Hi Harold,
I think I understand what you are after. The generic answer is that IF
you have a
uniform trace and you know the dimensions and material properties, you
can then
predict the actual shape of the curve, which we can approximate with a
constant
slope, but this approximation quickly breaks down with longer traces or
heavier
losses. In practice what we found that the biggest challenge was the
nonuniformity
due to glass-weave effect, etching inconsistency and occasionally the
proximity
of other features (antipads, via barrels, etc). You may find the
following DesignCon
paper useful for further details:
http://www.electrical-integrity.com/Paper_download_files/DC13_Determining_PCB_Trace_Impedance.pdf
Also, just as a reminder: conductive and dielectric losses create tilts
with opposite signs.
Regards,
Istvan Novak
Oracle
On 7/6/2016 5:22 PM, Harold Wang wrote:
Hi Experts,
When performing TDR simulation using S-parameteres, we know that the final
steady-state impedance (Zfinal) is equivalent to the DC impedance of the
DUT and the initial impedance (Zinit, t=0) is represented by the impedance
at the highest recorded frequency in the S-parameter data.
When doing a TDR on an uniform lossy transmission line, we expect the
impedance plot to raise from Zinit to Zfinal due to resistive loss in the
transmission line. Can we predict the slope of this rise? For instance, if
I moved 50ps into this uniform lossy transmission line, what is the
impedance rise that I should expect? What frequency point does that map to
in the S-parameter data?
The end goal is to separate the natural rise of an uniform lossy
transmission line from true impedance increase due to structural change.
Thanks,
Harold
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