OPTO ISOLATED (FM) INFORMATION TRANSFER 
Sometimes it is necessary to transfer an analogue signal from one system to another without making any electrical connections.
This can be done with two phase-locked-loops in an fm system using light as the transmission medium. Because of the high degree of electrical isolation obtained, low level signals can be transmitted without interference, even if there is a large potential difference between the sending and receiving circuits.
This can be done with two phase-locked-loops in an fm system using light as the transmission medium. Because of the high degree of electrical isolation obtained, low level signals can be transmitted without interference, even if there is a large potential difference between the sending and receiving circuits.
The circuit is shown above right. Transmitter is an NE 567 phase-locked-loop IC operating as a voltage controlled oscillator which drives the LED section of an opto-coupler.
The LED will flash at the operating frequency of the oscillator which is in turn dependent on the input signal level and the values of A, and C.
The output signal from the opto-coupier drives an amplifier which provides an output of sufficient amptitude (50 to 200 mV) to drive the receiving NE 567 phase-locked-loop. The receiver operates as an fm detector which demodulates the output ot the opto-coupler to provide the original input signal. The inherent non-linearity of the transfer function in the two phase-locked-loops cancel one another out to give an extremely linear information transfer.
Rooselvet Ramirez CAF
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