Abstract: An apparatus for detecting noise in communications channels in electrical power transmission systems and generating squelch signals, for transfer-trip receivers for performing protection and control functions, detects changes in a period of an analog signal and generates a signal in response to such changes. The apparatus receives an analog signal, generates zero-pulses corresponding to zero-transitions of the analog signal, generates an error pulse when an interval between successive ones of the zero-pulses either exceeds a preselected long window interval or is less than a preselected short window interval, and generates, in response to the error pulse, a squelch signal.