I was looking at the PCIE specification, and I don't understand the requirement for coupling capacitors. For 2.5GT/s, the standard requires AC coupling capacitors of 75nF to 265nF. I tried to analyze the number like in this article in High-Speed Digital Design Online Newsletter. The line is terminated at both ends, the capacitor faces 100Ω, the RC time constant is 7.5µs to 26.5µs, or 19000 to 66000 bit intervals. Since we're using 8b/10b encoding and there are at most 3 ones or zeros in a row, the RC time constant only needs to be large compared with 3 bit intervals, so the capacitance should only need to be large compared with 12pF. The requirement of at least 75nF seems unnecessarily large. Why is that? If the capacitance were allowed to be around 100pF, the capacitors can even be made into the chip.
The requirement for coupling capacitors can be found here, page 357, section 4.3.13.