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

Thank you all for your help and advice!!

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


-----Original Message-----
From: Scott McMorrow [mailto:scott@xxxxxxxxxxxxx]=20
Sent: Wednesday, March 22, 2006 12:50 PM
To: Grasso, Charles
Cc: si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] 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=20
area, or RF connector transition.  This can be inductive, capacitive,=20
and in many cases is actually a resonant structure that rings at high=20
frequency.  This can be eliminated or reduced by better test=20
fixture/connector design, lower edge rate on the TDR, and, if the=20
ringing is symmetric, can be averaged out.  Unfortunately, launch=20
ringing is generally never symmetric, and as such will color the TDR=20
measurement of a trace by shifting the reflected impedance following the

launch structure.  This is where software like Tektronix iConnect earn=20
their money.  Using reverse scattering methods, the impedance=20
discontinuity of the launch can be theoretically removed.

2) The impedance "wiggle" seen in a TDR measurement, which is due to=20
effects such as laminate weave Er variations, laminate press thickness=20
variations, and trace etch variations.  This can generally be averaged
out.

3) DC resistive losses cause a slope in the TDR measurement.  This can=20
be partially removed by computing the slope and using linear=20
extrapolation back to the launch point.  However, even in a perfectly=20
matched coupon launch with no ringing, and with no variation in=20
impedance due to materials or manufacturing, this extrapolation will=20
always indicate a higher than actual impedance.  Why?  Because there is=20
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=20
the rise time of the pulse itself, which together determine the ultimate

resolution of impedance in any TDR measurement, without some sort of=20
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=20
the launch point.  If you compute the rise time degradation of the=20
reflected pulse, you can use that to compute the upper frequency=20
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(r) is the registered service mark of
Teraspeed Consulting Group LLC



Grasso, Charles wrote:
> Hi all,
> =20
>
> 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.
>
> =20
>
> 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??
>
> =20
>
> 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
>
> =20
>
>
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