[SI-LIST] Re: Differential TDR

Zhenggang,

In case of symmetrical differential excitation (or probes), only the
differential mode will be excited and propagating along the transmission
line, regardless of how the excitation structure is implemented, with or
without connection to the ground or reference planes. The differential mode
impedance does not depend on the excitation - it is property of the
transmission line geometry with the reference planes (does not matter ground
or power). However the transition from the probes to the differential mode
will be different in two cases and may cause the impedance transformation
and differences in the observed impedances. If probes are not connected to
the reference plane, the transition into the differential mode may take
longer line segment - simply because of zero currents in the plane at the
location of the probes must transition into the differential mode with the
currents in the reference plane (reflection and radiation are side-effects
of such transition). The observed impedances may also be different due to
the impedance transformation effect in the transition area. In general, the
closer the current distribution in excitation structure to the current of
the mode to be excited, the shorter the transition area and the closer to
the unit the impedance transformation coefficient. Note, that all those
parameters can be effectively estimated with a 3D full-wave field solver for
a particular stackup and measurement setup.

Best regards,
Yuriy Shlepnev
www.simberian.com 

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of ZHENGGANG CHENG
Sent: Saturday, December 13, 2008 11:03 PM
To: istvan.novak@xxxxxxx
Cc: olaney@xxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Differential TDR

Thanks for your reply.

If there is Gnd connection from differential probe to DUT, the measured
Zdiff will be still the same as that measured w/o gnd connection?

My concern is: w/o gnd connection to DUT, the signal return path will be
each other, (i.e., p reference to n, and n reference to p); when gnd is
added, signal might partially refer to gnd, the return path might be
different from w/o gnd situation. Then will the Zdiff still the same?

ZG
2008/12/13 Istvan Novak <istvan.novak@xxxxxxx>

> ZG, Orin,
>
> If the probe is really part of a TDR setup, I dont think high-impedance
> probe is an option.
>
> If the DUT is passive and well balanced, the differential probe without
> ground connection
> measured the differential response.  It can not, however, measure the
> common-mode response
> without a ground connection to the DUT.  So to answer the original
> question, without a ground
> connection to the DUT, only the differential impedance can be measured.
>  And since without
> ground connection the single-ended impedance can not be measured, both
> Z_single and
> Z_mutual will remain unknown.
>
> Regards,
>
> Istvan Novak
> SUN Microsystems
>
>
> olaney@xxxxxxxx wrote:
>
>> 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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