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,
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
CAE Engineer of EDA Department
Digital Equipment Corp. Taiwan Branch
TEL: 1-886-3-3900000 ext. 2152