The effect of DC Gain adjustment can again be seen in Figure 3-5, which shows the eye diagrams for each DC Gain level tested after the redriver.

Figure 3-5 Redriver Output Eye Diagram vs. Different DC Gain Settings

As DC Gain is adjusted, variations in eye shape and opening can be seen. Note that adjusting DC Gain affects the eye height (VEO) of each eye diagram, but this adjustment does not greatly impact the eye width (HEO) of each eye diagram.

Comparing each eye diagram against the 0dB result, the following can be observed. At +2dB DC Gain, the DC level (outer eye) increased, but the eye height (VEO) was reduced. At -4dB DC Gain, the DC level and eye height were both reduced. Eye width (HEO) was slightly reduced as well, but in some cases it is observed that negative DC Gain settings marginally improve eye width. Finally, at -6dB DC Gain, the DC level is further reduced and eye width is reduced as well. Again note that in some cases, this may marginally improve eye width. In summary, it can be observed that optimum eye opening occurs when 0dB DC Gain is used, confirming the data sheet recommendation of using 0dB DC Gain.

Ideally, a channel that is equalized with the correct EQ Index and DC Gain value allows a channel signal to be replicated, giving the PCIe receiver a strong push to optimize the link based on what it perceives to be a relatively short low-loss application. This will result in a consistent training result optimized to the PCIe receiver strengths and characteristics.