[SI-LIST] : SMA launch

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From: Michael Allen (mallen@teraconnect.com)
Date: Mon Jun 04 2001 - 10:44:32 PDT


Group,
        I have a problem here we are trying to launch an SMA connection
inside a multilayer PC board. I have found that the 50 ohm impedance of
the board drops to around 28 ohms. I have discovered that the power and
ground layers around the through hole via are minimum distance from the
via barrel. This can account for some of the impedance reduction due to
the capacitive coupling of the barrel to the power and ground planes.
Does anyone have other suggestions as to how to increase this impedance
to near 50 ohms. These lines are all striplines by the way so even in
the launch area I need a plane above and below the signal. Now, should
these planes also be pulled back from the barrel but that would cause a
discontinuity in the signal. I realize that this is a complex problem
and that there may be many different solutions. Thank you in advance.

Mike Allen
mallen@teraconnect.com

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