[SI-LIST] Re: Differential TDR

  • From: "ZHENGGANG CHENG" <zhenggang.cheng@xxxxxxxxx>
  • To: istvan.novak@xxxxxxx
  • Date: Sat, 13 Dec 2008 23:03:14 -0800

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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