I. Introduction
The speed of serial links across copper backplanes has seen a steady rise over the past few years. Backplane serial links need to be able to handle increased channel losses at these higher speeds while still being capable of supporting legacy backplanes that were originally designed for 1–3-Gb/s operation. Advanced equalization techniques are required to remove intersymbol interference (ISI) due to loss mechanisms in copper traces drawn on PCBs. The loss mechanisms include those due to skin effect, dielectric loss, and reflections from impedance discontinuities. Equalization techniques that provide high frequency boost to compensate for channel losses also boost noise or crosstalk, which degrades overall performance. Traditional decision feedback equalization (DFE) architectures have both feedback and feedforward equalizers in the RX. In order to ease design and maintain backward compatibility, the feedforward equalizer is moved to the TX. The transceiver described here has both transmit (TX) equalization in the form of programmable de-emphasis filter (FF) and receive (RX) equalization in the form of DFE to compensate for channel losses. DFE uses clean decisions of previously received symbols to remove ISI in the current symbol. Since it does not boost high-frequency noise such as crosstalk or wideband noise to equalize the channel, this technique can be suitable for backplane environments with high channel count. DFE is vulnerable to error propagation because an error made during a decision will influence future decisions through the feedback equalizer. However, the target bit error rate (BER) in backplane applications is already very low , and the degradation due to error propagation is acceptable in most cases.