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

I1

. .+

I2

Vi R2L2L1

R1C1

R2’

12

2

ISO 14443 Timing

‘1’ - ISO 14443

‘0’ - ISO 14443

Frequency Period

Carrier 13.56 MHz 74 nsSub-carrier =Carrier/16 847 KHz 1.18 sBit rate = Sub-carrier/8 105.9 KHz 9.44 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

≈ XC2/R≡RC

≈C

Conditions: Valid at a single frequency Valid for Q >> 1

~

C2I1

. .+

I2

Vi R2sL2L1

R1C1

R2s’

12

Convert to a series resonant circuit

5

Hi toLo

Lo toHi

Assume both primary and secondary resonant at the excitation frequency 0

s2R

M1R

1

i 2

I

V Secondary resistance is switching between two values R2s and R2s’

.R2

L2L1

k.R1

Vi

.R2XC

ω0.MR1

Vi

R2sω0M

R1

ViI1 2

2

22

where XC =1/w.C2

2

2

22

2

R1

R2.

L2

L1k.Vi.

.R2L2L1

k.R1

.R2L2L1

k.

Vi.ΔR2

ΔI1

for k <<1

6

Modulation Depth

22

R1

R2.

L2

L1kVi.

ΔR2

ΔI1

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

I1

. .+

I2

Vi R2L2L1

R1C1

R2’

12

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

0 0.1 0.2 0.30

50

100

150

Coupling coefficient

Mod

dep

th m

A

Both Reader and Tag tuned to 13.56 MHz

L1= 306 nH C1= 450 pF Q1= 8.7L2= 2755 nH C2 = 50 pF Q2 = 33.5 (unloaded)

R2 switched between 5000 and 500 ohms

Steady State Analysis – no transient considerations

9

0 20 40 60 80 1001

10

100

1000

Parameter: k

C2 pF

Mo

d d

epth

mA

0.02

0.08

0.2

Effect of Tank Capacitor in Tag

10

High value of R2: 5000 ohms

k = 0.05

0 20 40 60 80 1000

20

40

60

Parameter: Switched resistance

C2 pF

Mod

dep

th m

A

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 MHzCurrent decreases during switching

16

Pulse Merge

Tag f0 13.7 MHz Tag f0 14.0 MHz

17

1 2 3 4 5 6 7 8 90 10

-300

-200

-100

0

100

200

300

-400

400

Time usec

Rea

der A

nten

na C

urre

nt m

A

1 2 3 4 5 6 7 8 90 10

-300

-200

-100

0

100

200

300

-400

400

Time usec

Rea

der A

nten

na C

urre

nt m

A

13.56 MHz

13 MHzGood TransientModulation Index compromised some

1 2 3 4 5 6 7 8 90 10

-300

-200

-100

0

100

200

300

-400

400

Time usec

Rea

der A

nten

na C

urre

nt m

A

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

scsc

Modulation subjected to asymmetric response

21

Carrier

Carrier

Modulation depth is reduced

+sc

-sc

scsc

Carrier

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 currentsReduction of effective MDetuning

+

V -V

29

C

2

1

C: Common C-1: Compensated Mode – 4 turns C-2: Uncompensated – 8 turnsBlue Dot: Via

NOT TO SCALE30

Effect Of Metal

31

Tag and Reader Application

Acting as ReaderOrActing 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 Zs

D.F. Sievenpiper, “High Impedance Electromagnetic Surfaces”, Ph.D. Dissertation, University of California, Los Angeles, 1999

j1

Zs = conductivity = skin depth

38

Equivalent Circuit and Phasor Diagram

~

.I3

R3L3

L1

R1

C1I1

V

+

.MetalR0

Reader

Vi = [R1+R0 + j(L1-1/C1)].I1 – jM13.I3 0 = [R3 + jR3].I3 – jM13.I1

1.3R

13M.

2

j13 II

L3=R3 10

30

45◦

resultant

39

Mitigation with Ferrite

B0

I

Metal

B

Ferrite

Bending increases with• r• thickness

Ferrite: High permeability, poor conductivity

40

Bending Angle

1 10 10020

40

60

80

100

Angle in air deg

Ang

le in

fer

rite

deg

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 1r

1r

43

44

• Antenna Design Issues

• Effect of Metal