Adaptive Equalizer

A characteristic of all cabling is that as length increases, high frequencies in the transmitted waveform will attenuate. This characteristic, as a function of frequency, can be modeled by:

Where ks and kd are the skin and dielectric loss constants of the particular cable, l is the cable length, and f is frequency. The effect of this attenuation is that the baseband pulse data stream will loose its sharp corners and appear to be somewhat smoothed. This effect can cause an increase in received bit error rate due to inter-bit interference and because of reduced noise margins due to eye closure. To mitigate these effects it is necessary to compensate for this frequency and cable length dependent attenuation. GVIF systems include an equalizer integrated into the receiver IC for this reason. Due to the relative sizes of the skin effect and dielectric loss constants, in the region less than 1 to 2 Gbps an effective model of the inverse of the above square root frequency dominated characteristic is:

The GVIF equalizer is self adaptive, automatically compensating for changes in the loss characteristics of the cable – perhaps due to different lengths or to a change in connectors used. Figure 4-4 provides both a block diagram of the equalizer circuit.

Figure 4-4: GVIF Adaptive Equalizer for Transmission System Losses

The overall effect of the equalizer is to mitigate the effects of high frequency attenuation on the received signal. Figure 4-5a shows a received waveform with no equalization and figure 4-5b shows the same signal after compensation with the GVIF equalizer. As can be seen from the figure, the post-equalizer waveform data value is much more discernable and should therefore result in lower errors in the bit decision process.

Figure 4-5: Pre and Post Equalizer Waveforms