[SI-LIST] Re: Impedance measurement using TDR
- From: Bob Patel <whizplayer@xxxxxxxxx>
- To: steve@xxxxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Mon, 12 Nov 2001 10:26:36 -0800 (PST)
Hi Thanks a lot for all the responses I have received
so far and I conclude that I need to get TDR filters
from Agilent in order to do my measurements.
Thanks this has been very useful.
Bob
--- Steve Corey <steve@xxxxxxxxxxxxxx> wrote:
>
> 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
> >
> > __________________________________________________
> > Do You Yahoo!?
> > Find a job, post your resume.
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> >
>
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