[SI-LIST] Re: TDR and line losses

  • From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
  • To: <Suresh.Subramaniam@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 21 Nov 2003 14:10:41 -0800

Yes - that's exactly what you should observe.  Be careful, though, that
you TDR from both ends - sometimes a transmission line's impedance will
actually vary along its length, mimicking the effects of loss.  If you
TDR from both ends, you should always see the voltage going up as time
increases.  If you see the voltage going down when TDR'ed in one
direction, you may be seeing actual impedance variation on that trace.

As you infer, the amount of rise will vary with the rise-time, since
some losses (the dominant ones at 26ps) are frequency-dependent.

A TDT would show a significant increase in the risetime.

Jeff Loyer


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Suresh Subramaniam
Sent: Friday, November 21, 2003 2:01 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] TDR and line losses



If I TDR (rise time 26ps) a long lossy transmission line, should I
expect
the impedance profile to gradually creep up to a higher value (assuming
I
start out with a 50 Ohm impedance?). In other words, how does the TDR
take into account the effect of losses?

Thanks
Suresh



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