Real-Time Impedance Characterization Method for RFID-Type Backscatter Communication Devices

Abstract

This paper presents a Radio Frequency measurement method that allows to determine electrical characteristics like scattering parameters or impedance of contactless passive chips, using common measurement devices as oscilloscope, personal computer and directional coupler. The measurements obtained using this method are as accurate as those obtained with vector network analyzer (VNA) measurements, while providing several improvements. First, the method makes it possible to measure a chip's impedance in activation state, which requires large input power, not compatible with VNA measurements. Secondly, this method realizes a real time measurement, which consists in impedance measurement as a function of time. Therefore, this real-time measurement makes possible to detect and measure the impedance changes of an actual contactless smart card's integrated circuit during a load modulation communication at different powers. Finally, this real time impedance measurement gives important information on chips electrical characteristics that can be used to optimize NFC (Near Field Communication) devices design in order to reduce the power losses within an RFID (Radio Frequency Identification) tag.

Study on the Reading of Energy-Harvested Implanted NFC Tags Using Mobile Phones

“…Another important aspect not considered in the literature is that the load impedance and loaded quality factor depend on IC impedance. The IC impedance, therefore, depends on the power received and presents nonlinear behavior [38]- [40]. This nonlinearity must be taken into account in both the simulations and the system design.…”

“…Fig.16 shows simulations of the IC equivalent resistance and capacitance as a function of the coupling coefficient for the 2-coil system. Due to the nonlinear behavior of the NFC IC [38]- [40], R IC resistance depends on the power received. The values of R IC obtained are below the limit imposed by the maximum quality factor to ensure the bandwidth required for the backscattering communication given by (12).…”

“…The excitation power level of a commercial VNA is often limited to 20 dBm (over 50 ). A modified VNA setup with an external amplifier and a reflectometer [38]- [40] to characterize the IC under similar power conditions to the actual operation with a reader is required. Experimental results obtained in the above studies for different ICs show the power dependence of the impedance obtained here in the simulations.…”

A Low Complexity Design Framework for NFC-RFID Inductive Coupled Antennas

“…The first step of inductive antennas design is to select an electrical model for the antenna and to determine the value of the complex impedance of the chip Z L for which the design of the coil will be optimized. This value can be found in data sheets or measured using measurement methods as [22], making sure the measured impedance corresponds to the chip impedance when it is in a working state. The choice for including the parasitic capacitance C par is often due to the angular frequency ω of the application.…”

“…As mentioned in section II, this information can be found in the datasheet of the chip, or can be measured directly. In [22], the authors present a method to determine a PICC chip impedance by sending a voltage wave to the chip and analysing the reflected wave, as Vector Network Analysers would do, but with a much higher power. Such high power is necessary to be able to supply the chip, so the measured impedance value corresponds to the working state of the chip.…”