[SI-LIST] Re: Impedance measurement using TDR
- From: Steve Corey <steve@xxxxxxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Sat, 10 Nov 2001 15:50:01 -0800
Bob -- you are asking the correct questions regarding measurement and
modeling using TDR waveforms. To answer your questions in order:
1. Your colleague is correct in that your launch cable should ideally be
longer than your DUT. This is not so much to allow the TDR step to
settle as it is to keep any multiple reflections which occur within the
launch cable from affecting your measurements. If you have a
high-quality launch cable, and have done a good job of connecting to your
device, these effects will be second-order or smaller. If you are
measuring any different DUT response for varying lengths of launch cable
(other than simply one being delayed with respect to the other) then it
is probably a result of loss in your launch cable.
2. You are correct that discontinuities will be present and even
dominant at higher edge rates which may not be either at lower edge
rates. You are also correct in that the best picture of what a trace
will look like to your circuit is a measurement taken at the risetime
your circuit will be seeing in practice. Since the risetime of a TDR
step cannot be easily varied, we encourage our customers to filter their
TDR measurements to the risetime in which they are interested. This can
be done on the TDR scope itself, or can be done after the fact.
You can find a lot of useful information on TDR measurements at our web
site, http://www.tdasystems.com
-- Steve
-------------------------------------------
Steven D. Corey, Ph.D.
Time Domain Analysis Systems, Inc.
"The Interconnect Modeling Company."
http://www.tdasystems.com
email: steve@xxxxxxxxxxxxxx
phone: (503) 246-2272
fax: (503) 246-2282
-------------------------------------------
Bob Patel wrote:
> Hi! I was conversing with my colleague about the way
> impedance measurements are done using a TDR and the
> rise times that are employed.
> Our discussion is regarding impedance measurement and
> looking at discontinuity on a trace using TDR.
> My colleague says that I should use a longer coaxial
> cable for injecting the signal from TDR into the short
> trace and that way I can mitigate the settling effects
> of the TDR.
> Well, then the real discontinuities on the trace will
> get masked out because the signal will settle down and
> I could see this effect for the same length of the
> trace except the launch point was far away and so the
> signal could settle down. Then the question arises how
> do you look at discontinuity and if you do with a fast
> rising edge then you could but this will not show up
> with the slower or actual rise time of your signals.
> My thinking is leading me to the fact that in a PCB
> there can be a lot of discontinuity but they could be
> will not be present with the rise time of your signal.
> I conclude that for a trace which is going to carry a
> signal of rise time 100ps then the impedance of the
> trace and any discontinuity has to be measured with
> a signal whose rise time = Fknee.
> I needed to know how should the impedance be measured
> and also look at the discontinuity at the same time.
> Bob
>
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