# Chip Talks Back Tag sends signal back to Reader 1.

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Chip Talks Back Tag sends signal back to Reader 1

Load Modulation Concepts 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 ↓ ~ C2 I1I1.. + I2I2 ViVi R2 L2 L1 R1 C1 R2’ 1 2 2

ISO 14443 Timing ‘1’ - ISO 14443 ‘0’ - ISO 14443 Frequency Period Carrier 13.56 MHz74 ns Sub-carrier =Carrier/16847 KHz1.18  s Bit rate = Sub-carrier/8105.9 KHz9.44  s 9.44  s 1.18  s 3

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

Heuristic Analysis ≈ XC 2 /R ≡ R C ≈C Conditions: Valid at a single frequency Valid for Q >> 1 ~ C2 I1I1.. + I2I2 ViVi R2s L2 L1 R1 C1 R2s’ 1 2 Convert to a series resonant circuit 5 Hi to Lo Lo to Hi

Assume both primary and secondary resonant at the excitation frequency  0 Secondary resistance is switching between two values R2s and R2s’ where XC =1/w.C2 for k <<1 6

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 7

Approximations ~ C2 I1I1.. + I2I2 ViVi R2 L2 L1 R1 C1 R2’ 1 2 If XC2 ~ R2’, then equivalent series capacitance becomes > C2 f02 ↓ and may be < operating frequency  Self-impedance of Tag: Inductive  Transient behavior: slow 8

More Detailed Analysis (Numerical) 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 Steady State Analysis – no transient considerations 9

0.02 0.08 0.2 Effect of Tank Capacitor in Tag 10

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

Measurement of Load Modulation  L1 C2 13.56 MHz C1 Scope Tag NFC Forum PD as Reader Query command 12

Bit duration = 9.44  s  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  s 14

H= 3.65 A/m 15

Excitation Frequency = 12 MHz Current decreases during switching 16

Pulse Merge Tag f0 13.7 MHz Tag f0 14.0 MHz 17

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

Bandwidth Requirement 19

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 20

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

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 Selected close to 13.56 MHz Tag 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) 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 26

Switched Resistance 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 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 pF is common 28

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

C 2 1 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 Acting as Reader Or Acting as Tag Antenna could be close to metal Requirement of Tag to be attached on or close to metallic surfaces 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 B(t) E(t) Coil 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

Loop Metal Magnetic Field from a Current Carrying Loop 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 Z s D.F. Sievenpiper, “High Impedance Electromagnetic Surfaces”, Ph.D. Dissertation, University of California, Los Angeles, 1999  = conductivity  = skin depth 38

Equivalent Circuit and Phasor Diagram ~. I3I3 R3 L3 L1 R1 C1 I1I1 V +. Metal R0 Reader Vi = [R1+R0 + j(  L1-1/  C1)].I1 – j  M13.I3 0 = [R3 + jR3].I3 – j  M13.I1  L3=R3  10  30 45 ◦  resultant 39

Mitigation with Ferrite B0B0   I Metal B  Ferrite  Bending increases with  r thickness Ferrite: High permeability, poor conductivity 40

Bending Angle  r =30  r =100 41

  r.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 (  r’’) adds additional loss and need to be maintained within limits 42

Image Approach PEC Ferrite Image current of source current 43

44 Antenna Design Issues Effect of Metal

Measurement of low load modulation 45