Transcript Presentation 3

Chip Talks Back
Tag sends signal back to Reader
1
Load Modulation Concepts
I1
C1
+
Vi
I2
R1
~
L1
2
. .
1
C2
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 ↓
2
ISO 14443 Timing
Carrier
Sub-carrier =Carrier/16
Bit rate = Sub-carrier/8
Frequency
Period
13.56 MHz
847 KHz
105.9 KHz
74 ns
1.18 ms
9.44 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
XC2/R
≈C
Lo to
Hi
Convert to a series resonant circuit
I1
C1
+
Vi
~
I2
R1
L1
C2
2
. .
1
L2
R2s
R2s’
Hi to
Lo
5
Assume both primary and secondary resonant at the excitation frequency w0

Vi
wM 2
 R1 
I1
R 2s
I1 
Secondary resistance is
switching between two
values R2s and R2s’
Vi
Vi
Vi


2
2
2


ω0M
ω0.M


R1 
R1 
.R2 R1  k. L1  .R2
2
R2s
XC
L2 

where XC =1/w.C2
2

L1 
k.
 .R2
L2 

2

ΔI1
L1   R2
 Vi.
 Vi.  k.
  . 2 for k <<1
2
2
ΔR2
L2   R1








R1   k. L1   .R2
L2  


 


6
Modulation Depth
ΔI1
 2 L1 R2
 Vi.k
. 2

ΔR2
 L2 R1
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
I1
C1
+
Vi
~
I2
R1
2
. .
L1
C2
L2
1
R2
R2’
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)
Steady State Analysis – no transient considerations
Modulation Depth: Difference in current in Reader Coil due to
switching in Tag
- for 1V excitation in Reader
150
Mod depth mA
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)
100
R2 switched between 5000 and 500 ohms
50
0
0
0.1
0.2
0.3
Coupling coefficient
9
Effect of Tank Capacitor in Tag
Parameter: k
Mod depth mA
1000
0.2
100
0.08
10
1
0.02
0
20
40
60
80
100
C2 pF
10
Effect of Switched Resistance
High value of R2: 5000 ohms
k = 0.05
Parameter: Switched resistance
60
Mod depth mA
500 ohm
40
2000 ohm
20
0
3000 ohm
0
20
40
60
80
100
C2 pF
11
Measurement of Load Modulation
C1
Query
command
Tag
L1
13.56
MHz
C2
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 f0 13.7 MHz
Tag f0 14.0 MHz
17
Effect of Tag Resonant Frequency
k= 20%
400
Reader Antenna Current mA
300
200
100
0
13.56 MHz
-100
-200
-300
-400
400
0
1
2
3
4
Reader Antenna Current mA
300
5
6
7
8
9
10
Time usec
200
100
13 MHz
Good Transient
Modulation Index compromised some
0
-100
-200
-300
-400
400
0
1
2
3
4
5
6
7
8
9
10
Reader Antenna Current mA
300
Time usec
200
100
14.2 MHz
0
-100
-200
-300
18
-400
0
1
2
3
4
5
Time usec
6
7
8
9
10
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
Modulation subjected to asymmetric response
sc
sc
sc
Carrier
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)
• Unloaded Q on PCB
can be high (~50) but
loaded (output
resistance of chip)
brings loaded Q
down.
Tag
• Limited by
– Bandwidth
– Close range operation
(Blind Spot)
• ESR of tag coil
matched to ESR of
chip-capacitor combo
for maximum power
transfer
– Matching network used
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
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
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
Zs 
1 j

 = conductivity
 = 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
I1
C1
R1
R0
L1
+
V
~
.
Metal
.
L3
I3
wL3=R3
45◦
F10
R3
Vi = [R1+R0 + j(wL1-1/wC1)].I1 – jwM13.I3
0 = [R3 + jR3].I3 – jwM13.I1
F30
Fresultant
1  j wM13
I3 
.
.I1
2
R3
39
Mitigation with Ferrite
Ferrite: High permeability, poor conductivity
Metal
B
q2
Ferrite
m
m0
q1
B0
Bending increases with
• mr
• thickness
I
40
Bending Angle
Angle in ferrite deg
100
mr =100
80
60
mr =30
40
20
1
10
100
Angle in air deg
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 (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
42
Image Approach
PEC
Ferrite
mr  1
Image current 
of source current
mr  1
43
• Antenna Design Issues
• Effect of Metal
44