[SI-LIST] Re: Differential TDR

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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