The discussions about calculators began with helpful
suggestions that they can be used as aids, and then
Arpad Muranyi's "caution" about our embedded microstrip
calculations.
The bottom line is that Arpad is correct. The two most
significant clues are
(a) Zo should drop as the embedding increases (Polar's
calculator (Version 1.0) does this while ours does not)
(b) The limit of Zo with infinite embedding should
approach asymmetric stripline with one plane infinitely
far away (which neither calculator does.)
I will include below what caused the confusion about these
calculations.
Our calculator will be revised in a couple of days. I will
post when the revision is available. We will also put a .pdf file on
our web page with more detail about how the correction factor
for Er should be applied in this case.
To my knowledge, this is the only error in the calculator.
Doug Brooks
President
UltraCAD
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Discussion of the problem
IPC-D-317 says (p22) "The equations for embedded microstriplines
are the same as in the section on [uncoated] microstrip, with a
modified effective permittivity." Unfortunately, this is very
ambiguous, at best, and actually incorrect.
For those who have IPC-D-317 you can look
at the equation for microstrip (5.32) and for the correction factor
(5.18) and assume that means a simple substitution. In fact, I
believe the correction factor (5.18) must be plugged into
equation 5.29 and then equation 5.32 REDERIVED (not just substituted
into.) This is the source of the problem. When this is done,
the more pleasing direction of Zo as the embedding increases,
including to the limit of infinity, is obtained.
Thanks for everyone's interest.