ms ‘ 1’ - ISO 14443 ‘ 0’ - ISO 14443 1. 18 ms 3
Current through Reader Coil Bit duration = 9. 44 ms 105. 9 Kb/s 0 1 4
Heuristic Analysis C ≈ ≡ R Conditions: Valid at a single frequency Valid for Q >> 1 XC 2/R ≈C Lo to Hi Convert to a series resonant circuit I 1 C 1 + Vi ~ I 2 R 1 L 1 . . C 2 2 1 L 2 R 2 s’ Hi to Lo 5
Assume both primary and secondary resonant at the excitation frequency w 0 Secondary resistance is switching between two values R 2 s and R 2 s’ where XC =1/w. C 2 for k <<1 6
Modulation Depth Increases with • Low R 1 (High Reader Q) • High R 2 (Low tag chip dissipation – High Tag Q) • High k (coupling coefficient) • Higher C 2 (Lower L 2) (Tag tank capacitance) Above relationship is approximate – need to use with caution Detailed analysis/simulation is often necessary 7
Approximations I 1 C 1 + Vi ~ I 2 R 1 . . L 1 2 L 2 C 2 1 R 2’ If XC 2 ~ R 2’, then equivalent series capacitance becomes > C 2 f 02 ↓ and may be < operating frequency Self-impedance of Tag: Inductive Transient behavior: slow 8
More Detailed Analysis (Numerical) Steady State Analysis – no transient considerations Modulation Depth: Difference in current in Reader Coil due to switching in Tag - for 1 V excitation in Reader Both Reader and Tag tuned to 13. 56 MHz L 1= 306 n. H C 1= 450 p. F Q 1= 8. 7 L 2= 2755 n. H C 2 = 50 p. F Q 2 = 33. 5 (unloaded) R 2 switched between 5000 and 500 ohms 9
Effect of Tank Capacitor in Tag 0. 2 0. 08 0. 02 10
Effect of Switched Resistance High value of R 2: 5000 ohms k = 0. 05 500 ohm 2000 ohm 3000 ohm 11
Measurement of Load Modulation C 1 Query command Tag L 1 13. 56 MHz C 2 Scope ~1 W NFC Forum PD as Reader 12
Bit duration = 9. 44 ms 105. 9 Kb/s 0 1 13
Tag at 5 mm (H = 7. 3 A/m) from PD-3 Sub carrier = 13. 56/16 MHz = 847. 5 KHz ≡ 1. 18 ms 14
H= 3. 65 A/m 15
Excitation Frequency = 12 MHz Current decreases during switching 16
Pulse Merge Tag f 0 13. 7 MHz Tag f 0 14. 0 MHz 17
Effect of Tag Resonant Frequency k= 20% 13. 56 MHz 13 MHz Good Transient Modulation Index compromised some 14. 2 MHz 18
Bandwidth Requirement 19
• Trade-off between Q (range) and Bandwidth (data rate) – ISO 14443 : 106 Kb/s, < 10 cm – ISO 15693 : 26. 5 Kb/s, < 30 cm • Sub-carrier – Higher with higher data rate – ISO 14443 : 847 KHz – ISO 15693 : 484 KHz 20
Modulation subjected to asymmetric response sc sc sc Carrier +wsc Carrier sc -wsc +wsc Carrier -wsc Modulation depth is reduced 21
• Load Modulation – Approximate theory – Numerical solution (steady state) – Illustration of simulation • Transients – Measurement • Bandwidth 22
Antenna Design Issues 23
Parameters Considered • • Resonant frequency Q-factor Switched resistance Tank inductor and capacitor 24
Resonant frequency Reader Tag Selected close to 13. 56 MHz Sometimes higher than 13. 56 MHz • Less detuning (choking) effect for multi-tag scenario • Pulse merge 25
Q factor Reader • Limited by – Bandwidth – Close range operation (Blind Spot) Tag • Limited by – Bandwidth – Close range operation (Blind Spot) • Unloaded Q on PCB • ESR of tag coil can be high (~50) but matched to ESR of loaded (output chip-capacitor combo resistance of chip) for maximum power brings loaded Q down. transfer – Matching network used 26
Switched Resistance Reader • NA Tag • Modulation depth increases with low R 2’ • Too low R 2’ tends to make Tag inductive during switched state and may degrade transient response 27
Tank Inductor, Capacitor Reader • Large L (low C) helps to increase M (power transfer) Tag • Large L (low C) helps to increase M (power transfer) • Large C (low L) – might help load modulation – Less spread in manufacturing (reduced effect from parasitics) 15 to 50 p. F is common 28
Compensated Antenna Motivation: Stray capacitance creating common mode currents Reduction of effective M Detuning + V -V 29
C 1 2 C: Common C-1: Compensated Mode – 4 turns C-2: Uncompensated – 8 turns Blue Dot: Via NOT TO SCALE 30
Effect Of Metal 31
Tag and Reader Application Requirement of Tag to be attached on or close to metallic surfaces Acting as Reader Or Acting as Tag Antenna could be close to metal 32
Automated Inventory with ‘Smart Shelf’ HF system allows more precise location than UHF • HF Reader antenna laid out on metal shelves need spacers – Wasted space – Inconvenience Reader Antenna 33
Eddy (Surface) Currents on Metal Coil B(t) E(t) 34
Current Carrying Coil near a Metal Sheet ~ Metal Magnetic field has only tangential component over perfect conductor -no normal component Surface (eddy) currents are generated on metal to satisfy above boundary condition 35
Magnetic Field from a Current Carrying Loop Metal 36
Performance Degradation • Magnetic field generated by eddy current opposes excitation field • Total flux linked by coil ↓=> Inductance ↓=> Resonant frequency↑ (Mistuning) • Flux linked by secondary loop ↓ => Deterioration in power and signal transfer 37
Surface Impedance Zs s = conductivity d = skin depth D. F. Sievenpiper, “High Impedance Electromagnetic Surfaces”, Ph. D. Dissertation, University of California, Los Angeles, 1999 38
Equivalent Circuit and Phasor Diagram Reader I 1 C 1 R 0 L 1 + V ~ . Metal . L 3 I 3 w. L 3=R 3 45◦ F 10 R 3 Vi = [R 1+R 0 + j(w. L 1 -1/w. C 1)]. I 1 – jw. M 13. I 3 0 = [R 3 + j. R 3]. I 3 – jw. M 13. I 1 F 30 Fresultant 39
Mitigation with Ferrite: High permeability, poor conductivity Metal B q 2 Ferrite m m 0 q 1 B 0 Bending increases with • mr • thickness I 40
Bending Angle mr =100 mr =30 41
§ mr. t determines shielding effectiveness § Low cost dielectric spacers help, but need to be much thicker than ferrite for same performance § 0. 1 mm ferrite sheet (FK 03 – NEC Tokin) allows Tags to be installed on metal surfaces. Dielectric spacers need few cm gap § Loss in ferrite (mr’’) adds additional loss and need to be maintained within limits 42
Image Approach PEC Ferrite Image current of source current 43
• Antenna Design Issues • Effect of Metal 44
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