Feed-Forward Equalization

Figure 1: FFE creates a number of delayed versions of the
input signal that are then added back to the signal with
proper weights

In addition to continuous time linear equalization (CTLE), another means of improving signal quality at the receiver end of a high-speed serial data link is known as feed-forward equalization (FFE). In terms of implementation, FFE is not unlike the pre-emphasis filtering that is done on the transmitter side. An FFE implementation looks for all intents and purposes like a digital finite impulse response (FIR) filter.

Continuous Time Linear Equalization

Figure 1: A CTLE implementation at the receiver end of a
serial-data channel seeks to boost higher frequencies while
not boosting noise any more than necessary

We've been looking at the broad topic of debugging high-speed serial links, and in that context, we're also touching on ways to improve signal performance on the receiver side. One of those ways is to implement continuous time linear equalization (CTLE).

Serial-Data Channel Emulation and S Parameters

Figure 1: Higher data rates + "same old" channel media
= degraded signal quality at receiver

Serial data rates have risen but propagation media for the channel remain unchanged, and that results in greater attenuation to the frequencies of interest. We could ignore these losses at lower frequencies, but now that rise times are so much faster, that's not an option. Channel effects now intrude into design margins to the point where eyes deteriorate and bit-error rates become unacceptable.