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

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From: Steve Corey (steve@tdasystems.com)
Date: Thu Apr 27 2000 - 11:48:27 PDT


Thought I'd chime in with my $0.02 on this thread:

1. As several people have witnessed and even derived, either approach 1 or 2 can be used to get even and odd characteristic impedance and delay values. As MikeT from Tek has pointed out, method 2 extracts rather than measures these terms from single-ended measurements, assuming that the DUT meets a certain set of criteria, such as symmetry. Of course, even single-ended impedance "measurements" are extracted from voltage measurements, and rely on a certain set of assumptions based on knowledge of the DUT and electromagnetic theory, such as the
existence of a frequency-independent Zo over our range of operating frequencies.

2. If traces are not symmetric, it makes more sense to deal with the more general term of mutual impedance, since much of the mathematics and "rules of thumb" surrounding differential design are based on symmetry, z11 = z22. A foray into eigenvalue-based TLine theory, e.g. "Analysis of Multiconductor Transmission Lines" by Clayton R. Paul, will quickly convince anyone that the "even" and "odd" modes are only defined for (reasonably) symmetric pairs.

3. Floating a ground plane while measuring a coupled pair allows you to measure an impedance value, but not the value defined as "differential impedance" in circuit-board design. In classic transmission-line theory (see the reference mentioned above) this is analyzed as a three-conductor system, in which the ground plane is the third conductor. Twisted pair is a two-conductor system, assuming all other conductors are far enough away as to not affect the fields. Practically speaking, if the ground plane is floating during measurement, whatever current
would return through the ground plane is forced to take a higher-impedance path, which as Fred pointed out is the cause of the higher values of impedance measured. In short, if you plan to use the ground plane to conduct return current when your circuit is in operation, you need to allow it to return current during measurement as well.

==

I would encourage anyone interested to look at the reference I cited above or one of the many others which cover multiconductor transmission line theory. A lot of very smart people have kicked around and refined this stuff over the last 50(?) years. Unfortunately, not all of them were natural-born writers... 8^(

  -- Steve

-------------------------------------------
Steven D. Corey, Ph.D.
Time Domain Analysis Systems, Inc.
"The Interconnect Modeling Company."
http://www.tdasystems.com

email: steve@tdasystems.com
phone: (503) 246-2272
-------------------------------------------

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?
>
> 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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