Pi filter cut off frequency

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Techman_7

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

1736338295258.png

1736398384024.png

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.
 

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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.
 

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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
 

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

C = C1*C3/(C1 + C3)

1736414945301.png
 

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