The Effects of De-Emphasis on Eye Diagrams

Figure 1: This eye diagram of a serial-data stream as measured
at the receiver shows the effect a lossy channel has
on signal quality

Having discussed what transmit pre-emphasis is all about and the various ways in which it's implemented, it might be useful at this juncture to look at some examples of its application and the salutary effect it can have on the signal's eye diagram at the receiver end of the channel. Recall that there are two variations on pre-emphasis: de-emphasis and pre-shoot, which use different taps of a three-tap finite impulse response (FIR) filter to emphasize the last bit or the first bit of a bit sequence, respectively.

Introduction to Channel Equalization

Figure 1: Transmit pre-emphasis pre-distorts signals
in anticipation of the channel's effects

A number of factors can cause degradation of performance in a high-speed serial data link. Among them are inter-symbol interference (ISI) jitter, attenuation, reflections due to impedance mismatches, and insertion losses, to name a few. But fear not: There are techniques one may use to compensate for these losses known as equalization. We'll review the basics of channel equalization in today's post.