Thank You for your time and informative
I will try to explain my needs and then
to summarize my understanding of your comments.
developing a very low noise analog front-end with low pass
filter for high speed signal acquisition board.
corner frequency is at few hundred MHz (sorry, I can't be
more specific). It is a Bessel (constant group delay)
filter. I try to do it with discrete components (L-s and
C-s). Those are quite good components with high Q (>10)
and low production tolerances (<=2%). My problem is to
take parasitics of PCB layout into consideration in
selection of discrete L and C values. I plan to calculate
the L and C of PCB and reduce values of correspondent
discrete components by the same amount. Usually, PCB
calculators provide transmission line characteristic
inpedance Zo and wave propagation speed Vp. I use them to
calculate L=Zo/Vp and C=1/(Zo*Vp) per length's unit. So,
correct prediction of PCB absolute equivalent electrical
characteristics (Zo and Vp) is quite critical for me. Let
say, my design is sensitive to difference in trace
inductance of 0.5 nH vs. 0.7 nH and I want to be sure that
my prediction is reliable.
Now, to your comments.
1. As I understand, you agree that AppCAD is misleading in
prediction of impedance for CPW w/o GNDplane.
surprising for me, but I will take it as it is.
provided results from two more calculators, and they are
quite different. Zo of 55.0 ohms vs 63.5 ohms is a big
difference for me, as you already understand. You also
wrote that "chemandy" calculator is "worse" (or not ?) by
definition, because it does not take (or does ?) in
account the fringe fields etc. etc.
Which one would
you use for my specific need?
Can you or anybody
else provide any suggestion, especially at the higher
level of the task.
For example, may be, to use
significantly lower value of H will be better (i.e. to put
a GND plane closer to signal layer). It will increase the
Cpcb, which may be compensated by appropriate selection of
Cdiscrete with good !!! (0.1+-0.05 pF) resolution, but,
simultaneously, it will decrease Lpcb, and it will make
selection of Ldiscrete (which are available with not so
high resolution, but only with 10+-2 %) much easier.
Any other proposals/advices will be appreciated.
Thank You for your time.