FW: [SI-LIST] : One Differential To Another

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From: George Rasko (grasko@calient.net)
Date: Thu Dec 07 2000 - 17:21:13 PST


-----Original Message-----
From: George Rasko
Sent: Thursday, December 07, 2000 9:40 AM
To: 'RMELLISON@aol.com'
Subject: RE: [SI-LIST] : One Differential To Another

Richard,

I got a few comments back, generally along the lines of what you said:

- differentail impedance is differential impedance = there won't be
reflections
        if the impedances are same, no matter how that is achieved.

- that being said, common mode signals and noise will be reflected and not
attenuated
        if the common mode impedances are not the same.

- making a differential pair by putting opposite polarity signals on
opposite sides of
        a plane probably gives little common mode rejection, since the
electromagnetic
        environments on the opposite sides of the plane are likely
different.

Thanks for asking,

George

-----Original Message-----
From: RMELLISON@aol.com [mailto:RMELLISON@aol.com]
Sent: Thursday, December 07, 2000 5:40 AM
To: George Rasko; si-list@silab.eng.sun.com
Subject: Re: [SI-LIST] : One Differential To Another

George, I was wondering if you ever got an answer to your question on the SI

list.
In my opinion, the differential signal fidelity shouldn't be affected by
changing from a coupled pair to an uncoupled pair as long as the
differential
impedance stays constant. The coupled line would be less susceptible to
crosstalk from an aggressor trace, however. A tighter coupled pair would
have better performance in the presence of crosstalk (higher common mode to
differential mode crosstalk ratio) than a lightly coupled pair.
Richard Ellison

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