[SI-LIST] : Trace impedance measurement for dual stripline

John Lin - TAO (LinJohn@digital.com)
Wed, 11 Feb 1998 10:00:08 +0800

Hi,

Can somebody suggest how to make a coupon for a trace impedance
measurement with TDR?

Currently I use PCB stackup to do trace impedance control. For
measurement purpose, I design a coupon on PCB waste area of a
design. The coupon is 1cm wide and 14cm long PCB board. On the coupon,
there are traces - one trace for one signal layer.

Now, I have a coupon for 4 layer - a dual stripline structure,
G-S-S-P.

My problem is how to connect G and P planes.

I doubt that connecting G and P planes with two vias, which I am used
to do for a TDR's probe, is not right. My last measurement for this
coupon showed the impedance waveform of traces on inner layers like a
ramp - gradually increasing from 69ohms to 87ohms. The expected value
is 90 ohms.

Can somebody explain why the waveform looks like a ramp instead of
straight line. Does it relate to the imperfect connection of G and P
planes.

Best Regards,

JOHNLIN
CAE Engineer of EDA Department
Digital Equipment Corp. Taiwan Branch
Email: Linjohn@mail.dec.com
TEL: 1-886-3-3900000 ext. 2152