The first part makes the signal saturate at the given tension source for the operational amplifier (reverting the signal in the process)
That square frequency is source to an integrator, that makes the operation
Vout = (-1/Ri.Cf)* ∫ (Vin dt)
The extra resistor is for real case uses, being the one responsible for enabling the capacitor to release its energy from the first charge, otherwise it won't discharge
This operation results in a triangle wave that is feed to a derivator amp op, which has the following principle for conversion
Vout = -Cin*Rf* d(Vin)/dt
Rf being feedback resistor
The resistor before the capacitor is used so at high frequencies there is still impedance for the circuit entrance, since the impedance from a capacitor dissapear when frequency reach high values like a few hundred MHz
You can alter the first DC source so you can have an offset for detection
The alternate current source would be the entrance of any received signal
If you use this circuit don't forget it causes phase shifting on the original source
