Gas transmitters consist of an enclosure, sensor, and electronics that convert a signal from a gas sensor into an analog output signal. Electrical outputs for gas transmitters include analog current, analog voltage, and frequency. Analog current levels such as 4 – 20 mA are suitable for sending signals over long distances. Typically, a current is imposed on the output circuit proportional to the measurement. Feedback is used to provide the appropriate current regardless of line noise and impedance. Analog voltage outputs are simple, usually linear functions of the measurement. Frequency or modulated frequency outputs for gas transmitters include amplitude modulation (AM), frequency modulation (FM), sine waves, and pulse trains.