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

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From: Tom Dagostino (tom_dagostino@mentorg.com)
Date: Tue Oct 31 2000 - 16:22:28 PST


DC block and AC coupling are two terms for the same thing. One is a
negative and the other a positive -- half empty/half full.

Look at the problem as a voltage divider between the complex Xc of the
coupling cap and the Zin of the receiver. As long as enough of the signal
from the source is impressed across the receiver it should work.

I would also make sure the low frequency cut-off of the system matches your
bandwidth requirements.

Tom Dagostino
IBIS and Tau Modeling Manager
SDD
Mentor Graphics Corp.
503-685-1613
tom_dagostino@mentor.com

-----Original Message-----
From: owner-si-list@silab.eng.sun.com
[mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Golian, Alexander
Sent: Tuesday, October 31, 2000 1:45 PM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : High Speed AC Coupling

Help!

I am designing with an optical transponder that is designed to accept a PECL
level electrical signals at 2.5GB/s. The driver outputs CML level
electrical signals. I have to AC couple the signal to make the voltage
levels compatible. The vendor said any old 0.01uf capacitor would do, but I
can't beleive this. I would expect a general purpose capacitor at 0.01uf to
become increasingly inductive past 1GHz already. I would expect that the
capacitor would have to be good out to 12GHz as to not attenuate the first 5
harmonics. Does anyone out there have a good suggestion? I have seen some
very wide band DC blocks. What, if any, is the difference between a DC
blocking cap, and a AC coupling cap?

Thanks

Alex

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