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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
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
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