[SI-LIST] Re: Q on TDR coupon measurements

Charles,

I'll follow on to what Ray and Lee have already said:

There are several "things" that color a TDR measurement of a test coupon:

a.) The launch ringing that occurs at a poorly designed test contact 
area, or RF connector transition.  This can be inductive, capacitive, 
and in many cases is actually a resonant structure that rings at high 
frequency.  This can be eliminated or reduced by better test 
fixture/connector design, lower edge rate on the TDR, and, if the 
ringing is symmetric, can be averaged out.  Unfortunately, launch 
ringing is generally never symmetric, and as such will color the TDR 
measurement of a trace by shifting the reflected impedance following the 
launch structure.  This is where software like Tektronix iConnect earn 
their money.  Using reverse scattering methods, the impedance 
discontinuity of the launch can be theoretically removed.

2) The impedance "wiggle" seen in a TDR measurement, which is due to 
effects such as laminate weave Er variations, laminate press thickness 
variations, and trace etch variations.  This can generally be averaged out.

3) DC resistive losses cause a slope in the TDR measurement.  This can 
be partially removed by computing the slope and using linear 
extrapolation back to the launch point.  However, even in a perfectly 
matched coupon launch with no ringing, and with no variation in 
impedance due to materials or manufacturing, this extrapolation will 
always indicate a higher than actual impedance.  Why?  Because there is 
also a concurrent actual change in impedance due to internal inductance, 
which is related to skin depth.

4) AC resistive and dielectric losses cause an additional coloration of 
the rise time of the pulse itself, which together determine the ultimate 
resolution of impedance in any TDR measurement, without some sort of 
normalization and reverse scattering process.

Bottom line is that the closest measurement of the actual impedance of a 
trace is made with an exceptional launch, that has no ringing, near to 
the launch point.  If you compute the rise time degradation of the 
reflected pulse, you can use that to compute the upper frequency 
resolution of your measurement.

As for "average mean", who knows?


Scott



Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax

http://www.teraspeed.com

Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC



Grasso, Charles wrote:
> Hi all,
>  
>
> In researching the "state-of-the-art" for TDR measurement I ran across
> an interesting measurement that I would welcome comments on from the
> group.
>
> After the coupon has been probed, the impedance is only measured over a
> small portion (<25%) of the resultant waveform and then using
> an "average mean* technique for reporting the impedance values.
>
>  
>
> In the opinion of the group - is this a valid technique? What are the
> reasons for only using a small portion of the waveform ? Does anyone
> know what an *average mean* value is??
>
>  
>
> Thanks in advance!!
>
> Best Regards
> Charles Grasso
> Senior Compliance Engineer
> Echostar Communications Corp.
> Tel: 303-706-5467
> Fax: 303-799-6222
> Cell: 303-204-2974
> Pager/Short Message: 3032042974@xxxxxxxx
> Email: charles.grasso@xxxxxxxxxxxx;
> Email Alternate: chasgrasso@xxxxxxxx
>
>  
>
>
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