# Low Pass Filtered Noise Power Spectra, Showing Low Power @ Lowest Frequencies

White noise is being passed through a voltage follower, then filtered through a simple first order low pass RC circuit (C=0.5 micro Farad, varying resistance between 2k-10k, ohm depending on RC value tested). This is then recorded using an A2D device. I am then calculating the power spectra using the following python code applied to the acquired raw data:

def FFTF2(raw_data):
result = abs(np.fft.fft(raw_data)*np.conj(np.fft.fft(raw_data)))
return result[0:(len(result)/2)]


The result is the following:

This is at an RC of 1ms. As you can see the very low frequencies are missing power. Its as if the signal is going through a bandpass filter. The effect is not as noticeable at higher RC values.

This is the unfiltered noise:

If I use a different op amp in the voltage follower circuit I get the following at different RC values:

RC of 5ms:

RC of 2.5ms:

RC of 1ms:

What could be causing this "cutting" at the lowest frequencies? Does the power spectra of this filtered noise look "ok", or should the decay be smoother?

• Two ideas off the top of my head: 1) it's random (can you take more data and see if the effect persists?); 2) there's some current leakage through your capacitor (just a hunch that this would cause you to lose power at low frequencies). – tok3rat0r Jul 24 '15 at 14:19
• This question might receive better attention, a better answer in the Electrical Engineering stack – docscience Jul 24 '15 at 14:21
• @tok3rat0r: 1) The sample sets used are 100,000 samples @ 10,000 a second. I have tested maybe 50+ sample sets, all show this problem to some varying degree, depending on the op amp used in the voltage follower. 2) I have tested 2 different RC circuits, both show this issue. – gline Jul 24 '15 at 14:26
• @docscience: Maybe it could be moved there if the moderators think it would be better there? I wasn't sure where to post it. – gline Jul 24 '15 at 14:28
• OK, so most likely to be some non-ideal component behaviour (finite resistance of capacitor or capacitance of resistor). As @docscience says, probably a better fit for EESE. – tok3rat0r Jul 24 '15 at 14:29