[SI-LIST] : Via signal integrity

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From: Gaboian, Jerry (j-gaboian@ti.com)
Date: Mon Aug 07 2000 - 06:15:44 PDT


  I greatly appreciate some pointers in using vias Vs. running lines
exceeding critical length.

  We design bench evaluation boards for transceivers with rise times around
250 Ps and 3v pp. The keep out
for the socket on the top side does not allow component placements with in 1
inch of the output of the DUT.

One of the parameters measured for the data sheet is channel to channel
crosstalk. In the past I have
put a 450 ohm resistor in series with the output, drive about 3 inches of
etch on the board, then go directly
to the 50 ohm input of a scope. I am aware of the losses on the board and
cabling and realize probing
with a 4 Ghz probe close to the DUT is a preferred method but have not
mastered the mechanics in keeping the probe in place (that's another issue).

The question is for crosstalk characterization, is it best to put a via as
close as possible to output pin then
place my 450 ohm resistor next to it for a (critical length divided by 6) or
do not use a via and run the trace
1 inch on the top side, then put the 450 ohm resistor in series. (The later
will cause some overshoot)

My past experience with the vias is the cross talk between them made it very
difficult to determine
if it was the via or DUT outputs. There are 16 outputs.

I do not have the luxury in simulating any of the above.

Any suggestion would be appreciated.

Best regards,
Jerry Gaboian

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