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Chip Talks Back Tag sends signal back to Reader

Chip Talks Back Tag sends signal back to Reader. Load Modulation Concepts. I 1. I 2. 2. R1. C1. 1. C2. +. L1. V i. L2. R2. R2 ’. ~. How does I 1 change when switching takes place in secondary (Tag) ?. Let R2’ < R2 When switch moves from position 1 to 2:

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Chip Talks Back Tag sends signal back to Reader

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  1. Chip Talks Back Tag sends signal back to Reader

  2. Load Modulation Concepts I1 I2 . . 2 R1 C1 1 C2 + L1 Vi L2 R2 R2’ ~ How does I1 change when switching takes place in secondary (Tag) ? Let R2’ < R2 When switch moves from position 1 to 2: Current in secondary ↑ Current in primary ↓

  3. ISO 14443 Timing Frequency Period Carrier 13.56 MHz 74 ns Sub-carrier =Carrier/16 847 KHz 1.18 ms Bit rate = Sub-carrier/8 105.9 KHz 9.44 ms 9.44 ms ‘1’ - ISO 14443 ‘0’ - ISO 14443 1.18 ms

  4. 0 1 Current through Reader Coil Bit duration = 9.44 ms  105.9 Kb/s

  5. Convert to a series resonant circuit I1 I2 C2 . . 2 R1 C1 1 + L1 Vi L2 R2s R2s’ ~ Heuristic Analysis Conditions: Valid at a single frequency Valid for Q >> 1 ≈ XC2/R ≡ C R ≈C Lo to Hi Hi to Lo

  6. Secondary resistance is switching between two values R2s and R2s’ where XC =1/w.C2 for k <<1 Assume both primary and secondary resonant at the excitation frequency w0

  7. Modulation Depth Increases with • Low R1 (High Reader Q) • High R2 (Low tag chip dissipation – High Tag Q) • High k (coupling coefficient) • Higher C2 (Lower L2) (Tag tank capacitance) Above relationship is approximate – need to use with caution Detailed analysis/simulation is often necessary

  8. I1 I2 . . 2 R1 C1 1 C2 + L1 Vi L2 R2 R2’ ~ Approximations If XC2 ~ R2’, then equivalent series capacitance becomes > C2 f02 ↓ and may be < operating frequency • Self-impedance of Tag: Inductive • Transient behavior: slow

  9. More Detailed Analysis (Numerical) Steady StateAnalysis– no transient considerations Modulation Depth: Difference in current in Reader Coil due to switching in Tag - for 1V excitation in Reader Both Reader and Tag tuned to 13.56 MHz L1= 306 nH C1= 450 pF Q1= 8.7 L2= 2755 nH C2 = 50 pF Q2 = 33.5 (unloaded) R2 switched between 5000 and 500 ohms

  10. 0.2 0.08 0.02 Effect of Tank Capacitor in Tag

  11. 500 ohm 2000 ohm 3000 ohm Effect of Switched Resistance High value of R2: 5000 ohms k = 0.05

  12. Measurement of Load Modulation C1 Tag L1 13.56 MHz Query command C2 Scope ~1 W NFC Forum PD as Reader

  13. 0 1 Bit duration = 9.44 ms  105.9 Kb/s

  14. Tag at 5 mm (H = 7.3 A/m) from PD-3 Sub carrier = 13.56/16 MHz = 847.5 KHz ≡ 1.18 ms

  15. H= 3.65 A/m

  16. Excitation Frequency = 12 MHz Current decreases during switching

  17. Pulse Merge Tag f0 14.0 MHz Tag f0 13.7 MHz

  18. Effect of Tag Resonant Frequency k= 20% 13.56 MHz 13 MHz Good Transient Modulation Index compromised some 14.2 MHz

  19. Bandwidth Requirement

  20. Trade-off between Q (range) and Bandwidth (data rate) • ISO 14443 : 106 Kb/s, < 10cm • ISO 15693 : 26.5 Kb/s, < 30 cm • Sub-carrier • Higher with higher data rate • ISO 14443 : 847 KHz • ISO 15693 : 484 KHz

  21. Modulation subjected to asymmetric response sc sc sc sc Carrier Carrier +wsc +wsc Carrier Carrier -wsc -wsc Modulation depth is reduced

  22. Load Modulation • Approximate theory • Numerical solution (steady state) • Illustration of simulation • Transients • Measurement • Bandwidth

  23. Antenna Design Issues

  24. Parameters Considered • Resonant frequency • Q-factor • Switched resistance • Tank inductor and capacitor

  25. Reader Selected close to 13.56 MHz Tag Sometimes higher than 13.56 MHz Less detuning (choking) effect for multi-tag scenario Pulse merge Resonant frequency

  26. Reader Limited by Bandwidth Close range operation (Blind Spot) Unloaded Q on PCB can be high (~50) but loaded (output resistance of chip) brings loaded Q down. Matching network used Tag Limited by Bandwidth Close range operation (Blind Spot) ESR of tag coil matched to ESR of chip-capacitor combo for maximum power transfer Q factor

  27. Reader NA Tag Modulation depth increases with low R2’ Too low R2’ tends to make Tag inductive during switched state and may degrade transient response Switched Resistance

  28. 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 pF is common Tank Inductor, Capacitor

  29. Compensated Antenna Motivation: Stray capacitance creating common mode currents Reduction of effective M Detuning + -V V

  30. C 1 2 C: Common C-1: Compensated Mode – 4 turns C-2: Uncompensated – 8 turns Blue Dot: Via NOT TO SCALE

  31. Effect Of Metal

  32. 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

  33. Reader Antenna 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

  34. Eddy (Surface) Currents on Metal Coil B(t) E(t)

  35. 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

  36. Loop Metal Magnetic Field from a Current Carrying Loop

  37. 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

  38. Surface Impedance Zs s = conductivity d = skin depth D.F. Sievenpiper, “High Impedance Electromagnetic Surfaces”, Ph.D. Dissertation, University of California, Los Angeles, 1999

  39. Vi = [R1+R0 + j(wL1-1/wC1)].I1 – jwM13.I3 0 = [R3 + jR3].I3 – jwM13.I1 ~ Equivalent Circuit and Phasor Diagram Reader I1 C1 . R1 R0 Metal L1 + . I3 V wL3=R3 F10 45◦ L3 R3 Fresultant F30

  40. Metal B q2 Ferrite m Mitigation with Ferrite Ferrite: High permeability, poor conductivity m0 q1 B0 Bending increases with • mr • thickness I

  41. mr =100 mr =30 Bending Angle

  42. 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 (FK03 – 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

  43. Ferrite Image current of source current Image Approach PEC

  44. Antenna Design Issues • Effect of Metal

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