A 0.18 um CMOS feed forward equalizer for 10 Gb/s data transmission over multimode fiber

Abstract

Intermodal Dispersion (IMD) in multimode fibers resulting in Intersymbol Interference (ISI) is the fundamental limiting factor to higher data rates and longer transmission distances in high-speed optical links. Equalization using a Finite Impulse Response (FIR) filter structure with variable gain and tapped-delay elements presents an effective way to compensate for this dispersion induced ISI. This paper presents the design of a 4-tap FIR filter structure with focus on the design of the gain block and delay line. The tap coefficients are extracted using the channel characteristics. The gain block is implemented using a modified Gilbert cell multiplier with improved gain linearity and the delay line is implemented using an LC ladder structure. The integrated FIR filter structure is implemented in a standard 0.18 um CMOS process. The equalizer is used to equalize a 10 Gb/s On-Off-Keying (OOK) signal after transmission through 500 m of multimode fiber (50 um core).