Transmitter Equalization for 4Gb/s Signalling

To operate a serial channel over copper wires at 4Gb/s, we incorporate an 4GHz FIR equalizing filter into a differential transmitter. The equalizer cancels the frequency-dependent attenuation caused by the skin-effect resistance of copper wire giving a frequency response that is flat to within 5% over the band from 200MHz to 2GHz even over wires with 6dB of high-frequency attenuation. All but the last stage of the transmitter operates at 400MHz. The transmitter output stage uses a stable 10-phase 400MHz clock to sequence an array of drivers that implement the FIR filter. This paper introduces the concept of digital-signal equalization, describes the system design and circuit design of our equalizing transmitter, and presents simulation results from a 4Gb/s 0.5μm CMOS transmitter.