[SI-LIST] Re: Questions on GND Ref Plane and Diff pair coupling
- From: Ravinder.Ajmani@xxxxxxxxxxxxxx
- To: "Yassin, Adnan" <adnan.yassin@xxxxxxxxx>
- Date: Tue, 18 Mar 2008 16:50:02 -0700
Hi Adnan,
In a two layer card, loosely coupled can't provide the desired
differential impedance. In this configuration you will need tighter
coupling between the lines. If you have a 2D field solver then you can
experiment with this. Just move the Ground plane about 500 mils away from
the signal planes, and you can determine what trace separation will be
needed for your impedance target.
Regards, Ravinder
Server PCB Development
Hitachi Global Storage Technologies
Email: Ravinder.Ajmani@xxxxxxxxxxxxxx
"Yassin, Adnan" <adnan.yassin@xxxxxxxxx>
03/18/2008 04:32 PM
To
<Ravinder.Ajmani@xxxxxxxxxxxxxx>, "shemshak" <shemshak1@xxxxxxxxx>
cc
<si-list@xxxxxxxxxxxxx>, <si-list-bounce@xxxxxxxxxxxxx>
Subject
RE: [SI-LIST] Re: Questions on GND Ref Plane and Diff pair coupling
Hi Ravinder,
What if there is no GND plane (e.g., 2 layer PCB), is the loosely
coupled scheme still favored over the tightly coupled one?
Thanks,
-Adnan
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Ravinder.Ajmani@xxxxxxxxxxxxxx
Sent: Tuesday, March 18, 2008 1:18 PM
To: shemshak
Cc: si-list@xxxxxxxxxxxxx; si-list-bounce@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Questions on GND Ref Plane and Diff pair coupling
Hi Shemshak,
The coupling between the two lines is much smaller than the coupling
each
line has with the adjacent plane. Hence any discontinuity in the return
plane will affect signal propagation. Absence of a ground plane will
also
result in higher crosstalk.
If you are not short on routing space then loosely coupled traces are
better than the tightly coupled. In such a configuration you need not
route traces next to each other, and it is easier to maintain equal
length
around vias etc.
Regards, Ravinder
Server PCB Development
Hitachi Global Storage Technologies
Email: Ravinder.Ajmani@xxxxxxxxxxxxxx
shemshak <shemshak1@xxxxxxxxx>
Sent by: si-list-bounce@xxxxxxxxxxxxx
03/18/2008 12:37 PM
To
si-list@xxxxxxxxxxxxx
cc
Subject
[SI-LIST] Questions on GND Ref Plane and Diff pair coupling
Hi all,
I have two questions which I really appreciate your feedback on them.
1) If we have differentail pairs which they can reference to each other
(one signal and the other as the return path), then do still need to
have
GND plane adjacent to these signals for the return path? I think the
answer is yes, we need GND plane, but why?
2) When do we use loosly coupled vs tightly coupled differential pairs?
When would be better to use one over the other.
thanks,
Shemshak
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