Some Studies on Delayed Feedback Circuits

This paper attempts to describe a new method by which periodic signals in noise can be detected easily without the aid of an expensive correlator. In principle, the circuit consists of a delay network which is inserted as a part of a unidirectional loop connected with the main circuit through a hybrid coil. The delay time must be equal to, or an integral multiple of, the signal period to be detected. The delay time also must be longer than the inverse figure of the preceding network bandwidth. Successive noises will be superimposed one after another at random in circulation, while the signals are superimposed in phase. This means an improvement in the signal-to-noise ratio. Theoretically, the ideal basic delayed feedback circuit gives an improvement in the snr of 7.66 db, and in the apparent Q of 9.06. Further improvement can be expected by the use of active circuits. Possible applications are for producing artificial reverberation, a narrow-band filter, and a short-time memory circuit. The response of the delayed feedback circuit is like a comb filter. The circuit also can extract complicated periodic waves without an appreciable distortion.