Linear equalization filter for PMMA fiber channels

The use of PolyMethylMethAcrylate, Step Index Plastic Optical Fiber (PMMA-SI-POF) channels for high speed data communications is impeded by its intrinsic large intermodal dispersion. The resulting Inter-Symbol-Interference (ISI) makes the successful data recovery at the optical receiver a daunting task. One possible approach to mitigate the ISI problem is the use of an electrical equalization filter to be placed after the Optical-Electrical Conversion block (OEC). To serve this purpose, a 2 biquad, CMOS gm-C filter with individually tunable low and high frequency gain is presented. As the equalization should be independent of fiber length and variation prone OEC characteristics, the filter's tuning range needs to be sufficiently large to accommodate for fiber lengths up to 150 m and OEC bandwidths down to 100 MHz. The presented filter consists of 2 stages; the first has a fixed bandwidth of 150 MHz and a DC gain range of 26 dB. The second stage implements 0 to 24 dB additional signal boosting in a frequency band centered at 110 MHz, which enables the overall system bandwidth to be placed at approximately 100 MHz. The filter was used in a fully integrated, adaptive linear feedforward equalizer implemented in a 3.3 V supply, 0.35 μm CMOS process, targetting a 150 Mbps link.