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

From: Fred Balistreri ([email protected])
Date: Tue Apr 25 2000 - 09:05:59 PDT

Pat, in approach 2 while injecting a signal in trace 1 what is
being done to trace 2? Is is driven with a complement, grounded,
left open etc. To the degree that the traces are coupled method
2 will have some problems if the second trace is not taken care
of.

Best Regards,

"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
>
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```--
Fred Balistreri
[email protected]
http://www.apsimtech.com
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