Abstract: A detector circuit (10) is programmed to produce an output signal when an input signal is at a predetermined frequency. The input signal is provided to an inverter (14) to produce an inverse signal. The input signal together with the inverse signal are provided to two switches (16, 18). The switches (16, 18) are driven by first and second drive signals produced by a drive generator (60). The output terminals (24, 28) of the switches are alternately connected to the input terminals in synchronism with the corresponding drive signals. The output signals from the switches (16, 18) are transmitted through low pass filters (26, 30) and the resulting signals are combined to produce a summation signal by a summation circuit (36). The summation signal is passed through an absolute value circuit (40) to produce an absolute value signal. The absolute value signal is examined by a threshold detector circuit (44) which produces an output when the absolute value signal exceeds a preset threshold.

Abstract: A homing system is provided for guiding a missile (10) towards a metal target. The metal target is illuminated by a radar unit (12) with a transmitted RADAR signal of predetermined frequency. The target reflects the RADAR signal and produces HARMONIC signals having frequencies that are harmonic with the predetermined frequency. An antenna (14) is mounted in the missile for receiving signals, including the HARMONIC signals produced by the target. A wave guide bandpass filter (18) receives signals from the antenna and passes selected HARMONIC signals to a filter output. A detector (34) is responsive to selected HARMONIC signals passed by the filter (18) to produce STRENGTH signals proportional to the signal strength of the selected HARMONIC signals. The STRENGTH signals are received by an error circuit (48) that produces ERROR signals indicating the direction of the target relative to the missile (10).