Automatic Selectivity Control

A receiving system is described in which the selectivity automatically varies with the sensitivity. The variation in selectivity is obtained through the use of several triodes whose plate-to-cathode impedance is shunted across a number of the receiver tuned circuits. The circuits with which these selectivity control tubes are associated are described. Circuit diagrams illustrate the method by which the control-grid potentials of the automatic selectivity control tubes are caused to vary with the strength of received signals. A manual control is described which permits the user to increase the selectivity over that determined by the strength of the received signal. Curves are given showing the change in over-all selectivity with sensitivity, the relation between fidelity and sensitivity, and distortion as a function of power output.