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PHASE LOCKED LOOP APPLICATIONSANALOG LIGHT-COUPLED ISOLATORS (565,567)The analog isolator shown in Figure 8-55a is basically aFM transmission system with light as the transmissionmedium. Because of the high degree of electrical isolationachieved, low-level signals may be transmitted withoutinterference by great potential difference between thesending and receiving circuits. The transmitter is a 565used as a veo with the input applied to the veo terminal7. Since the light emitting diode is driven from the 565veo output, the LED flashes at a rate proportional to theinput voltage. The receiver is a photo transistor whichdrives an amplifier having sufficient gain to apply a 200mVpeak-to-peak signal to the input of the receiving 565,which then acts as a FM detector with the output appearingat pin 7. Since the output has a twice carrier frequencyripple, it is best to keep the carrier frequency as high aspossible (say, 100 times the highest modulation frequency).Because of the excellent temperature stability of the 565,drift is minimal even when dc levels are being transmitted.If operation to dc is not required, the output of the receivercan be capacitively coupled to the next stage. Also, a 566can be used as the transmitter.LIGHT-COUPLED ANALOG ISOLATORS+V +V +v +V +V~ 10INPUT7 565 4 t--I~+-'£OUTPUTDC LEVELADJUST'::"'-V -V -v -V~OUTPUTIFILTER CAP.Figure 8-55aFigure 8-55b shows that the 567 may be used in the samemanner when operation from 5V supplies is required. Here,the output stage of the 567 is used to drive the LEDdirectly. When the free-running frequency of the receiving567 is the same as that of the transm itting 567, thenon-linearity of the two controlled oscillator transferfunctions cancel so that highly linear information transferresults.LIGHT-COUPLED ANALOG ISOLATORS+5 +5+5OUTPUTDC LEVELADJUSTFigure 8-55b58