Nonlinearities in the instruments for measurements in the frequency domain and their impact on the results obtained

The design of instruments for measurements in a given frequency domain is based on a functional network consisting of a selective circuit, a weighting circuit and an indicating circuit. A most important problem in the instrument design is the nonlinearity that is inherent to active networks. Problems arising in the measurements of discrete spectra are identified. Factors are discussed which limit the use of the whole dynamic range indicated in the specifications of the instruments. The results of a two-signal test, designed to define the real dynamic range for some applications of an instrument, are reported and discussed. The test described is suitable for defining false responses of the instrument obtained from the third-order nonlinearity.