# How to determine the corner frequency needed?

I want to design a second order sallen key low pass filter. I want to attenuate a 100Hz signal to around 10%. I need to use a 2nd order low pass filter. How could I calculate the corner frequency such that I will achieve this 10% attenuation at 100Hz? In this case a Butterworth Filter has to be used.

• A lowpass of 2nd order is determined not only by its corner frequency but also the form of the pass-band (maximally flat oder with a certain peaking). Hence, you have to select one of the corresponding approximations (Bessel or Butterworth or Chebyshev I or Chebyshev II ).
– LvW
May 31 '18 at 8:33
• In this case a Butterworth filter is going to be used May 31 '18 at 8:39
• Didn't you ask this same question on dsp.SE? I can't find it, did you delete it? There was a comment in there... May 31 '18 at 8:48

A 2nd order low-pass filter will normally reduce the high frequencies at 40 dB per decade therefore, you need to have the filter's cut-off frequency half a decade below 100 Hz at 31.62 Hz ($\sqrt{10\times 100}$).
This was modelled with an RLC low pass filter interactive tool having a butterworth response ($\zeta$ = 0.7071). Make sure that if you are using a sallen key filter the op-amp has a suitable GBWP or it might not meet your expectations.