moisture indicator

Certain types of passive RC networks show a resonance effect, that is to say, their output voltage is slightly larger than the input voltage at a specific frequency . Two typical networks that demonstrate this effect are the lowpass filter of figure 1 a and its highpass counterpart of figure 1 b. Figure 2 indicates that both filters have a 6 dB per octave roll-off with a slight hump around the break frequency (curves a and b respectively). The `gain` near the break point is roughly 0.8 dB. Since the slope changes from positive to negative this would indicate that somewhere near the break the phase angle is zero. Further RC net- works can be added (figures 1 c and 1 d) which results in a more pronounced magnification.