# [SI-LIST] Re: characteristic impedance at DC

*From*: "Nick Luther" <Nick.Luther@xxxxxxxxxx>*To*: "mohammad haaeri" <haaeri@xxxxxxxxx>*Date*: Mon, 9 Apr 2012 18:45:58 -0500

Aha! Now we are zeroing in on the question! Just to be clear, the TDR is making an AC measurement. The TDR pulse has an edge and the rise time of that edge is related to its bandwidth. I assume the authors are claiming that they can make better low frequency Z0 measurements by TDR than they can with a VNA or frequency domain technique and are claiming the usefulness of that information, and you're looking to discuss how the information would be useful. I haven't read the paper. If that's the true scope of our discussion, I don't have an example when I've been concerned with the characteristic impedance of a transmission line at a very low frequency. The reason I can cite is that at very low frequencies the transmission line I'm working with is always electrically very short, so transmission line effects are negligible, and the characteristic impedance isn't really seen on either side of the channel. -Nick From: mohammad haaeri [mailto:haaeri@xxxxxxxxx] Sent: Monday, April 09, 2012 6:27 PM To: Nick Luther Cc: Weston_Beal@xxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: Re: [SI-LIST] Re: characteristic impedance at DC Thanks Nick, I understand characteristic impedance and its usefulness at AC (not DC). What about very low AC frequencies? Also, I'm reading a paper in DesignCon 2006 "Implementation of Broadband transmission line models with accurate low-frequency response for high speed system simulations" and they are calculating characteristic impedance at low frequencies with TDR which is not very high or infinity as the formula) says and not DC resistance as Weston and Todd are saying! You can read the paper On Mon, Apr 9, 2012 at 4:16 PM, Nick Luther <Nick.Luther@xxxxxxxxxx> wrote: One quick clarification: Looking at the two terminals at the input to a transmission line (consider a center conductor and shield in a coax cable), and with nothing at the other end (open), you likely will see an open circuit at DC, just like the "infinite" impedance that Mohammad predicted using his general equation. So, in a strict sense, Mohammad was correct. I want to point out to Mohammad that he is using the equation correctly, and this is how he should be interpreting the result that he calculated. The characteristic impedance is what an AC source would effectively see at those same two terminals. Weston and Todd's replies explain why the characteristic impedance calculation at DC isn't useful. Mohammad: I hope that Weston and Todd's replies help you understand this concept. The Wikipedia link should be very useful. I think you'll find a lot of additional literature with a Google search. Nick -- Nick Luther, P.E. Design Engineer Plexus Engineering Solutions Ship To: 55 Jewelers Park Drive Mail To: PO Box 677 Neenah, Wisconsin 54957-0677 USA http://www.plexus.com/ -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Beal, Weston Sent: Monday, April 09, 2012 5:50 PM To: haaeri@xxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: characteristic impedance at DC Characteristic impedance applies to propagating EM waves, so it does not apply to DC. Or, if you look at the general equation that you cited, you can see that when jw = 0, the impedance approaches the line resistance. Somewhere in college I remember seeing a graph of the general impedance equation compared to actual measurement at low frequency. They are the same above some kHz, but diverge as frequency decreases. Weston -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of mohammad haaeri Sent: Monday, April 09, 2012 3:31 PM To: 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 with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: http://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: http://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: http://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: http://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: http://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: http://www.freelists.org/archives/si-list Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu

**Follow-Ups**:**[SI-LIST] Re: characteristic impedance at DC***From:*mohammad haaeri

**[SI-LIST] Re: characteristic impedance at DC***From:*Muhammad Nazir

**References**:**[SI-LIST] characteristic impedance at DC***From:*mohammad haaeri

**[SI-LIST] Re: characteristic impedance at DC***From:*Beal, Weston

**[SI-LIST] Re: characteristic impedance at DC***From:*Nick Luther

**[SI-LIST] Re: characteristic impedance at DC***From:*mohammad haaeri

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