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

From: Bob Lewandowski ([email protected])
Date: Tue Apr 25 2000 - 12:32:06 PDT

I think Mike's p.s. needs to be capitalized and put up front. If they only make
a single measurement of each line and not the second which shows the coupling
effect of one line on the other, the math won't yield the correct answer.
Differential TDR is a 2 port measurement which must include the cross terms. A
single measurement of each line only works if they are totally isolated like two
coaxes. The time alignment is also a very big issue in this measurement, and
probably the biggest reason to use method 1, because cable and connector delay
mis-matches can be adjusted out within the instrument. Something that is
extremely difficult to get right with method 2. The 1-2 vs 2-1 business can be
an issue if there is magnetic material involved with the t-lines. (Not likely)
Comparing the results can provide a check on the time delay alignment.

---Bob Lewandowski
Vixel Corp.

Mike Jenkins wrote:

> Pat,
>
> I've used both approaches. From a mathematical viewpoint,
> they are equivalent (assuming the system is linear and
> time-invariant -- which are pretty good assumptions).
>
> >From a practical point of view, Approach 1 is a bit more
> expensive. However, the processing is "canned" inside
> the instrument, so less error prone. Approach 2 gives
> good insight to principles that anyone using dif'l signals
> ought to understand (e.g., coupling between lines lowering
> the impedance). But the chances of screw-ups are higher.
> For instance, if the two waveforms to be subtracted are
> not time-aligned, it's a mess.
>
> Hope that helps.
>
> Mike
>
> ps: You didn't explicitly state it for Approach 2, so
> I will note that TWO signals must be recorded for
> each measurement, the "rho(11)" reflection and the
> "rho(12)" coupling (although recording rho(21) in
> the second measurement is redundant, since it equals
> rho(12) theoretically).
>
> "Zabinski, Patrick J." wrote:
> >
> > 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
> --
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Mike Jenkins Phone: 408.433.7901 _____
> LSI Logic Corp, ms/G715 Fax: 408.433.7461 LSI|LOGIC| (R)
> 1525 McCarthy Blvd. mailto:[email protected] | |
> Milpitas, CA 95035 http://www.lsilogic.com |_____|
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
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