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I calculated value of inductors and capacitors by using online calculators and document that I attached to post. I want to obtain 17 kHz cut off frequency. According to document and online calculator value of capacitors and inductor are given below. Since I have restriction in component heights in my PCB, I used these values L1 and L2 4.7uH, C1 and C3 4.7uF and C2 is 22 uF. According to these values I am getting 17Khz cut off frequency in LTSpice. According to document values that I used in LTSpice is not correct, but I get correct cut offf frequency. If cut off freqeuncy of filter depends on formulas how I can get correct cut off freqeuncy in LTSpice with wrong component values? Regards.
Yes, for a fifth order filter as most others we need to know both the input impedance and the output impedance.
For the values given with zero input source resistance we get about 25826.55Hz for the -3db point while with an input source resistance equal to 1 ohm to match the output resistance we get about 29390.16Hz for the -3db cutoff point.
Also the zero ohm source resistance has a MUCH different response curve being partly like a bandpass filter rather than a low pass filter, while the input resistance equal to 1 ohm response is more like just a low pass.
Yes, 1 ohm is used in filter designs that are meant to be scaled up later. It's actually the "normalized" version, normalized to 1 ohm.
For example, in that design if we have C=50uf and we want 50 ohms instead of 1 ohm then the new value of C is C/50=1uf. For an inductor, if the value is 1uH with 1 ohm then the new L with 50 ohms is 50*1uH=50uH.
There's also the assumption that Rin=Rout.