EQUALIZATION - Advanced Signal Integrity for High-Speed Digital Design - page 507

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Figure 12-10 Example interconnect channel impact on a 10-Gb/s data eye: (a) trans-
mitter output; (b) receiver input.

interference (ISI). Since the disparity of losses between low and high frequencies
is what causes ISI, the slope of the loss curve tends to outweigh its magnitude.
We want the equalizer to counter the low-pass effect of the interconnect, which
causes the high-frequency signal content to be attenuated much more severely
than the low-frequency content. Thus, the desired operation of the equalizer is to
amplify the signal in such a way as to perfectly counter the attenuation at each
frequency. This is expressed mathematically as

H eq (f ) = H − 1

channel (f )

(12-14)

Simply stated, the ideal equalizer has a transfer function that is the inverse of the
channel transfer function, making it a high-pass filter. Figure 12-12 illustrates
this concept, showing that the ideal equalizer counters not only the ampli-
tude distortion caused by the interconnect, but the phase distortion as well. As