I'll attempt a wild guess at what might be going on with your =
measurement, not really being familiar with your particular instrument. =
The differential TDR AC couples two step functions into the traces you =
are measuring. The coupling capacitors are the same value. But if on =
the second board a conductance path to some DC potential (ground plane =
or something else) exists on each trace and is not the same, then the =
step function excitation will have a very slow RC time constant--that is =
what you may be seeing as a change in characteristic impedance.
If this explanation goes down in flames I won't be surprised.
Charles Hill, consultant
chuckh@altaeng.com
----------
From: Mark McKee[SMTP:mmckee@cisco.com]
Sent: Tuesday, December 09, 1997 3:22 AM
To: si-list@silab.Eng.Sun.COM
Cc: mmckee@cisco.com
Subject: [SI-LIST] : TDR Measurements
I hope someone can add some insight into a phenomena that I've been =
unable to
explain. Any theories would be appreciated
A brief background. If more data or information is needed, let me know.
I have 2 boards. Different stackups (as described below). Both have
differential lines designed with the same target impedance (102 ohms).
Both have (finished) 4.5 mil line widths (as measured by the venders
after cross-sectioning) and are using 1/2 oz copper (that I believe is
actually 0.6 mils thick after a roughening process that both venders =
use.
Board 1 has differential trace lengths of roughly 20 inchs (actually 2 =
10"
traces that I've jumpered together). The traces start with a set of =
vias
that get the signal pair to the internal layer, and they make a straight
run to the 10" via pair. The second 10" run is identical (i.e. the =
lines
are straight with no vias in between. The nearest lines are 50 mils =
away.
The D.C. resistance of a single 20 inch trace is roughly 7.2 ohms. =
Using an=20
11801 with 6150 probes and a kludged miniture semi-rigid coax tips to =
make=20
contact with the differential pairs, the differential impedance is 109 =
ohms. =20
The impedance (within an ohm) is 109 ohms from the start of the line to =
the
end (except for the discontinuity at the probe tips and at the jumper =
where
the 2 10" traces are connected).
I connected the 2 10 " traces together to get the lines longer than =
board 2.
But the measurements for the 10" are identical (except the D.C. =
resistance
is about 3.5 ohms for the single 10 inch trace)
Board 2 has differential trace lengths of roughly 16 inchs (single run, =
same
layer from connector via to connector via). The nearest traces are 20 =
mils=20
away and run in parallel for the full 16 inchs (this is a bus). The =
D.C.=20
resistance is 5.6 ohms. Using the same measurement technique, the =
differential
impedance is 111 ohms at the beginning of the line, BUT, the impedance=20
increases from 111 to 119 from the start of the line to the end in a =
very
linear fashion. When I go to the other end of the trace and repeat the=20
measurement, I get the same reading (111 at the start and increasing to =
119
and the other end). =20
So the question is:
Why does board 2 exhibit this increasing impedance effect? Is it =
possible
that the materials are so different (terrible) that I'm seeing =
significant
losses or other effects?
The following describes the stackups and line spacing:
Board 1
=20
--------------------------- gnd
4 mils dielectric core
-- -- signal layer (4.5 line, 8.5 mil space, 4.5 lin)
8 mils dielectric prepreg
-- another signal layer, 50 mils away
4 mils dielectric core
--------------------------- gnd
Board 2 (I do not know yet which is core and which is prepreg if this is
significant)
--------------------------- gnd
6 mils dielectric
-- -- signal layer (4.5 line, 8.5 mil space, 4.5 line
8 mils dielectric=09
--------------------------- gnd