[SI-LIST] Re: Impedance measurement using TDR

Bob,
before spending any money on additional hardware see if your TDR machine of
choice includes the "normalization" feature.  As far as I know this is only
available from HP/Agilent.  Normalization is mathematical post-processing of
the TDR waveform to remove discontinuities in the measurement fixture(it
requires an additional calibration step using short and 50ohm terminators at
the point of connection to your DUT).  It also allows you to filter the
original TDR step to a gaussian step with any specified risetime from 10ps
to several nanoseconds.  The entire TDR waveform including reflections
receives the same filtering, so this is exactly what you need to take
measurements at the risetime of your logic.

Jeremy Plunkett
Signal Integrity Engineer
ServerWorks Corporation
jeremy@xxxxxxxxxxxxxxx


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Bob Patel
Sent: Monday, November 12, 2001 10:27 AM
To: steve@xxxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Impedance measurement using TDR



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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