# Re: [SI-LIST] : Differential TDR "Measurements"

From: Bob Lewandowski ([email protected])
Date: Wed Apr 26 2000 - 10:52:53 PDT

Vinu,

The problem with using a coaxial probe to make a differential TDR measurement is
that the drive impedances in a single ended system are not balanced. The center
conductor looks like a 50 ohm source at all frequencies, but the outer conductor
source impedance is essentially indeterminate as a function of frequency. It is
a zero ohm source at dc and the common mode impedance of the outer conductor
(inductive @ < 1/4 wavelength) at other frequencies. A true differential source
has balanced impedances.

---Bob Lewandowski
Vixel Corp.

Vinu Arumugham wrote:

> Fred Balistreri wrote:
>
>> NO, this is the wrong approach. For one thing the planes are left floating
>> under this scenerio.
>
> When a single-ended TDR measurement is performed on a differential pair, the
> instantaneous voltage on the plane between the lines will always be half way
> between the true and complement voltages due to the voltage divider formed by
> the two transmission lines (each trace to the plane). This is also the case
> when a differential TDR is performed on the same traces. In other words, if
> the instantaneous voltage on the plane is the same for both measurements, it
> seems to me that leaving the plane unconnected should make no difference to
> the measurement.
>
>> This would work for twisted pair cable with no shield and
>> gnd far away such as in inches at least.
>>
>> A PCB gnd plane plays an important part in determining Zo diff. If not
>> properly accounted for the resulting error is large.
>>
>
> Since this is not some calculation where we are ignoring the plane but a
> physical measurement where the fields are affected by the presence of the
> plane, it seems to me that the plane is being accounted for.
>
>>
>> Best Regards,
>>
>> Vinu Arumugham wrote:
>>
>> > Is it not possible to perform a single-ended TDR measurement, with the
>> > probe
>> > ground connected to say the true trace and the signal being launched into
>> > the
>> > complement trace, to measure the differential impedance of the pair?
>> >
>> > Thanks,
>> > Vinu
>> >
>> > Dima Smolyansky wrote:
>> >
>> > > Pat and All:
>> > >
>> > > We have both instruments and use both approaches on interconnects, and
>> > they
>> > > both work. If your system is linear, either approach will work.
>> > > Interconnects are linear, except maybe for some exotic cases where we
>> > deal
>> > > with ferromagnetic or something like that. If your system is non-linear
>> > > (e.g., an active device), the crosstalk approach (approach #2) breaks
>> > down.
>> > > Again, both approaches should work fine for PCB interconnects.
>> > >
>> > > In my experience, errors in differential impedance measurement (as any
>> > other
>> > > impedance measurement) more often have to do with the fact that the TDR
>> > > users sometimes don't have a good interface from the TDR scope to the DUT
>> >
>> > > (cables and probes) and sometimes do not know how to best utilize
>> > > calibration capabilities in the instrument. Also, picking a specific
>> > point
>> > > on the TDR trace where the measurement is taken can cause discrepancies.
>> > > Averaging over a short region of TDR trace may help reduce these
>> > > discrepancies; we participated in the industry round robin on impedance
>> > > measurements where averaging over a region removed practically all
>> > > discrepancies between the instruments and methods.
>> > >
>> > > Hope that helps,
>> > >
>> > > -Dima
>> > > ===================
>> > > TDA Systems, Inc.
>> > > 11140 SW Barbur Blvd., Suite 100
>> > > Portland, OR 97219
>> > > (503) 246-2272
>> > > (503) 246-2282 (fax)
>> > > (503) 804-7171 (mobile)
>> > > http://www.tdasystems.com
>> > > The Interconnect Modeling Company(TM)
>> > >
>> > > ----- Original Message -----
>> > > From: Brent DeWitt <[email protected]>
>> > > To: <[email protected]>
>> > > Sent: Monday, April 24, 2000 8:16 PM
>> > > Subject: RE: [SI-LIST] : Differential TDR "Measurements"
>> > >
>> > > > Pat,
>> > > >
>> > > > I'm not an SI guru (and I don't play one on TV) but;
>> > > >
>> > > > My first impression is that your method is the more confident one.
>> > Your
>> > > > method appears to "wrap up" more of the variables than the fab house.
>> > How
>> > > > is the fab house estimating the line to line coupling in the
>> > differential
>> > > > pair? It's possible to do accurately with a good model, but I would
>> > > > question if their model is rigorous.
>> > > >
>> > > > Best of luck Sir!
>> > > >
>> > > > Brent DeWitt
>> > > > Datex-Ohmeda
>> > > >
>> > > > > -----Original Message-----
>> > > > > From: [email protected]
>> > > > > [mailto:[email protected]]On Behalf Of Zabinski,
>> > Patrick
>> > > > > J.
>> > > > > Sent: Monday, April 24, 2000 6:45 PM
>> > > > > To: [email protected]
>> > > > > Subject: [SI-LIST] : Differential TDR "Measurements"
>> > > > >
>> > > > >
>> > > > > We're working more and more with differential signals,
>> > > > > and subsequently dealing with more differential printed
>> > > > > circuit boards (PCBs). Over the past few years, we've
>> > > > > had difficulty with several PCB vendors
>> > > > > trying to obtain a controlled impedance 100 ohm
>> > > > > differential pair.
>> > > > >
>> > > > > The problem generally boils down to "who's measurement
>> > > > > do we believe"? We measure one impedance, while the
>> > > > > PCB vendor measures another.
>> > > > >
>> > > > > We've done some digging, and there appears to be two
>> > > > > approaches to measuring differential impedance, and I'd
>> > > > > like to hear what folks have to say about them.
>> > > > >
>> > > > > Approach 1: inject two signals of opposite polarity,
>> > > > > one into the true and one into the complement. The
>> > > > > complement signal is substracted from the true, and
>> > > > > you read the impedance just like a single-ended
>> > > > > measurement.
>> > > > >
>> > > > > Approach 2: Inject one signal into the true trace and
>> > > > > record its signal. Then, inject a signal into the complement
>> > > > > trace and record its signal. Then, with the magic of
>> > > > > mathematics, compile these two different captured signals
>> > > > > into an effective differential measurement.
>> > > > >
>> > > > > The equipment we have in-house uses Approach 1, while
>> > > > > nearly every board vendor we work with uses Approach 2.
>> > > > > Can anyone shed some light into the accuracies, sensitivities,
>> > > > > etc. of these two approaches? Are there cases where one
>> > > > > approach is better/worse than the other?
>> > > > >
>> > > > > Thanks,
>> > > > > Pat
>> > > > >
>> > > > > -----
>> > > > > Pat Zabinski ph: 507-284-5936
>> > > > > Mayo Foundation fx: 507-284-9171
>> > > > > 200 First Street SW [email protected]
>> > > > > Rochester, MN 55905 www.mayo.edu/sppdg/sppdg_home_page.html
>> > > > >
>> > > > >
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>>
>> --
>> Fred Balistreri
>> [email protected]
>>
>> http://www.apsimtech.com
>>
>>
>

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