The Application of Modulation-Frequency Feedback to Signal Detectors

Positive modulation-frequency feedback to the load circuit of a signal detector may be adjusted to make the effective modulation-frequency admittance of the load circuit equal to its direct-current conductance, thus eliminating peak clipping and improving the efficiency of detection. Conversely, negative feedback to the detector tends to increase peak clipping. Incidental effects in the amplifier from which the positive feedback voltage is derived may correspond to positive or negative, voltage or current, feedback depending on the general circuit arrangement. Design formulas and some simple equivalent circuits are given and the major design considerations are outlined. Typical examples include a detector arrangement which provides automatic-volume-control voltages and has a high input impedance and a very low output impedance, as well as Varrell´s arrangement which is in agreement with the design procedure outlined in this paper. Attention is also drawn to the great care necessary in the design of detector-amplifier circuits, using multipurpose valves (vacuum tubes), to avoid distortion due to incidental negative modulation-frequency feedback to the detector.