Special purpose oscillators

In many sensor applications, the frequency variations of signals generated as sustained oscillations are used as a measure of the tracked non-electrical quantity. The variations of oscillation frequency are usually obtained by some mechanism influencing the imaginary parts of impedances constituting a resonant circuit, electrical or some electro-mechanical equivalent. Unfortunately, in all these cases, real parts of the resonant circuit are varied as well, frequently to a quite considerable degree, meaning that in real operation the dynamic impedance of the resonator varies notably. As a result, it is rather difficult to keep the oscillations-generating circuit (i.e., oscillator) operating in optimum mode. In this connection, the optimum operating mode is defined as the state in which the condition for sustained self-oscillation is just fulfilled, without overdriving any of the active and passive devices involved. This paper discusses methods for solving this problem.