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.