An improved method for introducing self-tuning features in an active filter for detection of weak signals in a noisy background is described. The filter, which works at low frequencies (100 Hz-10 kHz), is provided with facilities for search and tracking. During an experiment, the filter was able to detect a sinusoidal signal of 1 mV r.m.s. in a 10kHz-bandwidth background noise of 3 mV r.m.s. © 1969, The Institution of Electrical Engineers. All rights reserved.