[SI-LIST] Re: TDR Measurement of a Long PCB Trace - Increased value over time
- From: wolfgang.maichen@xxxxxxxxxxxx
- To: "ryansatrom" <ryan.satrom@xxxxxxxxxxx>
- Date: Thu, 3 Dec 2009 12:33:47 -0800
Hello Ryan,
just imagine a loss-less line (Zo=50Ohm) with a small-value ohmic resistor
(resistance dR << Zo) somewhere in the middle. If you do a TDR of this
line, the resistor will cause a small positive reflection, because there
is an impedance mismatch between the section before (50 Ohm) and after
(total impedance = Zo + dR, i.e. series combination of the second line
segment and the resistor); you'll see a small step in the TDR reflection
profile, with a height proportional to the line impedance:
rho = (Zo + dR - Zo) / ( Zo + dR + Zo) = dR / (2*Zo +dR) ~ dR / (2*Zo)
Now place a larger number of those resistors alogn the line, each in equal
distance from the previous one. You'll get a series of such steps
(higher-order reflections will be negligible because we assumed dR << Zo).
In the limit (infinite number of infinitely small resistors infinitely
close together) this will appoach a straight line with an upwards slope
proportional to the ohmic resistance per unit length.
In your trace the ohmic resistance can be caused by two effects - DC
resistance and skin effect (the latter is just aggravated DC resistance
due to current redistribution).
For further reading I'd recommend Howard Johnsons book "High-speed signal
propagation" which talks about this upwards slope as well.
Wolfgang
"ryansatrom" <ryan.satrom@xxxxxxxxxxx>
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12/03/2009 11:29 AM
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Subject
[SI-LIST] TDR Measurement of a Long PCB Trace - Increased value over time
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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