[SI-LIST] Re: Differential TDR
- From: "ZHENGGANG CHENG" <zhenggang.cheng@xxxxxxxxx>
- To: istvan.novak@xxxxxxx, shlepnev@xxxxxxxxxxxxx
- Date: Sun, 14 Dec 2008 16:01:05 -0800
Istvan/Yuriy,
Thanks for your replies.
The subsequent question is: if there is a big slut on the reference plane,
do you think differential TDR will see this discontinuity?
The concern is:
Assuming everything still sysmetrical, there is still no fluctuation on the
gnd reference plane, it seems differential TDR might not see the slut on
reference plane. However, the single-ended impedance can see the
discontinuity, and Z_mutual may still be unchanged, then Zdiff=2*(Z11-Z21)
should change, so there should be some Zdiff discontinuity.
ZG
2008/12/14 Istvan Novak <istvan.novak@xxxxxxx>
> ZG,
>
> As Yuriy also pointed out, you still will measure the differential
> response, even if there is no ground connection. If we have a single
> 'ground' connection for a balanced circuit, the 'ground' will have no
> fluctuation due to a differential excitation, so whether you connect it or
> not, it will not change the result. Unbalanced circuits, however (for
> instance, if the two single-ended impedances are different, or if there is
> noticeable skew between the legs), will result in a residual signal on its
> 'ground' and therefore its stray coupling to cable and instrument ground
> will alter the result. Bottom line: for well-balanced circuits and if you
> need only the differential response, the ground connection is not needed.
> For all other cases you need the ground connection.
>
> Regards,
>
> Istvan Novak
> SUN Microsystems
>
>
> ZHENGGANG CHENG wrote:
>
>> Dr. Novak,
>> 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 <mailto:
>> 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 <mailto: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 <mailto: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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