Just for kicks, measure the capacitance permutations between your lines.
Also what does Z[11] and Z[22] do?
... Richard Mellitz, NCR
>----------
>From: Mark McKee[SMTP:mmckee@cisco.com]
>Sent: Tuesday, December 09, 1997 1:22 PM
>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
>11801 with 6150 probes and a kludged miniture semi-rigid coax tips to make
>contact with the differential pairs, the differential impedance is 109 ohms.
>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
>away and run in parallel for the full 16 inchs (this is a bus). The D.C.
>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
>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
>measurement, I get the same reading (111 at the start and increasing to 119
>and the other end).
>
>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
>
>
>--------------------------- 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
>--------------------------- gnd
>