Suppose we have the following construction:
Zo
/\/\/\+

>
< Z1
>

GND
Anticipating the type of line termination
reflection can be calculated by using
Z1  Z0
Tau = 
Z1 + Z0
Case 1: Z1 = Z0
Therefore, Tau = 0
No reflections.
Case 2: Z1 >> Z0, Z0 = 0, or Z1 = infinity
Therefore Tau ~ 1
Any reflections are positive in nature.
Case 3: Z1 << Z0, Z1 = 0, or Z0 = infinity
Therefore, Tau ~ 1
Any reflections are negative in nature.
What Mr. Cheong has demonstrated is that vias
are of a Case 3 nature.
I have strongly suspected that vias are also of
a differentiator model when Z0 is not equal to Z1.
With Z0 = Z1 it appears as a classic delay line.
Zo Z1
/\/\/\+/\/\/\




GND
Discussion?
Regards, Doug McKean
At 09:46 AM 7/12/99 0500, mcheong@molex.com wrote:
>I agree with Ron's email on via being nothing but a capacitive stub.
>
>One can verify that by measuring a typical via with a TDR.
>Run a length of trace with an open via in the middle and launch a TDR signal
>(~35pS) from one end. Make sure the line is terminated in its characteristic
>impedance. You will discover a negative reflection (capacitive
discontinuity).
>
>The equivalent capacitance can be derived by utilizing the reflected voltage,
>risetime, incident voltage and Zo.
>
>Michael Cheong
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