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.

Rise-Time Degradation and ISI Jitter

Figure 1: Shown are the signals from two extreme bit patterns
overlaid on top of each other with no interconnect in the channel

In discussing inter-symbol interference (ISI), the phenomenon in which information "leaks" from one bit to subsequent bits, we've identified a couple of root causes of ISI jitter. The first is reflection losses caused by impedance discontinuities, while the second is group delay dispersion, a consequence of the differing propagation speeds of different frequencies through a given material. We looked at these forms of distortion in both the time and frequency domains.

Inter-Symbol Interference (or Leaky Bits)

Figure 1: Inter-symbol interference, or ISI jitter, is the result
of information from one bit "leaking" to subsequent bits

In reviewing the subject of debugging high-speed serial links, one important aspect of signal integrity we must touch on is inter-symbol interference (ISI). ISI is the phenomenon in which information from one bit "leaks" to some subsequent number of bits.