1.1.1.4 Decision Feedback Equalizer

In the receiver front end, an optionally enabled 5-tap decision feedback equalizer (DFE) is available to equalize the lane response in conjunction with the CTLE. The DFE allows better compensation of transmission channel losses than a linear equalizer of CTLE, by providing a closer adjustment of filter parameters. The tap values of the DFE are the coefficients of this filter that are set by the adaptive algorithm.

The DFE mitigates lane noise or inter-symbol interference (ISI) caused by reflections or cross-talk without amplifying the high-frequency noise within the data. The DFE-based operation uses current bit information to cancel ISI for the following bit through a feedback mechanism, allowing the following bits to be correctly sampled. Using taps to delay and multiply the symbols, the DFE effectively cancels out interference on the analog signal. Similar to the CTLE operation, the DFE has an automatic mode. When the DFE is used in automatic mode, the CTLE can be in automatic mode as-well.

The operation is nonlinear, allowing it to overcome the notch response that the CTLE cannot perform. The DFE also includes an automatic calibration that finds the best possible tuning to match the transceiver lane to the system channel.