Ultra Broadband Predistortion Circuit for Radio-over-Fiber Transmission Systems

摘要

A dual Schottky diode-based ultrabroadband predistortion circuit (PDC) is designed and experimentally verified to linearize both directly and externally modulated radio-over-fiber (RoF) transmission systems. A monolithic dual Schottky diode is used in the PDC to reduce the footprint and improve the bandwidth. The measured S-parameters show that the circuit is able to work for up to 40 GHz. The theory of third order intermodulation distortion (IMD3) generation by the PDC is established, and calculated IMD3 agrees well with the measured. Two measures are used to verify the PDC: spurious-free dynamic range (SFDR) and error vector magnitude (EVM) for WiFi signal at 2.4 GHz. For a directly modulated RoF transmission linearized by the PDC, the SFDR is improved by ~12 dB. For an externally Mach-Zehnder modulator (MZM) modulated RoF transmission linearized by the PDC, the SFDR is improved by higher than 5 dB from 2 to 30 GHz, and in particular higher than 12 dB from 2 to 5 GHz. For 2.4-GHz WiFi signal over the externally modulated RoF transmission linearized by the PDC, the EVM for WiFi signal is improved by up to 5.1 dB for back-to-back and 3.5 dB for 20-km SMF transmission.