[SI-LIST] Re: Measuring propagation delay on PWB interconnects
- From: Ritchey Lee <leeritchey@xxxxxxxxxxxxx>
- To: jeff.w.loyer@xxxxxxxxx
- Date: Wed, 17 Apr 2002 17:13:05 -0700
Clever!
Lee
Loyer, Jeff W wrote:
> 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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