[SI-LIST] Re: Differential TDR
- From: olaney@xxxxxxxx
- To: zhenggang.cheng@xxxxxxxxx
- Date: Sat, 13 Dec 2008 14:27:29 -0800
If you mean a hi-z diff probe, then the line needs to be terminated with
matched resistors to ref ground. If dual 50 ohm probes into 50 ohm
inputs, the instrument itself provides termination. The instrument
doesn't need to "know" what the single ended impedance is to measure
Zdiff. That's not to say that Zse or Zcm aren't also available from the
typical setup. The most important consideration for Zdiff accuracy is
that measurement path delays be identical for both probes (automatic for
a unitary diff probe), and that the probe grounds be tied to each other
(not necessarily to other grounds) at the measurement end (also automatic
for a unitary diff probe). Also note that Zdiff can be well defined even
if Zcm is not, hence the successful application of unshielded twisted
pairs dangling in air and over heating ducts in the form of CAT5 LAN
cabling. The large swings in Zcm barely affect Zdiff.
Orin Laney
On Sat, 13 Dec 2008 12:22:54 -0800 "ZHENGGANG CHENG"
<zhenggang.cheng@xxxxxxxxx> writes:
> Hi all,
> When using differential probe to do the TDR for differential pair,
> if the
> probe only has P and N tip no Gnd tip, does the TDR gives accurate
> values?
>
> Here is the concern:
>
> Zdiff =2*(Z_single-Z_mutual), if no Gnd tip is used, how does the
> TDR know
> what's the single-ended impedance, and consequently how does TDR
> know the
> Zdiff?
>
>
> Thanks,
>
> ZG
>
>
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