A.C. capacitors in PCIE spec

[Haier]

When study PCIE 3.0 spec, I find it describes that the A.C capacitors must be placed on the Transmitter side of an interface that permits adapters to be plugged and unplugged.
Also I know the coupling capacitor always placed on the transmitter side.

But I don't know why. Anybody please help me to understand this, or give some advice such as which book I should study.
Thank you!

 

[chuckey]

Most amplifiers that supply an output have a DC bias on their output. if you just put a load on the RF, you are shorting this bias voltage to earth by the resistance of the load. Putting a capacitor in series lets the RF through but stop the DC level being shorted out.
Frank

 

[FvM]

The "capacitors on TX" can be found through all PCIe specifications version 1 to 3.

AC coupling does reduce the effect of DC offsets, in so far it's reasonable. And it's possible due to the DC balanced signalling.

Capacitors are placed on the TX side in PCIe just by convention. The specification also requires a DC path (<200k) on the RX side, apparently to avoid capacitors to charge up and possibly destroy receivers or transmitters on plugging.

 

[Haier]

Thanks for your reply.
It is placed on TX side just by convention?
I thought it was for some kind of SI concern.
Otherwise the AC capacitors could be placed anywhere easy for layout, why make a convention to limit. Is there any advantage to place on Tx side?