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

From: Mike Jenkins ([email protected])
Date: Fri Apr 28 2000 - 19:26:20 PDT

Dave,

I think there was a breakdown in the communication to
the expert you quoted. If you have a single-ended TDR
PLUS A SINGLE-ENDED SCOPE CHANNEL TO MEASURE THE
COUPLING FROM THE LINE DRIVEN BY THE TDR, then
"Approach 2" is perfectly valid. I would appreciate
knowing if his assertion does not change under that
assumption.

Regards,
Mike

Dave Hoover wrote:
>
> Patrick,
>
> I asked a TDR Guru from Tek (MikeT) what his
> enterpretation of approach 2 is and he sent the
> following:
>
> "Dave,
>
> "Approach 1" is the ONLY correct way to measure
> differential impedance.
>
> "Approach 2" is NOT differential impedance. It is two
> regular or single-ended TDR measurements. I don't
> know what "magic of mathematics" is referred to, but I
> do know that there is no way to compute true
> differential impedance using two regular TDR
> measurements. What CAN be done, is to MODEL a change
> in single ended impedance and CORRELATE that to a
> change in differential impedance. This is called
> modeling, and like all modeling, it is only as good as
> the model, and the ASSUMPTIONS that go into the
> model.
>
> Differential Impedance is DEFINED as two times the odd
> mode impedance. Odd mode impedance is DEFINED as the
> impedance measured on one of the lines, while the
> other line is being driven by an equal amplitude,
> opposite polarity TDR pulse. There is NO question
> about it, there is exactly one correct way to MEASURE
> it.
>
> In my opinion, modeling has a place in printed circuit
> board manufacturing - in DESIGN!
>
> Quality Control Test should be about making
> independent measurements, that make NO ASSUMPTIONS
>
> (Dave speaking here) I wasn't quite sure what you
> meant with the approach 2 until I saw MikeT's
> response.
> If MikeT describes that test correctly, then I
> understand it better. Fabricators used this method
> for years before we had any way to TDR test
> differential signals. Main reason being was we only
> to be done for each design lot (part number). This was
> time consuming and awkward. Then decent equipment came
> around with multiple sampling heads that allowed us to
> really test differential (odd mode). I have to say I'm
> in agreement with MikeT's response.
>
> Dave Hoover
> (BTW - I co-chair IPC-2141 - Controlled Impedance

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