Thank You for your time and informative
I will try to explain my needs
and then to summarize my understanding of your comments.
I am developing a very low noise analog front-end
with low pass filter for high speed signal acquisition
The 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
Now, to your comments.
As I understand, you agree that AppCAD is misleading
in prediction of impedance for CPW w/o GNDplane.
Quite surprising for me, but I will take it as it
2. You 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.