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Mun Leng Ng Auto-ID Lab @ Adelaide School of Electrical & Electronic Engineering University of Adelaide Australia [email protected]. ANALYSIS OF CONSTRAINTS IN SMALL UHF RFID TAG DESIGN. Introduction on RFID. What is RFID? RFID basic components:. RFID Tag. Consist of - PowerPoint PPT Presentation
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ANALYSIS OF CONSTRAINTS IN SMALL UHF RFID TAG DESIGN
Mun Leng Ng
Auto-ID Lab @ Adelaide
School of Electrical & Electronic Engineering
University of Adelaide
Australia
Introduction on RFID
• What is RFID?• RFID basic components:
HO STCO M PUTER
RFID REA DER RA DIO W AV ES RFID TAG SO N O BJE CTS
RE ADERAN TEN NA
RFID Tag
• Consist of
• Basic requirement:- Compact- Reliable- Inexpensive
M ATCHINGNETW O RK
RFID CHIP(LO AD)
The Constraints
• Bandwidth limitation on impedance matching
• Tag antenna size constraint towards impedance matching
Bandwidth Limitation
Bode-Fano Limit
V S
R S
Z IN
LO SSLE SSM ATC H IN GN E TW O R K
RC
RC
π dω
1ln
0
Bode-Fano Limit (cont)
Ref
lect
ion
coef
fici
ent,
||
A n g u la r f re q u e n c y,
1
| | in b an d
1 2
fRC2
1
inbande
Application of Bode-Fano Limit on RFID
• Allocated bandwidths for RFID:
• Others:China: 917 – 922 MHz (Temporary license can be applied)Australia: 918 – 926 MHz
Application of Bode-Fano Limit on RFID
• 3 different cases considered:
Ref
lect
ion
coef
fici
ent,
||
Frequency, f (in M H z)
1
| | m a x
902 928
f
(a)
Ref
lect
ion
coef
fici
ent,
||
Frequency, f (in M H z)
1
| | m a x
865 868 902 928 950 956
f 1 f 2 f 3
(b )
Ref
lect
ion
coef
fici
ent,
||
Frequency, f (in M H z)
1
| | m a x
f
865 956
(c)
Application of Bode-Fano Limit on RFID
• Assume R = 1 k, C = 1 pF
• R = 10 k, C = 1 pF (for less power consuming tag chip in practice)
Tag Antenna Size Constraint
A Simple Loop Antenna
• Single-turn circular loop antenna:
R
L
r
Overall CircuitR
L
r
R C
Equivalen t circu it o floop an tenna
Equivalen t circu it o fR FID tag ch ip
V s
Qant = (2f L)/Rr Qchip = f0/f
Results
• When Qant = Qchip: - D = 9.65 cm - Impedance of antenna approximately 5 times impedance of chip
• If R = 10 k, C = 1 pF: - D = 4.48 cm - Smaller difference between tag antenna and chip impedances
A Simple RFID Tag
Butterfly antenna
(a modified bow-tie)
Copper foil tuning strap for
impedance matching
A Simple RFID Tag (cont)
• Simplified equivalent circuit:
R CV s R 'r C '
N ew equ ivalen t c ircu it o fbu tterfly an tenna
Equivalen t c ircuit o fR FID tag ch ip
L m a tc h
Lossless m atch ingnetw ork
Another Simple RFID Tag
• Consists of a circular loop antenna with a very simple matching network
FRONT REAR
Conclusion
