RE: [SI-LIST] : High Speed AC Coupling

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From: Ingraham, Andrew (Andrew.Ingraham@compaq.com)
Date: Wed Nov 01 2000 - 04:35:38 PST


DC block and AC coupling are essentially the same.

But a lot depends on where the terminations are. If the impedance to
terminate the line comes after the cap, then it is much more important to
have a low impedance to all useful harmonics, not only to faithfully couple
the waveform to the receiver, but also to terminate the line without a lousy
reflection coefficient. That is why DC block capacitors must have very wide
bandwidth; the (AC) termination is on the other side.

On the other hand, if the termination is before the cap, and the load seen
on the other (receiver input) side is a relatively hi-Z, then the cap's
impedance can be higher without adversely affecting either the coupling
efficiency or the reflection coefficient. What's important here is
impedance magnitude, not phase angle (inductive vs. capacitive).

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