抄録
The generaly used noise filters are consisted of L and C, therefore their impedances are reactance. Then they may be series resonated with the connected circuits and lose filter action at the resonated frequencies.
Other case, the filter connects in main lines, have many resonances. If the pulse current (such as burst noises) flows in it, the damped oscillation will be occur namely the disturbance noise.
Here introduce the noise filter applied the principle of constant impedance, which construted from the bridged. T.
Let the series arm is Z1, and the shunt arm is Z2, and the resistance arm are both R.
If select their constants as R2=Z1•Z2 it's input impedance will be the constant R.
Apply this principle to noise filter.
As it's character must be low pass, than set Z1=L, and Z2=C.
It's input impedance will be the constant R=√L/C for wide frequeney range (ex. 400kHz_??_30MHz), even if the condition connected any impedances between its onput terminals.
The value of R should be selected same as the characteristic impedance of main lines. Constant impedance means resistance or loss element which don't resonate with any circuits. When the filter connected to main lines, the noise energy in it, will be faded away.
Other case, the main lines with the filter will lose resonance action, then even if the pulse current flow in it, there are no damped oscillation or no noise.
There are two kinds of noise mode, namely normal and common. Then the practical filters must be modefied to two types.
