If the sensor system need an active supply, we can use only a single pair of cable to carry both the power supply and the output signal. Not only simplify the wiring, converting analog voltage level to frequency modulated pulse improve the noise immunity as well. This is the figure of the circuit;
The diode is employed to prevent the capacitor voltage to be discharged when the transistor is grounding the supply voltage source to send a zero pulse. If the sensor or input system need a supply, we can tap the power from the 1uF capacitor, as long as it need only small current. [Circuit diagrams source: Microchip Application Note]
