USB3.x and AC-Coupling Caps on the RX lines

Question

In the USB3.2 Specification (Sept. 22, 2017) Table 6-18 and Table 6-22 give some characteristics for the TX and RX lines.

Basically, I understand that the TX lines have AC-Coupling capacitors to block DC and let just the AC signals pass. How is the min/max (75nF to 265nF) decided? What if my design has 10nF? Could this somehow effect USB3 enumeration?

The spec also mentions that having AC-Coupling on the RX lines is optional (because the device-side already has series capacitors on its Tx lines, which connect to the Host's Rx lines, so it isn't needed for DC blocking).

But it still states you can have up to ~330nF at the Rx side...I don't really get where this comes from.

This note states:

We recommend 330nF capacitors in order to meet the minimum capacitance requirement (75nF to 265nF) for RX detection.

I don't understand this note, and how having this capacitance can help with Rx detection...(I am working on a USB3 design where we are seeing some enumeration/fallback issues, and wondering if it might have something to do with these capacitance values)

Answer 1

The minimum and maximum capacitancr are decided upon what kind of characteristics the transmitted data has, whether considered in time or frequency domain.

At least 75 nF of capacitance is necessary to pass low enough frequencies, or long bit strings of same bit without transitions so that there is not too much droop. Smaller capacitance will have faster droop and will track common mode or DC bias voltage changes faster.

At at most 265 nF is enough that there is no reason to have higher capacitance, it does not need to pass lower frequencies or longer bit strings that have no transitions. Higher capacitance also takes longer to charge from idle state. It will also be large enough that common mode voltage or DC bias voltage changes slow enough that it is not misinterpreted.

The reason why a 330nF capacitor is suggested is that capacitors have tolerance. A cheap ceramic X7R capacitor typically used for high speed AC coupling might typically have +/- 20% tolerance. As it happens, 80% of 330nF is 264 nF, so they specify a large enough capacitor to include the tolerance of the capacitor.

If you have 10nF capacitors in your design, that is way out of specification, almost a factor of 10 below the minimum specified capacitance and it could easily cause the problems you are seeing.

Answer 2

AC coupling capacitors are essentially high pass filters with a cutoff frequency of \$\frac{1}{2\pi R C}\$ where \$R\$ is the input impedance after your blocking capacitor to whatever network/chip comes next. If your capacitor is too small for the network's input impedance, it will basically filter out the content of you are trying to transmit.