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. > > http://careers.yahoo.com > > > ------------------------------------------------------------------ > > To unsubscribe from si-list: > > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' > in the Subject field > > > > or to administer your membership from a web page, > go to: > > //www.freelists.org/webpage/si-list > > > > For help: > > si-list-request@xxxxxxxxxxxxx with 'help' in the > Subject field > > > > List archives are viewable at: > > > //www.freelists.org/archives/si-list > > or at our remote archives: > > > http://groups.yahoo.com/group/si-list/messages > > Old (prior to June 6, 2001) list archives are > viewable at: > > http://www.qsl.net/wb6tpu > > > > ------------------------------------------------------------------ > To unsubscribe from si-list: > si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in > the Subject field > > or to administer your membership from a web page, go > to: > //www.freelists.org/webpage/si-list > > For help: > si-list-request@xxxxxxxxxxxxx with 'help' in the > Subject field > > List archives are viewable at: > //www.freelists.org/archives/si-list > or at our remote archives: > http://groups.yahoo.com/group/si-list/messages > Old (prior to June 6, 2001) list archives are > viewable at: > http://www.qsl.net/wb6tpu > > __________________________________________________ Do You Yahoo!? Find a job, post your resume. http://careers.yahoo.com ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu