From: Larry Miller (email@example.com)
Date: Tue Oct 24 2000 - 13:21:31 PDT
I agree with Jim. It is very handy to be able to just clip out one message
and get the whole thread, rather than spendin a lot of time rummaging
through e-mail archives to put together a history.
The Information Ages's Mantras:
"Bandwidth is free.
Storage is Free."
> -----Original Message-----
> From: Jim Freeman [SMTP:firstname.lastname@example.org]
> Sent: Tuesday, October 24, 2000 10:43 AM
> To: David Instone
> Cc: Ron Miller; email@example.com
> Subject: Re: [SI-LIST] : Re: Diff probes for tdr.
> Hi David,
> I personally like the history of the thread being carried forward. The
> are so many and diverse that it is hard to remember what is going on in
> each thread.
> It takes 3 or 4 responses before the question becomes exciting and the
> history helps
> to jibe the arguments.
> Also I hope you are not aware of the major interpretetion of the
> acronym SOB. It
> is considered an epithet in the US.
> Jim Freeman
> David Instone wrote:
> > Ron,
> > I think I tend to disagree,
> > 1. If you are making a TDR measurment on an unpowered unit which is
> > connected only to the the TDR set then the only current source is from
> > the TDR set.
> > 2. I believe that one major TDR manufacturer does differential
> > measurements by firing a pulse down one line of the pair (and at the
> > same time noting the cross talk to the other) and then firing the pulse
> > down the other pair and finally calculating the differential impedance.
> > This is essentially two single mode measurements for which the ground
> > (or reference plane/s) is most definitely required.
> > Which brings me to a further point, If the lines are running between
> > two return planes (eg Vcc and Gnd) or change return plane (ie swap from
> > one side of the pcb to the other) then make sure that either all the
> > decoupling caps are in place (best method) or place a short in all their
> > places.
> > Finally a plea to all, SOB! save our bandwidth. Please trim off previous
> > messages more than two deep. This one was 5 levels deep before I
> > trimmed it!
> > Ron Miller wrote:
> > >
> > > Hi Jeff
> > >
> > > As you say there is current in the ground plane. There are many
> sources for
> > > this current so for critical signals stay away from it. Measurement
> implys that
> > > accuracy is important so decouple stray sources. That means do not
> connect the
> > > ground unless needed. For differential signals ground is not needed.
> > >
> > > Ron
> > --
> > Regards
> > Dave Instone. Compliance Engineer
> > Test Systems, MP24/22
> > Xyratex, Langstone Rd., Havant, Hampshire, P09 1SA, UK.
> > Tel: +44 (0)23-92-496862 (direct line)
> > Fax: +44 (0)23-92-496014
> > http://www.xyratex.com Tel: +44 (0)23-92-496000
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