Here is the schematic :

Here is the following assertion :

"Small Signal Effect: A reduction in CTR by 2x will cause the DC gain and crossover frequency of the for-ward converter’s voltage feedback loop to drop by 2x, assuming all other parameters are constant. Likewise, an increase in CTR by 2x, assuming no change in other parameters, will cause the DC gain and the crossover frequency of the voltage feedback loop to increase by 2x. The voltage feedback loop must be designed ensuring that at CTR(MAX) (maximum CTR of the opto-coupler), the crossover frequency of the feedback loop stays well within the Nyquist frequency of the system (= switching frequency / 2). A good rule of thumb is to design the voltage feedback loop’s crossover at about 1/10 of the switching frequency for an opto-coupler at the nominal value of CTR. "

I do not understand why the crossover frequency is function of CTR and why CTR = a*crossoverfrequency. Could please tell me the relation between the two parameters ? If you have any documentation about this, it would be great to read it.

Thank you very much and have a nice day !