From: Fred Balistreri ([email protected])
Date: Tue Apr 25 2000 - 16:40:33 PDT
Vinu, your argument is theoritically correct. However the
problem is in the measurement. If you make the measurement
as you suggest and leave the plane floating you will measure
a Zo that is higher than the original method 1 discussed. The
same thing is true if you measure (tdr) a stripline structure and only
gnd one plane to the instrument. The amount of error seems
dependent on lots of factors. It's not uncommon to get a 10%
difference.
My guess is that when you float the plane you create a high
impedance path from the plane to the source (instrument).
The return current still gets to the source but is coupled through
a small capacitor, thus the impedance measured is slightly higher
than the theoritical. If there are other explanations my ears are
open. The bottom line is we know from measurements that this
is true. Foating planes in TDR measurement is not a good idea.
Best Regards,
Vinu Arumugham wrote:
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