Abstract: A calibrator for use in calibrating a two wire DC transmitter includes an oscillator and a light emitter which produce a time varying light signal of known frequency. No physical connection is required between the calibrator and the transmitter, other than a path for the light signal. The transmitter includes a light sensor which is responsive to the presence of the light signal and which produces a time varying calibration signal in response to the light signal. Calibration adjustment circuitry in the transmitter is used to adjust the output of the transmitter when the calibration signal is provided, thereby calibrating the transmitter.