Biography
Enrollment Date: 2010
Graduation Date：2013
Degree：M.S.
Defense Date：2013.05.28
Advisors：Chun Zhang
Department：Institute of Microelectronics，Tsinghua University
Title of Dissertation/Thesis：Design and Implementation of a Leakage Cancelling System For RFID Reader
Abstract:
RFID（Radio Frequency Identification）has much superiority which contains non-contact, long reading and writing distance and repeatable wipe. Because of low tag cost, passive UHF RFID has an advantage in the middle or long distance. But it will meet the problem of carrier leakage, so there will be a part of pure carrier leaks from transmitter to receiver, which brings the phase noise of the pure carrier and the noise of the transmitter to the receiver.Due to large leakage carrier power, the low noise amplifier of the receiver may saturation and can’t work, which will reduce the receiver sensitivity. Cancelling the leakage carrier, which means improving the isolation between the transmitter and receiver and improving the receiver sensitivity, has become an important direction of UHF RFID research.At the same time, the high-speed read/write of UHF RFID requests to adjust the system quickly.This paper uses the reflection cancellation technique, adopts a directional coupler as isolation device and devises an adaptive leakage carrier cancelling system for UHF RFID. This paper has carried on the detailed theoretical arguments about the theory and carried on the detailed system, which is composed of directional coupler, impedance tuning circuit, measurement module and control module. Then analysing each module in detail and puts forward the specific goals and requirements. According to the need of control module, this paper has studied the the control algorithm. Drawing lessons from the steepest descent method which is for the multivariable unconstrained problem and conducting theoretical derivation, this paper devises the automatic control algorithm similar to the steepest descent method and tests it in the FPGA. The adjusting time is on an average of 100 ms and the working efficiency is improved by 94%.This reseach has conducted two practical application tests with the adaptive leakage carrier cancelling system by the discrete devices and reusing the receiver.By the discrete devices, the measurement module is composed of directional coupler, low noise amplifier and power detector. Leakage carrier power measurement is completed by power detector. The control module uses the FPGA, adopting automatic adjustment control algorithm. Measured result is that the isolation of system can reach more than 60 dB within 896~905MHz, peak value can reach 70 ~ 80 dB.By resuing the receiver, measurement module multiplexes receiver and the leakage carrier power measurement is completed by the downconverter of the receiver, four differential dc ouputs of the downconverter is as a symbol of leakage carrier signal after processed. The control module is still performed by FPGA. This way is more suitable for integration of the dedicated reader chip and the cost is lower. Measured result is that the isolation of system can reach more than 50 dB within 896~905MHz, peak value can reach 60 dB.