[SI-LIST] Fw: Re: Re: Relevance of Common Mode Return Loss
- From: olaney@xxxxxxxx
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 19 May 2008 14:03:50 -0700
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Small change: Rdm needs to be center tapped to ground for the CMC to do
its magic for the receiver. This is independent of whether Rcm is
implemented or not. I'm not going to bother tweaking the schematic.
Orin
Since we don't want them, common mode signals can be treated differently
than for differential mode. Given that common mode is undesirable, at
the transmit end we often use a deliberate mismatch (CMC) to reflect this
signal back to the transmitter. This energy can be absorbed by the
transmitter if there is an adequate common mode backmatch, or it can be
left to ring between the driver and choke if that is considered harmless.
At the receiver, the intent of providing a common mode termination is
simply to prevent unwanted CM energy from returning up the line, giving
it an additional chance to radiate. If the common mode signal is
terminated before the differential signal passes through a CMC to reach
the DM termination at the receiver, then the best of both worlds is
achieved: the CM signal is both absorbed and suppressed. The receiver
common mode range becomes much harder to violate. For coding with a zero
at DC (accepts AC coupling), a center tapped inductor is an easy way to
provide the CM termination:
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