The Empirical Analysis of Complex Electric Waves

In many problems dealing with electric waves, particularly those encountered in electrical communication, it is necessary to examine with some care the several sinusoidal components present. Both qualitative and quantitative methods are in use. A convenient qualitative examination may be made by heterodyning the complex wave with a sinusoidal wave of variable frequency and providing means for detecting any low frequency beat currents which may be produced. In practically all methods for the quantitative measurement of the individual components of a complex wave, means are provided for selecting a single component and for preventing other components from reaching the indicating apparatus. The general arrangement of the selecting and measuring elements depends upon the character of the information sought. If the problem involves an exploration of the frequency range of the complex wave, primarily for the purpose of determining what components may be present, automatic analyzers are available which cover the field point by point and plot its spectrum. There are also analyzers which permit the field to be covered by heterodyning with a variable frequency so as to cause the resulting wave to be swept across a fixed selective circuit. For the precise measurement of any single component, particularly a component of small amplitude in the presence of others of larger amplitudes, analyzers have been developed which employ the well known heterodyne principle which utilizes selectivity at a frequency below that of the unknown wave, thus effecting an appreciable increase in the percentage separation between adjacent components.