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

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From: Chris Bobek (cbobek@cadence.com)
Date: Fri Oct 22 1999 - 13:10:40 PDT


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
> >
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