CHAPTER 10 - Resonance Methods of Measuring L, C, R, and Q

Abstract

This chapter explains resonance methods of measuring L, C, R, and Q. Bridge methods are largely confined to audio frequencies, though radio-frequency bridges are built. These instruments are expensive and their accuracy is not always all that can be desired. The detector has to take the form of a receiver with a heterodyne oscillator, and this further complicates the measurement. When such a bridge is used, it is advisable to use the substitution method. This method ensures that the conditions of the bridge shall change as little as possible during the test. The method of substitution can be used for measuring inductance using a variable inductor. It is more usual, however, to use a standard variable capacitor and a fixed inductor, in which case the coil under test is substituted for the known inductor in a resonant circuit and the circuit is retuned to the original frequency. The measurement of small capacitances calls for considerable ingenuity as a method must be found such that no allowance has to be made for stray capacitances. The best method is to measure the change in frequency of a negative resistance oscillator when the unknown capacitance is added to its tuning capacitance.