Circulator Design for UHF RFID Applications

• Main Authors: Huu-Hung Nguyen, 阮友興
• Other Authors: Chien-Wen Chiu
• Format: Others
• Language: zh-TW
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/09428127870091682042

碩士 === 國立宜蘭大學 === 電子工程學系碩士班 === 101 === The thesis aims at designing a high isolation circulator for UHF RFID reader applications. The circulator is a coupled-line directional coupler implemented by a parallel-line microstrip line structure. When the transmitter in the reader of a UHF RFID system sends EM waves to interrogate, the backscattered receiving EM wave from tags is working at the same frequency with the transmitter signals. The circulator of the reader must have a capability of very high isolation between the transmitting and receiving ports to avoid decreasing sensitivity of the receiver since larger transmitting signals will interfere with the weaker backscattering signals. The isolation of the circulator must be lower than -50 dB. In this thesis, how to improve isolation of a circulator is our research focus. A microstrip directional coupler is employed to design the circulator. The theory of coupled line which is based on the even- and odd-mode theory is studied. The relationship between the coefficient, mode impedances, and the correspondent geometry sizes of the coupled microstrip line is derived. A -20 dB coupled line directional coupler is designed and a practical structure is fabricated to test. The experimental study on the constructed prototypes validates the numerical accuracy. The conventional microstrip directional coupler has poor isolation due to its unequal phase velocity between even mode and odd mode, which propagate on the coupled line. In chapter 3, to improve the isolation, a dielectric overlay is added on the top of a quarter-wavelength coupled microstrip line to let the phase velocities consistent. We study the effective dielectric constant of the modes by changing the thickness of the added dielectric overlay to design the dielectric overlay. Simulation results demonstrate that the designed directional coupler with a dielectric overlay obtains a good isolation of -90 dB at 915 MHz. The simulation results are also verified by a FEKO simulation tool. In chapter 4, a single stub and dual stub added at the coupling port are proposed to improve the isolation. To make use of mismatch at the stub in the coupling port, the reflected signal from the stubs is cleverly employed to cancel the undesired signals reflected from the antenna coupling to the receiver port and the undesired coupling signal generated by the transmitter. The ideal makes the isolation between the transmitter and receiver greatly improved. By introducing the stubs, the proposed directional coupler has a maximum isolation of -68 dB at 915 MHz. Finally, measurements are performed to verify the validity of the simulation results.