Conversion detectors for superheterodyne sets are classified in two groups, containing two types each. These types are illustrated in Figs. 1(a), 1(b), 2(a), 2(b), 2(c), 2(d), and 2(e). It has been found possible to represent measured static characteristics (current versus applied voltage) of all types of detectors considered accurately by an equation of the type i = EAnea-V By using this expression for the static characteristics, conversion gain, distortion effects (modulation rise, modulation distortion, cross-modulation), and harmonics (causing whistling notes) could be calculated for all types of conversion detectors. An apparatus is described, permitting of easy measurements of conversion gain and harmonics. Measured and calculated data check as well as could be expected. Conversion gains of more than 400 were found with modern valve conversion detectors. It is pointed out that distortion may be determined by measuring the harmonics.