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 - 12:42:43 PDT


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 ****


TlineZin.jpg

**** 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