[SI-LIST] Re: characteristic impedance at DC
- From: Muhammad Nazir <muhammad.nazir@xxxxxxxxx>
- To: Nick.Luther@xxxxxxxxxx
- Date: Mon, 9 Apr 2012 17:03:52 -0700
Nick!
I agree with you 100%.
It never made me any sense to consider a 'piece of copper' as a
transmission line at 0 or 1Hz.
I knew prof. Jonghoo Kim (KAIST) personally and he believes that TDR is a
more accurate tool for S-parameters modelling at low frequencies
as compared to VNA. [Fine with me]
But Why bother with S-parametrs at low frequencies? That is a $1M question.
and bigger question (for a mathematician is]
What is the magnitude of SQRT[infinity] and What is the magnitude of
SQRT[0]?
By the way the Chc impedance is an AC term and holds good for certain BW
of Tline.
The lower limit of BW is often in KHz, which is way above 1Hz or DC.
Best regards.
-MN
On Mon, Apr 9, 2012 at 4:45 PM, Nick Luther <Nick.Luther@xxxxxxxxxx> wrote:
> 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
>
>
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