The D169 serves as a level detector and provides complementary outputs. The op amp is used to integrate the input signal Vin with a time constant of R1C1. The input (must be negative) causes a positive ramp at the output of the integrator which is summed with a negative zener voltage. When the ramp is positive enough D169 outputs change state and OUT 2 flips from negative to positive
The output pulse repetition rate Â£>, is directly proportioned to the negative input voltage Vin.