RE: [SI-LIST] : TDR Measurements

Leesa Macleod (macleod@waves.utoronto.ca)
Fri, 12 Dec 1997 09:05:08 -0500

Hi Mark and Chuck -
I've seen this effect when using TDR to look at a single line.
A quick way to check this is to decrease the TDR pulse repetition rate
and see if the "impedance" changes.

Regards,
Leesa
macleod@waves.utoronto.ca
PhD candidate
Electromagnetics Group
U of Toronto, Canada

>
> Mark,
>
> 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
>
>
>
>