Detector having a constant false alarm rate

DETAILED DESCRIPTION The present invention is an improved constant false alarm rate detector circuit in which the variable threshold circuit includes a bistable device such as a tunnel diode which has a first low voltage stable state and a second high voltage stable state and a variable threshold level which is sensitive to both noise and input signals applied to the input of the detector.
The variable threshold circuit also includes a control circuit which determines the range gating interval of the detector by providing a square range-gate pulse or gating pulse and controls the sensitivity of the bistable device by providing a reset pulse which sets the bistable device to its first low voltage stable state.
The reset pulse has a shorter pulse width or duration than the square gating pulse and may be produced by differentiating the leading edge of the square gating pulse.
The improved variable threshold circuit also includes a logic circuit coupled to the bistable circuit means which provides an output signal in response to switching of the bistable element from its first low voltage state to its second high voltage state which occurs during the interval established by the gating pulse provided by the control circuit.
The output pulse is coupled into a pulse stretcher which produces output pulses that are coupled into first and second counters.
The first counter has a preset threshold reprsented by M such that when the total number of pulses registered in the first counter is equal to or greater than M an output signal indicative of the presence of a target or an alarm condition is indicated.
The second counter provides an analog output signal proportional to the number of pulses produced by the pulse stretcher and the rate at which the analog output signal is varied is proportional to the rate at which the pulses are produced by the pulse stretcher.
The analog output signal is coupled through a series feedback circuit including a long time constant low pass filter which controls the d