3 General Guidelines for Optimal CTLE and Gain Selection

The following steps provide general guidance for selecting the redriver CTLE settings:

1. It is highly desirable to perform signal integrity analysis using actual media, connectors, vias, root complex, end point, and redriver S-parameter models. The aim is to provide margin to meet high requirements of bit error rate and eye dimensions.
2. Understand the overall channel media loss profile. Temperature, humidity, signal-to-noise ratio, and crosstalk all require additional margin for a stable operation.
3. Each end point or root complex requires a certain IO margin. It is desirable to add margin to ensure error-free operation over PVT.
4. The redriver mainly provides boost at its input to compensate for the signal loss. On the other hand, it is limited on its output signal conditioning. When there is high loss on its output (post-channel loss), there are two options for marginal signal conditioning:
   1. Over-equalize the input signal by 1-2 steps, such that the input signal is not compressed.
   2. Provide flat gain to increase eye height.
5. After understanding pre- and post- channel loss, use Table 2-3 to choose starting CTLE settings. Different root complexes, end points, or BIOS revisions can differ in performance. Thus, fine tuning can be required on top of the recommended values shown in Table 2-3.
6. Table 2-3 was measured using a PCIe recommended SI board. Actual applications can have a different loss profile. As such, CTLE indexes mentioned in this table can be a good starting point for an actual application.
7. Verify the CTLE settings for the downstream redrivers. Sweep the CTLE settings of the downstream redrivers up and down from the initially selected CTLE setting (while keeping the initially selected CTLE setting of the upstream redrivers) to determine the range of CTLE settings that allow a successful system link up, if testing an actual system, or meet the minimum predefined eye mask if performing system simulations or measuring eye diagrams.
8. Verify the CTLE settings for the upstream redrivers. Sweep the CTLE settings of the upstream redrivers up and down from the initially selected CTLE setting (while keeping the previously chosen CTLE setting of the downstream redrivers) to determine the range of CTLE setting that allow a successful system link up or meet the minimum predefined eye mask.
9. Another general, systematic method of tuning is as follows:
   1. Determine if the redriver use case is a system or AIC format. This can determine what end of the PCIe link (root complex or endpoint) that the redriver is most associated with, which is referred to as the main link partner for now.
   2. Perform TX compliance testing in the downstream direction from the main link partner. The objective is for the downstream redrivers to compensate for the pre-channel loss: their direct output needs to reproduce the signal originally transmitted by the main link partner as closely as possible. Once this is achieved, the CTLE settings for the downstream redrivers can be frozen.
   3. Next, perform IOMT or lane margining testing in the upstream direction. The objective is to tune the upstream redrivers such that the signal received by the main link partner is of acceptable quality. Different tools and criteria can be used for this process. Successful upstream tuning needs to result in a received signal that not only passes all requirements but has enough margin to compensate for some levels of noise, stress, and other unforeseen effects.
   4. This overall method is not always possible due to physical or software limitations, but is the most comprehensive approach to redriver tuning and is recommended if possible.