In OptiSystem a signal is generally expressed as a sampled signal, which is passed to a component in one chunk. To perform time domain calculations, like feedback, the sampled signal must be converted to individual samples, which consist only of a single signal value and its corresponding position in time. This allows OptiSystem to propagate the samples through different components at different times. The attached project file Electric_Phase_Locked_Loop.osd is an example of an electrical feedback system.
You can follow the tutorial on this example at: