[SI-LIST] Re: Differential TDR

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
>>
>>
>>     
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field


List technical documents are available at:
                http://www.si-list.net

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu

Other related posts: