Some experiments on the use of frequency modulation in electrical measurements

The paper describes experiments designed to measure capacitance change by the variation produced in the frequency or phase of a signal from an oscillator normally of high frequency-stability. The advantages of such a method are tabulated, and the factors limiting the sensitivity and stability noted. In the experiments the sensitivity was increased by a process of frequency multiplication and frequency changing. An initiating oscillator designed to reach a high frequency-Stability at one particular frequency of operation is described. The auto-synchronized oscillator was adopted as a frequency discriminator with a high sensitivity but with restricted linear operating range. Results are given which indicate that capacitance changes of 5×10¿4 ¿¿F were measurable with a possible error of ± 50%. Simple experiments to study the method for possible use in the measurement of capacitance changes occurring in about one millisecond are described.