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

About this list Date view Thread view Subject view Author view

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


I think there was a breakdown in the communication to
the expert you quoted. If you have a single-ended TDR
"Approach 2" is perfectly valid. I would appreciate
knowing if his assertion does not change under that


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
> about the design."
> (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
> had one sampling head. Yes, we could correlate back
> to a given reading and lot of PCBs. Correlation HAD
> 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

 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      |_____|    

**** To unsubscribe from si-list or si-list-digest: send e-mail to [email protected] In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. si-list archives are accessible at ****

About this list Date view Thread view Subject view Author view

This archive was generated by hypermail 2b29 : Wed Nov 22 2000 - 10:50:09 PST