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Research on the Predistortion Circuit of the RoF System

Author: ZhouXin
Tutor: YuJinLong
School: Tianjin University
Course: Communication and Information System
Keywords: RoF LiNbO3 electro-optic modulator predistortion ADS micro strip
CLC: TN015
Type: Master's thesis
Year: 2009
Downloads: 88
Quote: 1
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Abstract


With the development of microwave radio frequency technology being mature day by day, the problems of loss and interference caused by transmitting high frequency signals for a long distance become obvious. If the signals can be modulated to the optical carrier, and taking fibers as the medium, all the problems can be solved, this is Microwave Photonics technology. Microwave fiber-optic transmission system based on microwave photonics technology adopts Radio over fiber technology, its main idea is to make microwave signal being modulated to the optical carrier, then transmitting and distributing the signal after loading it to fiber.In the launch part of RoF system, one of the most important components is electro-optic modulator. Generally speaking, microwave signals use external modulator. The most commonly used external modulator-- LiNbO3 electro-optic modulator based on Mach-Zehnder structure, its modulation curve is sine curve.When the input signal exceeds the linear range, it will bring distortion. As a result, it will limit the dynamic range of the input signal. If there are some measures aimed at the nonlinearity, the performance of either the modulator or the whole RoF system will be improve greatly. The motive of this paper is to research the nonlinearity compensation of the external modulator. Among the various ways of compensation, the design chooses the predistortion for its simple structure, easy to control and low cost. This method is to use reverse diode in pairs to generate one signal similar to arcsine curve and achieve the effect of complementary to the modulator’s characteristic curve, improve the liberation of modulator, enlarge the dynamic range.The research of this paper starts from the nonlinearity of the modulator, deviating the capability in nonlinearity compensation of the circuit designed in theory. Then combining the circuit with ADS2008 to design and emulator its stability, the smoothness of the gain, VSWR and impendence matching during the working frequency band. Finally, finishing a preliminary design of micro strip of the impedance matching and the connections between the components for the circuit.

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CLC: > Industrial Technology > Radio electronics, telecommunications technology > General issues > Basic theory > Microwave and UHF
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