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

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From: Vinu Arumugham (vinu@cisco.com)
Date: Wed Apr 26 2000 - 12:14:59 PDT


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?

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)?

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 <bdewitt@ix.netcom.com>
>> >> > To: <si-list@silab.eng.sun.com>
>> >> > 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: owner-si-list@silab.eng.sun.com
>> >> > > > [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of
>> >> Zabinski, Patrick
>> >> > > > J.
>> >> > > > Sent: Monday, April 24, 2000 6:45 PM
>> >> > > > To: si-list@silab.eng.sun.com
>> >> > > > 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
>> >> zabinski.patrick@mayo.edu
>> >> > > > Rochester, MN 55905
>> >> www.mayo.edu/sppdg/sppdg_home_page.html
>> >> > > >
>> >> > > >
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>> >
>> > --
>> > Fred Balistreri
>> > fred@apsimtech.com
>> >
>> > http://www.apsimtech.com
>> >
>> >
>>

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