Wireline equalization using pulse-width modulation

Abstract

High-speed data links over copper cables can be effectively equalized using pulse-width modulation (PWM) pre-emphasis. This provides an alternative to the usual 2-tap FIR filters. The use of PWM pre-emphasis allows a channel loss at the Nyquist frequency of ~30dB, compared to ~20dB for a 2-tap symbol-spaced FIR filter. The use of PWM fits well with future high-speed low-voltage CMOS processes. The filter has only one `knob', which is the duty-cycle. This makes convergence of an algorithm for automatic adaptation straightforward. Spectral analysis illustrates that, compared to a 2-tap FIR filter, the steeper PWM filter transfer function fits better to the copper channel. This applies to both half-symbol-spaced and symbol-spaced 2-tap FIR filters. Circuits for implementation are as straightforward as for FIR pre-emphasis. In this paper new measurements are presented for a previous transmitter chip, and a new high-swing transmitter chip is presented. Both coaxial and differential cables are used for the tests. A bit rate of 5 Gb/s (2-PAM) was achieved with all cable assemblies, over a cable length of up to 130 m. Measured BER at this speed is <10-12