[SI-LIST] Re: characteristic impedance at DC
- From: mohammad haaeri <haaeri@xxxxxxxxx>
- To: shlepnev@xxxxxxxxxxxxx
- Date: Mon, 9 Apr 2012 17:33:38 -0700
Thanks Yuriy for your response, it makes sense.
I think what's been calculated for characteristic impedance at low
frequencies in "Implementation of Broadband transmission line models with
accurate low-frequency response for high speed system simulations" in
DesignCon 2006 is not correct (although it has improved the time domain
simulation results!). They are assuming only the transmission line under
test is frequency dependent and calculate its limit going to low
frequencies not the TDR cable (which is another transmission line and it
doesn't have 50ohms going to zero). (page 11, equation (5)). That's the
reason they are coming up with a characteristic of 25.7ohms at DC for a
transmission line!
On Mon, Apr 9, 2012 at 5:15 PM, Yuriy Shlepnev <shlepnev@xxxxxxxxxxxxx>wrote:
> Mohammad,
>
> See my answers below.
>
> Best regards,
> Yuriy
>
> Yuriy Shlepnev, Ph.D.
> President, Simberian Inc.
> 3030 S Torrey Pines Dr. Las Vegas, NV 89146, USA
> Office +1-702-876-2882
> Cell +1-206-409-2368
> Skype: shlepnev
> www.simberian.com
>
>
>
> -----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?
> YS: Yes, this is correct for a lossy line that does not have conductive
> losses in the admittance per unit length (technically in dielectric).
> Though, there is no waves at DC, for TEM mode we can calculate asymptotes
> of
> the impedance and admittance per unit length and the characteristic
> impedance at DC.
>
> How does behavior of L, R, G, and C (line parameters) change vs. frequency
> (at low and DC, and at very high frequency)?
> YS: It obviously depends on a transmission line type. See analysis for a
> microstrip line in this app note
> http://www.simberian.com/AppNotes/MicrostripImpedanceAndTDR_2009_04.pdf
> Impedance grows at lower frequencies if dielectric model has only
> polarization losses. In reality, there are some conductive losses in
> dielectric and thus the asymptote of the characteristic impedance ad DC is
> not infinity. As someone already noted, the low-frequency growth of the
> impedance has small impact on overall behavior of the line. It should also
> not be confused with the conductor resistance that is more important to
> account at DC. For a microstrip line, the impedance also grows at very high
> frequencies.
>
> Can Z0=sqrt(R+jwl/G+jwc) be used for all frequencies?
> YS: Yes, as long as the impedance (R+jwL) and admittance (G+iwC) per unit
> length are appropriately defined. The formula does not have limitations
> neither at low nor at high frequencies, though this is relatively
> complicated subject for a short posting.
>
> Thanks,
> mohammad
>
>
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