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

From: Marc Humphreys ([email protected])
Date: Tue Apr 25 2000 - 12:30:02 PDT

Pat,

You may want to look at these guys. I've talked to them in the past
needs.

You folks might find these interesting.

I think you'll find all their contact info on their web page, but
if not I have a couple of contacts over there that I can forward you.

Marc

---------------------------------------------------------------------
Marc Humphreys
Lucent Technologies
Core Routing - InterNetworking Systems
508-460-3355 x2128
e-mail: [email protected]
--------------------------------------------------------------------

> -----Original Message-----
> From: Zabinski, Patrick J. [SMTP:[email protected]]
> Sent: Monday, April 24, 2000 8:45 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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