Problems in Selective Reception

In this paper we are comparing from a theoretical standpoint the methods of attainment of the highest degree of selectivity required by present broadcasting conditions, and investigating the distortions introduced by receivers employing such methods in practice. The equations for magnification and phase differences introduced by one or more simple tuned circuits are first obtained in a convenient form: corresponding expressions are obtained for the band-pass circuit, and a note is appended upon circuits with reaction. We then investigate the effect of the detector by determining the audio output from a square law rectifier when an amplitude modulated wave of general form is applied to it: the effective selectivity of, and audio distortion initroduced by, the tuners previously discussed are thus obtained. The whole process is then repeated for the more difficult case of the linear detector and the harmonic distortion here calculated; the "demodulation" effect which occurs when two transmissions are received simultaneously is considered in theory and practice. The two rival systems of selective tuning-viz., the band-pass and the simple sharp tuner with audio tone correction-are compared as regards their ability to deal with the direct and also the heterodyne interference from unwanted neighboring transmissions: the probability of frequency and harmonic distortion in the resulting reception under practical operating conditions is also discussed, and it is shown in particular that the band-pass system is inferior on at least two of these four heads.