Modeling and Design of Small, Passive, and Standard-compliant Proximity Coupling Transponders

Proximity coupling (13.56 MHz) RFID transponders of today utilize small form factor antennas and are passive in terms of power supply. They are predominantly used in smart wearables (e.g., watches, bracelets, rings). Such transponder sizes demand careful design regarding energy transfer and longest achievable operating distance, so that compliance to standards (e.g., EMVCo) can be maintained.This work compares three design approaches for these transponders: IC-only tuning, optimized tuning, and power matching, along with their advantages, disadvantages, and reasoning behind the choice of right transponder components (antenna and IC). A simulation framework based on linear and nonlinear modeling is also presented. It offers more degrees of freedom in searching for the optimal design and predicts the most important figure of merit for these transponders: the minimum operating magnetic field strength (Hmin).Compared to measurements, our framework was able to predict Hmin with an accuracy of under 0.1 A/m. The three design approaches were applied on rectangular 20×25 mm transponders that were modeled within the framework and subsequently fabricated. Using power matching (Hmin=0.78 A/m, Q=27.64) resulted in a significant improvement over IC-only tuning (Hmin=1.71 A/m, Q=3.98) and optimized tuning (Hmin=1.31 A/m, Q=5.02). An even smaller (20×20 mm) transponder with power matching was still able to fulfill EMVCo compliance and achieve a reading distance of 7 cm with a typical payment reader.