Transmitting Frequency Signal and Power on 2-Wires

The existence of the low/high current modulation of a current source is sufficient to prove the continouos existence of the loop current. F1/F2 modulation of frequency shift keying is sufficient in a similar way, alone.

If you are referring to HART communication which has standard well defined two FSK frequency bands, then these can easily be added and sensed by transformer isolation. However, for wider range sensor frequency signals, frequency can be anything in time from 1Hz to 20kHz dynamic range that we have considered here. This requires new design methods to be implemented.

In the figure above, I have used loop current modulation using NPN transistor which limits the amount of maximum current to be switched to high level when signal is also high level. When signal is at low level, the loop current remains at the lowest power consumed by the oscillator circuit.

I have also used a diode to separate the power during modulation of the current as that will drop the voltage on the power / signal line and during high current switching oscillator is prevented to look at the power line voltages. However, when power line current goes to minimum and power line voltage recovers, the diode permits the current flow to charge the capacitor to power the oscillator circuit to the extent its voltage is dropped out before. This circuit also permits high dynamic range for the power supply in use from 5V to 15V but not above 15V. In industries, dynamic range of the voltage should be 10V to 30V. Perhaps a low dropout voltage regulator can be added which can take input as high as 40V and its output can be fixed to 2V to 5V range and output current of 200uA to <1mA will be ideal.

HART communication requires an intelligent circuit to be added at the node point to handle communication properly.