Adaptive Equalization, or “The Math is in the Chip”

Modern high-speed I/O equalization schemes typically include
both fixed (programmable) and adaptive components to ensure signal integrity
even in adverse system conditions. What tools are available to ensure that
these equalization techniques are working properly on a given system?

Equalization is a processing technique used to ensure signal
integrity on multi-gigabit SerDes I/O. At higher speeds, inter-symbol interference
(ISI) and crosstalk can distort signals to the extent that the eye (margin) on
a channel is essentially closed. Think of equalization as “the math inside the
chip” (on both the transmit and receive side) which helps reconstruct the
signal and provides sufficient margin that your board works properly. When done
well, and under good conditions, equalization can allow silicon receivers to
identify bit levels at a bus’s desired bit error rate (BER) even when the eye
diagram measured on the board’s interconnect is essentially closed.

Although different serial bus technologies like PCIe, SATA,
USB 3.0, etc. use different approaches, equalization can be classified in two
general categories: fixed and adaptive. Fixed techniques are “locked in” during
board configuration. A fixed receiver scheme such as CTLE (Continuous-Time
Linear Equalization) has limited tuning ability and is quite susceptible to
both silicon and board manufacturing variances, but has the benefit of a small
logic circuitry footprint in the chip. Adaptive equalization, such as DFE
(Decision Feedback Equalizer) and AGC (Automatic Gain Control) continually self-adjusts
based upon the unique characteristics of the system – board and silicon
process, temperature, voltage, and other variances. These adaptive technologies
are of course more expensive from a silicon footprint perspective.

It is worthwhile noting that equalization can actually have
an adverse effect on random noise, such as that induced by crosstalk. On the
transmitter side, pre-emphasis and de-emphasis can make crosstalk worse. On the
receiver side, CTLE and FFE (feed-forward equalization) amplify crosstalk
noise. DFE has no effect on crosstalk noise. A good discussion on this is in
the Test & Measurement World article, Crosstalk
problems are back

So, given the above, it’s highly worthwhile having tools based upon embedded instrumentation to
help set and tune the equalization parameters, and measure true margins at the
silicon level (not on the board interconnect itself). A great technical paper on the
effects of equalization on signal integrity is here: