[SI-LIST] Re: Measuring propagation delay on PWB interconnects
- From: "Loyer, Jeff W" <jeff.w.loyer@xxxxxxxxx>
- To: "'brian.butler@xxxxxxxxxxxxxxx'" <brian.butler@xxxxxxxxxxxxxxx>,si_list <si-list@xxxxxxxxxxxxx>
- Date: Wed, 17 Apr 2002 09:47:03 -0700
I believe that, for true accuracy, propagation delay measurements should be
relative vs. absolute measurements. I.E., measure the delay of a 5" trace
and compare that to the delay of a 4" trace. The difference is your delay
per inch. This eliminates any ambiguity related to choices of start and end
points, which are arbitrary.
If you don't have traces of different lengths on the same board, I believe
it's perfectly acceptable to measure a trace, cut the trace 1" shorter, and
measure that same trace.
Jeff Loyer
-----Original Message-----
From: Brian D. Butler [mailto:brian.butler@xxxxxxxxxxxxxxx]
Sent: Wednesday, April 17, 2002 7:44 AM
To: si_list
Subject: [SI-LIST] Measuring propagation delay on PWB interconnects
To all:
I wanted to get opinions on the best way to measure propagation delay
utilizing only a TDR technique (not TDT) on PWB interconnects (utilizing
only a single interconnect). The methods I have seen to-date are:
Note: Propagation delay is equal to the End point minus the Start point
along the TDR horizontal time axis. All values obtained are the
"round-trip" delay.
1) Start point is 50% of risetime curve of TDR probe - End point is 50% of
risetime curve at end of trace.
2) Start point is lowest point of capacitance dip (associated with launch
via) - End point is 3mvolts over nominal impedance on the risetime curve at
end of trace.
3) Start point is 40% of risetime curve of TDR probe - End point is 40% of
risetime curve at end of trace minus 3.2 * SQRT((Ristetime40%-60% at end of
trace)^2 - (Risetime40%-60% at end of probe)^2) - current IPC TM-650
method
4) Same as any above but utilize falltime curve (by probing a short and
shorting end of trace).
Any comments on the above methodologies or alternate methods are
appreciated.
Regards
Brian Butler
Introbotics Corp
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