Arpad, You make a good point - when I said the impedance was low, I should have specified which points I was probing. I'm talking about the near and far end of the conductor itself, not the characteristic impedance of the trace as seen looking outward from the driver. The following presentation seems germane to this discussion - I had a lot of fun creating it and like to drag it out whenever the opportunity presents itself: http://www.eda.org/ibis/summits/jun05/westerhoff.pdf Todd. Todd Westerhoff VP, Software Products Signal Integrity Software Inc. ? www.sisoft.com 6 Clock Tower Place ? Suite 250 ? Maynard, MA 01754 (978) 461-0449 x24 ? twesterh@xxxxxxxxxx ?I want to live like that ? -Sidewalk Prophets -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Muranyi, Arpad Sent: Monday, April 09, 2012 8:03 PM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: characteristic impedance at DC I have a problem with the statements that the impedance is very low, on the order of a few milliOhms or so at low frequencies or DC. Consider this circuit: [cid:image001.png@01CD1683.626893C0] How would you measure the impedance in the box on the right? You would measure it by measuring the voltage and current that is in the wire connecting the two boxes, right? What would you get if the box contained a capacitor and the source on the left would be a DC source? You would measure the full voltage of that source with 0 current, i.e. the impedance would be infinite. Why would the T-line exhibit a few milli Ohms, if the insulating material is not leaking significantly? I think that idea comes from measuring the series resistance of the T-line, from its left end to its right end. But that is not the load impedance a driver (such as the box on the left in the above diagram) would see... Thanks, Arpad =============================================================== -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Todd Westerhoff Sent: Monday, April 09, 2012 5:54 PM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: characteristic impedance at DC Mohammad, Characteristic Impedance (sometimes called surge impedance) is the impedance seen by a propagating wavefront. It's been covered at length multiple times, including this overview in Wikipedia: http://en.wikipedia.org/wiki/Characteristic_impedance DC impedance from a typical transmission line ranges from nil to a few ohms, depending on the line geometry and length. It's amenable to direct physical measurement in many cases. Here's a device that can be used to affordably make such measurements with 4 digits of precision: http://www.radioshack.com/product/index.jsp?productId!03176 Hope that helps, Todd. Todd Westerhoff VP, Software Products Signal Integrity Software Inc. * www.sisoft.com<http://www.sisoft.com> 6 Clock Tower Place * Suite 250 * Maynard, MA 01754 (978) 461-0449 x24 * twesterh@xxxxxxxxxx<mailto:twesterh@xxxxxxxxxx> "I want to live like that " -Sidewalk Prophets -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx<mailto:si-list-bounce@xxxxxxxxxxxxx> [mailto:si-list-bounce@xxxxxxxxxxxxx]<mailto:[mailto:si-list-bounce@freeli sts.org]> On Behalf Of mohammad haaeri Sent: Monday, April 09, 2012 6:31 PM To: si-list@xxxxxxxxxxxxx<mailto:si-list@xxxxxxxxxxxxx> Subject: [SI-LIST] characteristic impedance at DC Hi, What is the characteristic impedance of a transmission line at DC? If you are saying Z0=sqrt(Rdc/Gdc) at DC, since Gdc=0, and Rdc is not zero, therefore Z0 is infinite. Is it correct? How does behavior of L, R, G, and C (line parameters) change vs. frequency (at low and DC, and at very high frequency)? Can Z0=sqrt(R+jwl/G+jwc) be used for all frequencies? Thanks, mohammad ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx<mailto: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<mailto:si-list-request@xxxxxxxxxxxxx> with 'help' in the Subject field List forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list 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<mailto: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<mailto:si-list-request@xxxxxxxxxxxxx> with 'help' in the Subject field List forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list 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 forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list 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 forum is accessible at: http://tech.groups.yahoo.com/group/si-list List archives are viewable at: //www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu