Design Of The Electronicoptic And Optoelectronic Conversion Interface Chip For RoF System

Nonlinearity is a hot topic for RoF (Radio over Fiber) system. Most devices inRoF system will introduce a certain degree of nonlinear distortion such as laser,lightning modulator, fiber optic link and detector. In order to correct the nonlineardistortion in RoF system, this paper employed the lasers driving circuit with anologpredistortion to correct the nonlinear of the electro-optical conversion device; andthe automatic gain control optical receiver to correct the photoelectric conversionchip by circuit design, simulation and tape-out, and completed the following contentof research:A standard CMOS compatible laser drive circuit with the analog predistortionis designed. The laser driver circuit uses the UMC0.18μm CMOS process, the chiparea is1424μm×912μm. By Spectre simulation, the bandwidth of the laser drivecircuit is about2.15GHz, the gain is13.2dB.To correct the nonlinear characteristics of the laser, designed a nonlinear curvefor the analog predistortion circuit. As the MOSFETs have different I-Vcharacteristics under different width to length ratio and different bias voltage, thiscircuit use it as the current input of the laser drive circuit by adding severalMOSFET current to realize the predistortion characteristic in one chip.An optical receiver with automatic gain control was implemented in standardCMOS technology. The optical receiver circuit using the UMC0.18μm CMOSprocess design to achieve an area of850μm×800μm, working in the data rate of1.5Gb/s with clear eye diagram, The dynamic range of the gain is21.28dB, and havegood dB linear; the setup time of the gain modulation is less than50ns and thebandwith of the circuit is about1.8GHz.To correct the nonlinear characteristics of RoF system, a variable gain amplifiercircuit with an exponential function circuit takes the place of the amplitude limitamplifier. Through the negative feedback provided by the peak detection circuit andthe comparator, realize the amplifier automatic control. As the variable gainamplifier’s output signal also has nonlinear characteristic, employs the exponentialfunction circuit could correct the VGA’s output into dB linear. Through the Spectresimulation, the control voltage varies from0to1.8V, the gain of VGAchanges from -100dB to8.66dB accordingly. From0to about1.2V, the whole VGA module andthe optical receiver circuit have excellent dB linearity to the control voltage.