[SI-LIST] Re: Q on TDR coupon measurements
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Charles.Grasso@xxxxxxxxxxxx
- Date: Wed, 22 Mar 2006 14:50:08 -0500
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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