The wheel set shaft of a rail vehicle is protected in an effective and economical manner by a protection device. A noise detector is configured to execute a so-called \u201ccepstrum\u201d transform of a captured signal exposed to cell phone noise. With this configuration, the circuit of a virtual ground point and the following of after voltage-to-current conversion can be provided in common by using a cascode amplifier as a low-noise amplifier, making it possible to constitute a multi-band amplifier minimized in the connection loss resulting from interconnection.