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 mng@eleceng.adelaide.edu.au

Introduction on RFID What is RFID? RFID basic components:

RFID Tag Consist of Basic requirement: - Compact - Reliable - Inexpensive

The Constraints Bandwidth limitation on impedance matching Tag antenna size constraint towards impedance matching

Bandwidth Limitation

Bode-Fano Limit

Bode-Fano Limit (cont)

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:

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:

Overall Circuit Qant = (2f 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

Copper foil tuning strap for impedance matching A Simple RFID Tag Copper foil tuning strap for impedance matching Butterfly antenna (a modified bow-tie)

A Simple RFID Tag (cont) Simplified equivalent circuit:

Another Simple RFID Tag Consists of a circular loop antenna with a very simple matching network FRONT REAR

Conclusion