Folks: The TDR visual trace you see one the scope face (assuming you are using a=20 TEK, LeCroy or Agilent TDR) has only a mild dependence on risetime of the=20 excitation pulse. As the pulse travels down the interconnect, it loses=20 risetime and tends to smear (average) any effects which are proportional to= =20 a time derivative (parasitic or real capacitors and/or inductors for=20 instance). If you look at a coupon with the usual messy launch on each=20 end, the nearest the launch point will show a lot of discontinuity while=20 the far one will show much less. This is because the risetime is slowed by= =20 the losses. The lower the loss the interconnect, the larger the far end=20 discontinuity will affect the results. In the frequency domain, the presence to launch discontinuities causes=20 standing waves at specific frequencies corresponding to half and full=20 wavelength (in the dielectric) distances along the interconnect. For low=20 loss materials, the resonance effects are more pronounced. So, if you are using a surface pad to launch onto a PCB, the effect of=20 parasitics must be taken into account. To a first approximation, the=20 risetime loss is approximately equal to the time constant of the=20 parasitic. In the case of a shunt Capacitance, the transmission line=20 impedance is divided by two and for a series inductance, the transmission=20 line impedance is multiplied by 2. Thus for a capacitor, time constant =3D= =20 0.5*Zo*C and for an inductor, time constant =3D L/(2*Zo). As typical=20 numbers, C =3D (2-3) pF and Zo =3D 50 ohms, time constant ~ (50-75)ps. This= is=20 more than enough to corrupt the eye on a good 5 Gbps SERDES device. TDR traces are wonderful for seeing discontinuities for diagnostic=20 purposes, far better than the Return Loss. A TDR is not suited for=20 measuring propagation delay to any accuracy since the loss of risetime or=20 equivalently, the additional delay. These introduce a 2-way error into the= =20 measurement. For that measurement either a TDT through measurement is= required. Thanks for letting me share these ideas with you. ed At 12:27 PM 3/24/2006 -0500, Scott McMorrow wrote: >ah ... now we know where the term "averaging mean" method comes from. >Averaging refers to placing the TDR instrument into averaging mode. >Mean refers to the segment of the TDR waveform over which the mean is >measured. > >This method is reasonable, considering that most TDR coupon launches are >terrible, and attempts to remove the launch artifacts from the >measurement result. But, it is incorrect, and does not provide a >measure of the actual impedance of the PCB trace. > > >Scott McMorrow >Teraspeed Consulting Group LLC >121 North River Drive >Narragansett, RI 02882 >(401) 284-1827 Business >(401) 284-1840 Fax > >http://www.teraspeed.com > >Teraspeed=AE is the registered service mark of >Teraspeed Consulting Group LLC > > > >Loyer, Jeff wrote: > > One source for this kind of information: > > > > http://www.intel.com/design/chipsets/applnots/29817901.pdf=3D20 > > > > It's pretty old, though - much has been learned since 2000... > > > > Jeff Loyer > > > > > > -----Original Message----- > > From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] > > On Behalf Of Ivor Bowden > > Sent: Thursday, March 23, 2006 5:07 PM > > To: si-list@xxxxxxxxxxxxx > > Subject: [SI-LIST] Re: Q on TDR coupon measurements > > > > I've been enjoying the discussion on TDR measurements. While I > > understand the principle, I don't have experience with them beside > > evaluating TDR printouts. > > > > I was wondering if anyone knows a web site or other resource that has a > > variety of example waveforms and their proper interpretations? Including > > touching on some of the details Scott and Eric mention. > > > > tia, > > > > Ivor Bowden > > Curtiss-Wright Controls Embedded Computing > > > > > > ------------------------------------------------------------------ > > 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 FAQ wiki page is located at: > > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > > > > List technical documents are available at: > > http://www.si-list.org > > > > List archives are viewable at: =3D20 > > //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 > > =3D20 > > ------------------------------------------------------------------ > > 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 FAQ wiki page is located at: > > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > > > > List technical documents are available at: > > http://www.si-list.org > > > > 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 FAQ wiki page is located at: > http://si-list.org/wiki/wiki.pl?Si-List_FAQ > >List technical documents are available at: > http://www.si-list.org > >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 > > > >!DSPAM:4424319054777047185227! +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+ | NORTH EAST SYSTEMS ASSOCIATES, INC. | | ------------------------------------- | | "High Performance Engineering & Design" | +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+ | Dr. Ed Sayre e-mail: esayre@xxxxxxxx| | NESA, Inc. http://www.nesa.com/ | | 235 Littleton Road, Ste 2 Tel +1.978.392-8787 | | Westford, MA 01886 Fax +1.978.392-8686 | +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+ ------------------------------------------------------------------ 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 FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org 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