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

From: Bob Lewandowski ([email protected])
Date: Thu Apr 27 2000 - 10:34:50 PDT

Vinu Arumugham wrote:

> Bob,
>
> The inner conductor and outer conductor have a 50 ohm impedance w.r.t each
> other. Since these are the only two points of contact to the device under
> test, how does the common mode impedance of the outer conductor affect the
> measurements?

The problem is that the TDR or scope chassis (alias the earth) is connected to
the outside of the coax and unbalances the source. Visualize a very short coax
to see the effect. You are not driving the differential line connected to the
shield of the coax with an equal and opposite voltage, with respect to ground
(earth), compared to the center conductor of the coax. The device under test
will typically have large capacitances from each differential trace to a large
ground plane (the circuit board), with a large capacitance from the ground plane
to earth (you holding the circuit board). The net result is that the drive
signal with respect to the circuit board ground plane is principally applied to
the trace connected to the center conductor. The measured impedance will appear
higher because the negative differential trace is not driven equally and
opposite to the positive differential trace. The measured result will be
somewhere between the true differential impedance, and an impedance measurement
made with the coax shield tied to the ground plane of the circuit board under
test as well as to the negative differential trace.

>
> Are you saying that this measurement will be possible if the coax between the
> sampling head and the probe were replaced with a twisted pair (for this
> discussion, let's assume a perfect one)?
>

Yes, if you sourced the signal to the twisted pair cable from a balun.
Otherwise, one side of the twisted pair is connected to the TDR chassis. Again,
the length of the twisted pair comes into the act. At the frequency where the
electrical length of the twisted pair is 1/2 wavelength, the twisted pair is
unbalanced again. The twisted pair thing without the balun can be made to work
over a limited frequency range. Particularly if the device being tested has a
small capacitance to ground (earth).

---Bob Lewandowski
Vixel Corp.

>
> Thanks,
> Vinu
>
>
> Bob Lewandowski wrote:
>
>> 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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