[SI-LIST] TDR Measurement of a Long PCB Trace - Increased value over time
- From: "ryansatrom" <ryan.satrom@xxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 03 Dec 2009 19:29:59 -0000
Hello All-
When I run a simulation or take TDR measurements of a long PCB trace (>8inch),
I notice that as time passes, the impedance value increases.
I have attributed this to the loss of the line, and simulations verify this.
But I can't figure out why this is the case.
Essentially what I see is that re(S11) of a constant-impedance trace increases
over time, as the signal traverses down the line. And the amount of increase
is proportional to the loss of the line. For example, a 1/2oz trace will
increase at about twice the rate of a 1oz trace, due to the higher bulk
resistance of the thinner trace.
My expectation is that the more loss in a trace, the less amount would be
reflected back to the source, since the signal loss is being dissipated into
the conductor and dielectric, thus leaving less to make it back to the source.
This is not what the results are showing me. What am I missing?
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