# LC lowpass π-type filter design

I am trying to design a LC lowpass π-type filter for power filtering. The power being filtered in question is from a SMPS(LRS-75-24), which has large amounts of HF noise which cannot be filtered out by the LDO(a LM1084). However, several sources state a ferrite bead should be used in place of the inductor L due to its increasing resistance at higher frequencies.

How do I calculate the cutoff frequency $$\F_{c}\$$ given that there is no inductance listed as a property of ferrite beads? Inductance is needed to calculate

$$F_{c}=\frac{1}{2\pi\sqrt{LC}}$$

where L and C are the inductance and capacitance of the elements in a π filter.

Additionally, what is the frequency range of the HF noise from the SMPS?

simulate this circuit – Schematic created using CircuitLab

• "what is the frequency range of the HF noise from the SMPS?" -- that's an excellent question; what have you measured? (There is no "standard noise source", SMPS are far too diverse.) Can you measure it? -- if you don't have an EMC measurement setup, how else could you measure it: oscilloscope, radio receiver, etc.? Even the target circuit -- do you know you need a filter at all, what effect is noise having on your circuit? What about common mode noise, have you verified that a DM filter will solve the concern? Commented Mar 3 at 9:05
• The switching noise is usually in the 100 MHz ballpark and up. I.e. at least an order of magnitude higher than ripple harmonics. And it is both in common and diff. mode. If you filter only diff. mode, you can get common-to-differential mode conversion after the filter again. I.e. you should filter switch noise both in common-mode and in differential-mode. Commented Mar 3 at 9:56
• @TimWilliams I do have an oscilloscope but not the SMPS, shipping costs will go through the roof if i order them separately. Commented Mar 3 at 20:15