The antiparallel diodes limit the voltage to a maximum of 0.7v and the resistors in series limit the current, so you can always get the same signal voltage, than the opamp clean up and amplify and balance the signal so you get a perfect stable and clean square wave signal.. this goes to pin 14 in the pll...
To test it is easy you simply pulse your primary (in saturation) and work in the circuit until you get the right signal for the pin 14...
