Self-limiting principle

The reason why the proportional frequency detector has a limiting effect is that a large capacitor C is connected in parallel across the resistors R1 and R2. By utilizing the energy storage function of the large capacitor, when the input signal amplitude changes instantaneously, the voltage across C remains basically unchanged, thereby maintaining uc1 and uc2 basically unchanged.
The limiting effect of the proportional frequency detector can also be explained as being caused by the load effect of the high-Q loop.
(1) The unloaded Q0 value of the loop must be high enough so that when the detector input resistance Ri changes with the input voltage amplitude, it can cause a significant change in the loop Qe.
(2) It is necessary to ensure that the time constant (R1+R2)C is greater than several cycles of the parasitic amplitude modulation interference. The proportional frequency detector has over-suppression and blocking phenomena.

Add resistor R to reduce over-suppression and blocking