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?