The importance of coupling capacitance in high speed circuit design is analyzed in detail (II)

In low speed circuit design, this capacitor can be equivalent to an ideal capacitor. In the high frequency circuit, due to the existence of parasitic inductance and the impedance discontinuity caused by the plate, the capacitance can not be regarded as an ideal capacitor. Here the signal frequency is 2.5G, the channel length 4000mil and the AC coupling capacitance are located at the transmitter and receiver 200mil respectively. Let's look at the changes in the emulation of the eye map.

Obviously, when the AC coupling capacitance is close to the receiving end, the signal integrity is better than that at the transmitter side. My understanding is that the non ideal capacitor impedance is discontinuous, the signal reflected after the channel attenuation will be less than the direct reflected energy, so the vast majority of the serial links require the AC coupling capacitor to be placed at the receiving end. But there are exceptions, the author encountered this problem before board to board connection, and the PCIE specification found that if two boards were usually placed on the sending end, the other function of the AC coupling capacitor, overvoltage protection, was also used. For example, SATA, so it is usually required to place near the connector.

To solve the problem of placement, another problem that puzzles everyone is the choice of value. In this way, the equivalent resistance R of our entire serial link is fixed, then the selection of the AC coupling capacitor C will be related to the time constant (RC), the greater the RC, the greater the DC component, the lower the DC voltage drop. In this case, can the AC coupling capacitance increase indefinitely? It is obviously not.