In message <[email protected]>,
dated Wed, 22 Jul 2015, "ce-test, qualified testing bv - Gert Gremmen"
<[email protected]> writes:
Filter specifications should not be made in 50 ohm environments but
in much lower impedance ranges (0.5 -5 Ohms), so the product will
better fulfil the expectations of their users
See the less well-known CISPR 17 standard.
· Filter manufacturers need to be aware of the existence of
emission and immunity regulations and create fully bi-directional
filters, for lower input currents (due to lower power consumptions of
modern devices)
· Adequate input/output filters do not need high value touch
current capacitors. By using T-type of filters (LCL) instead of
PI-types (CLC) the values of decoupling caps can be heavily reduced (no
touch current), at increased cost of dual CM-inductors. But we are used
to that aren?t we?
I did a quick simulation of T and Pi filters with 1 ohm impedance and
normalized frequency, and I can't confirm that the capacitor in the T is
smaller than those in the Pi. However, the inductors in the T are
smaller than the one in the Pi.
This seem logical to me, since at cut-off (-3 dB frequency) the T filter
'matches' zero impedance source and load, and the Pi filter 'matches'
infinite impedances. So the T filter has a normalized C value of 2, with
two L values of 1 in parallel, so LC = 2 x 0.5 = 1. The Pi filter has an
L value 2 in series with two Cs of value 1, so LC is again 1.
· Shunting a low impedance signal with high value capacitors is
NOT the way to go. Attenuation needs a high series impedance followed
by a low ground path, I think we all agree about that.
This is a very important point.
--
OOO - Own Opinions Only. With best wishes. See www.jmwa.demon.co.uk
When I turn my back on the sun, it's to look for a rainbow
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
-
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