Sensor transmitters are measurement or signal conditioning packages that provide standard, calibrated outputs from sensors or transducers. Outputs types include current loops, variable voltage levels, frequency or pulse signals, timers or counters, relays, and variable resistance outputs. Sensor transmitters can also provide radio frequency (RF) signals and transistor-transistor logic (TTL) outputs. Current loops 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. Variable voltage outputs are simple, usually linear functions of the measurement. Frequency or modulated frequency outputs for sensor transmitters include amplitude modulation (AM), frequency modulation (FM), sine waves, and pulse trains.