[SI-LIST] Re: TDR measure / Simulation

What I think you are missing is that there is a round trip.  A feature on a
trace is counted twice, the first time when the signal passes through it and
the second time when it returns.  For example a 10 inch trace terminating in
an open should look something like 1.7 nsec in time.  But to get to the open
at the end of the trace the signal must propagate down the trace for 1.7
nsec and then return down another 10 inches of trace (1.7 nsec) before the
open is seen by the TDR head and digitized.

Tom Dagostino
Teraspeed(R) Labs
13610 SW Harness Lane
Beaverton, OR 97008
503-430-1065
tom@xxxxxxxxxxxxx 
www.teraspeed.com 

Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
401-284-1827
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of si09604@xxxxxxxxxxxx
Sent: Tuesday, November 27, 2007 4:29 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: TDR measure / Simulation


But I have the dielectric constant,distance etc of pcb. Is 
impossible to have it exactly.

The patterns material is metal and It has a typical 
propagation,no?



Missatge original de "Peterson, James F (EHCOE)" 
<james.f.peterson@xxxxxxxxxxxxx>:




Calibration is the key, and it isn't easy unless you have 
a long trace =
test run of which you know the exact length.=20

I've found TDR's do certain things really well, they 
measure =
reflections, impedance, and time very accurately.=20

But there are things they do that need calibration : 
distance and the =
dielectric constant. If you have a length that is really 
accurate then =
you can derive the Er and thus prop delay of your other 
traces. But on a =
regular board I could get no closer than 10-20% because of 
the =
parasitics that lengthened the edges (vias, stubs, 
probe/launch =
attachments).

If you don't have an accurate Er then prop times won't 
agree in your =
sims and measurements.

Jim Peterson
Honeywell

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-
bounce@xxxxxxxxxxxxx] =
On Behalf Of si09604@xxxxxxxxxxxx
Sent: Tuesday, November 27, 2007 10:23 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] TDR measure / Simulation

Hello,
I try to understand the TDR (Time domain reflection) in=20
the measure and in the simulation, to identify the=20
elements.=20
I have seen that The TDR measure is more long in time 
that=20
the simulation.=20

Do you know what happens?

Thank you.

B.R

N=FAria
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