[SI-LIST] Re: Impedance confusion

Lakshmi,

If your offset single or differential strip lines are sandwiched between two
planes and have rotational symmetry, the impedances are identical. If the
rotational or mirror symmetry is valid for the complete stackup, the
parameters of two symmetrical lines are identical unconditionally.

Note that if you have one signal layer closer to one plane and another
signal layer closer to another plane in the same stackup and there is
symmetry - the impedances will be identical. However, the strip line
transition from one signal layer to another with a via-hole may become a
substantial discontinuity due to the differences between the modal current
distribution in the planes even if the planes are reference planes for both
lines. Matched transition between two lines requires identity of the
cross-sections regardless of symmetry in general - the identity of the
impedances is not sufficient.

Best regards,
Yuriy Shlepnev
www.simberian.com 

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Lakshmi N. Sundararajan - PTU
Sent: Friday, November 21, 2008 12:18 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Impedance confusion

Hi Team,

Between these two stackup, 
a) SIGTOP-PP(5 mils)-SIGINNER1-PP(22 mils)-SIGBOT
b) SIGTOP-PP(22 mils)-SIGINNER2-PP(5 mils)-SIGBOT

While computing the impedance of the SIGINNER* layer,
it is offset stripline or edge coupled offset stripline.

Would the impedance be different between these two cases?
I ask this because, when board (a) is rotated it becomes
board (b) and hence I do not expect impedance to vary between
these two stackups. 

Can anyone please confirm/clarify on this.

Thanks,
-LN
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