Dear Nabila,
you may check the theory used in each component by referring to the datasheet that can be accessed by right clicking on the component and selecting the component properties, then choosing Help.
The BER Test set is suitable for advanced modulation where you have decoding process at the receiver. The problem when using BER analyzer with decoded signal is the removal of all distortions that the pulses acquire through the transmission in the system.
Regards,
Ahmad