One thing to note is that feedthrough caps only work if they have a
source impedance that is sufficiently high. If the source impedance is
low, then they are ineffective.
The figures you see for attenuation for feedthough caps, mains filters
and similar are all based on a 50 Ohm measuring system, where the input
is driven from a 50 Ohm source and the output is measured on a 50 Ohm
load (spectrum analyser).
Of course the mains impedance is fractions of an Ohm at 50/60 Hz, but it
is impossible to say what it is at high frequencies, but you can be
pretty sure it will not be 50 Ohms resistive except at a very few
frequencies that nobody can predict in practice. As such, the mains
filter that is supposed to give 50dB attenuation probably gives anything
but that. It might be higher or lower.
As such, I don't blindly trust feedthrough caps. Some are not just caps,
but have an inductor too, which is more effective, but I would if
possible put a resistor on the outside close to the cap, so the unwanted
energy is dissipated in the resistor.
One company I worked for took this 50 Ohm thing a bit too far in my
reckoning. They made filters capable of handling 100's of A at 1000's of
Volts. Someone wanted 100dB attenuation. This was achieved by having one
of the coils on a very high permeability core. As such, the inductance
was large, and the attenuation measured in a 50 Ohm system was large.
However, if you actually put 100's of A though it, the core would
saturate, the inductance drop and nothing like 100dB would have been
achieved. But of course it was most unlikely the client would actually
try to measured the attenuation whilst running 100's of A though it. All
they might do (if anything) was to put it on a spectrum analyser and see
the 100dB attenuation.
--
Dr. David Kirkby,
G8WRB
Please check out http://www.g8wrb.org/
of if you live in Essex http://www.southminster-branch-line.org.uk/
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