Pi filter cut off frequency

[Techman_7]

The cut off frequency for pi filter ckt shown below is -

But as per simulation in LTspice (AC analysis), the above equation does not meet the simulation cut off frequency.
So, is it the correct equation applicable for pi filter.

 

[FvM]

You are showing a formula for LC resonance frequency, LC filter cut-off is most likely in this order of magnitude but not the same.

Generally, specifying a filter without source and load impedance is useless. We don't know your measurement setup and results, please show.

 

[KlausST]

Hi,

I agree with FvM.

So let´s say the output side is high impedance, then
* if the input side is low impedance: then f_res = 1/ (2*Pi* sqrt(L1 * C2))
* if the input side is high impedance, then f_res = 1/ (2*Pi* sqrt(L1 * Cs)) .. with Cs = 1 / ( 1/C1 + 1/C2)) = series impedance of C1 and C2

in both cases talking about cutoff frequency is not correct .. because it´s un undamped resonance.

Klaus

 

[FvM]

In case of a Buttwerworth filter with matched input and output impedance, the formula actually fits.

C = C1*C3/(C1 + C3)