Re: [SI-LIST] : Effect of low Zo for unterminated lines

About this list Date view Thread view Subject view Author view

From: Dmitri Kuznetsov (vdm@iname.com)
Date: Fri Oct 22 1999 - 15:05:36 PDT


Chris,

The plot shows input impedance (as seen by the driver) of an
unterminated foot-long tline. At dc, an unterminated tline is almost an
open circuit. The impedance depends on the dielectric conductivity and
is usually very high, over 10 MOhm.

Dmitri

Chris Bobek wrote:
>
> Hi,
>
> Am I missing something here? I thought the impedance of a tline is close to 0ohm at DC. What does the circuit look like that produced the graph you emailed?
>
> Thanks,
>
> Chris
>
> Dmitri Kuznetsov wrote:
>
> > I believe this discussion may be missing an important point.
> >
> > The input impedance of a finite-length tline is not constant. It looks
> > like a comb filter with a period of 1/(2*delay). The value changes
> > between very small and a very high impedance for low-loss unterminated
> > lines (zero and infinity for lossless lines).
> >
> > The impedance is very high at dc, and gradually reduces to almost zero
> > at f=1/(4*delay). Then it repeats with a period of 1/(2*delay). I
> > attached to this email an input impedance plot for a foot-long line with
> > 1.3 ns delay. The dips in impedance are located at 192+n*384 Mhz, n =
> > 0, 1, 2...
> >
> > As you can observe, the load presented by an unterminated line depends
> > not so much on the characteristic impedance as on how the power spectrum
> > of the signal correlates with n/(4*delay). The average impedance will
> > be high for signal frequencies below 1/(4*delay) (shorter line, slower
> > driver case). It will also be high for majority of broad-spectrum
> > digital signals. But you could damage the driver by a periodic signal
> > (clock) with a frequency close to 1/(4*delay) + n/(2*delay).
> >
> > And of course, the best thing to do is to run a simulation and verify
> > that the average and pick current is within the driver specs. That's
> > why we, EDA vendors, are in business.
> >
> > Regards,
> > --Dmitri
> >
> > =======================================================
> > Dmitri Kuznetsov, Ph.D.
> > Principal Engineer
> >
> > ViewLogic Systems, Inc. e-mail: vdm@viewlogic.com
> > 1369 Del Norte Rd. Tel: (805)278-6824
> > Camarillo, CA 93010 Fax: (805)988-8259
> > =======================================================
> >
> > Chris Bobek wrote:
> > >
> > > Hi,
> > >
> > > I think I should restate my question. The original question wasn't just pertaining to an LVCH245, that was just an example. I'm trying to find out what the effects are on a driver when the Zo is low.
> > >
> > > As a general example, assume you had a 30ohm line being driven by a driver that was designed to drive a 50ohm load. The line is unterminated and suppose it's a foot long. Suppose there are multiple drops on the line. I've
> > > already seen simulations of this, but I'm wondering if this will damage the driver or not.
> > >
> > > Thanks for your help,
> > >
> > > Chris
> > >
> > > **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****
> >
> > ------------------------------------------------------------------------
> > [Image]
>
> **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

**** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****


About this list Date view Thread view Subject view Author view

This archive was generated by hypermail 2b29 : Tue Feb 29 2000 - 11:39:18 PST