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Amazing project professor Ajoy Raman. Perhaps for a future project integrating the AD5933/AD5934 to increase the measurement range? http://www.analog.com/media/en/technical-documentation/data-sheets/AD5933.pdf
Congratulations for your work! However, looking at your PLL schematic, all grounds have the same symbol (meaning there are connected together). In that situation, using two 2A output optocouplers hasn't too much sense. You've connected pins 3 and 5 to the same ground. In this configuration the optocoupler does not reject any noise, it's just a redundant device. Eventually, to keep clean the signal, it has to be separated ground between PLL and inverter... Do I miss something?