A solid-state capacitive humidity sensor has a capacitance given by:
C = 1.7 RH + 365pF
where RH is the percentage relative humidity. The sensor has an associated parallel resistance of 100 kΩ. The sensor is incorporated into a feedback oscillator system with a pure inductance and a maintaining amplifier. The oscillator is to give a sinusoidal output voltage at the natural frequency of the L–C circuit for a relative humidity between 5 and 100%.
(a) Draw a diagram of a suitable oscillator system.
(b) If the frequency of the output signal is to be 100 kHz at RH = 100%, calculate the required inductance.
(c) Find the gain and phase characteristics of the maintaining amplifier.