Adaptive threshold circuit

Abstract

A multi-channel adaptive threshold circuit usable to extract affirmative detection impulses from a plurality of detection circuits each having a different background noise level. The circuit comprises a plurality of analogous detection circuits whose outputs are coupled in parallel between a threshold circuit and threshold activation device such as a switch which grounds all outputs when in the ON position. The input to each detection circuit, which is a composite of sporadic affirmative return pulses and continuous background noise, is passed through a unidirectional conducting device, such as a base-to-emitter junction of a transistor, to charge a capacitor to a value which is a function of the continuous background noise. When the threshold activation device is turned off, the output plate of the capacitor of each detection circuit is applied to a fixed threshold circuit. Since a large value resistor coupled between the input plate and ground prevents rapid discharge of the capacitor, the potential across the plates remains substantially constant. The slow discharge results in a higher voltage on the input plate which causes the base-to-emitter junction to be back biased and thus nonconductive for almost all detection signal levels except high voltage pulses which are likely to indicate a positive detection return. Thus, each channel independently sets its own threshold level which is determined by the additional charge due to background noise across the capacitor just before the threshold activation switch is turned off. The signal threshold level in each detection circuit is then the sum of the additional potential across the capacitor of that detection circuit and the fixed threshold voltage established by the threshold circuit.