I've got an electronics circuit which was filtering between a Passive HPF stage (1uF and 330K) and an Active LPF stage (4.7uF and 10K). As you can see it was filtering between 0.48Hz and 3.38Hz, and it was working just fine as my signal of approx. 1.17 Hz was crystal clear on the plotter.

Now just to test whether I can eliminate this signal I altered the Passive HPF by changing the 1uF to 300nF which should mean that it eliminates all frequencies below 1.6Hz but strangely although the quality of the signal has gone down (Its like a child painted Mona Lisa, same basic structure, poor detail) the signal still exists. Why is that?


1 Answer 1


I think you mixed up Low Pass and High Pass in the second paragraph of your question. Adding a image of your signals would help.

In any case, a simple RC filter is not an on/off switch, the lower frequency signals still pass through, just attenuated. At the cut-off frequency, the attenuation is -3dB, so your output is ~0.7 (=1/sqrt(2)) the amplitude of the input. As your signal frequency is close to the cut-off frequency, the output is not very attenuated.

here is a simulation of your filters, I plotted it with linear scale, and not the usual logarithmic to show the slope of the cut-off. with your modified HPF, you would still get an output amplitude of 0.6 of your original signal at 1.17Hz (red line)

enter image description here

If you really want to make your signal go away, the cut-off frequency should be much higher (the attenuation for an RC filter is -20dB/decade so with a cutoff frequency 10 times higher, the attenuation is -20dB so an output of 10%). Or design a High pass filter with a steeper slope.

See wikipedia for more details on RC circuits.

  • $\begingroup$ Thanks for your answer and for pointing out the error in the second para, I made correction. $\endgroup$ Dec 23, 2022 at 12:59

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