EQUALIZATION - Advanced Signal Integrity for High-Speed Digital Design - page 506

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Figure 12-9 Sinc function.

12.1.4 Equalization Overview

Our study of high-frequency interconnects has established that they typically
impart a low-pass effect on high-speed signals, which causes amplitude and
phase distortion. The typical result is that the interconnect channel closes the data
“eye.” An eye diagram is constructed by superimposing numerous consecutive
bits in a data stream. The opening of the eye is a metric often used to judge the
quality of the signal integrity; an open eye generally indicates a recoverable bit
stream, and a closed eye indicates a distorted bit stream that is not recoverable.
The eye diagram is discussed fully in Chapter 13. Figure 12-10a shows an open
eye at the output of a transmitter. Figure 12-10b shows the eye at the receiver
after the data stream has propagated across a lossy interconnect. Note that the
interconnect has reduced the area of eye opening dramatically. To develop our
conceptual understanding further, we consider the waveforms in Figure 12-11.
Figure 12-11a shows the transmitter output for a bit pattern that contains sections
with alternating logic levels for each consecutive bit, along with sections that
maintain a given logic level for multiple bit positions. As a result, the signal
power spectrum spans a wide range of frequencies, with significant content from
dc to beyond the 5-GHz fundamental frequency. The high-frequency components
get attenuated much more severely than do the low-frequency and dc components,
as Figure 12-11b demonstrates. A bit pattern of alternating logic states (101010)
will have a fundamental frequency that is higher than that of bit patterns that
have fewer transitions (such as 110011001100).
Since losses increase with frequency, bit patterns that have a higher frequency
content will be attenuated more. In other words, for fast bit patterns, the time it
takes for the signal to “charge up the interconnect” and transition to its maximum
value is greater than the switching rate of the transmitter. This is the intersymbol