RF interference generally comes from two sources, internal and external. Internal sources are things in the product that generate electromagnetic noise in the same frequencies as the radio. This can include things like crystal oscillators, switching power supplies and regulators and motors. Internal sources usually reduce the sensitivity of the receiver, resulting in a reduction in system range. When this happens, the noisy components need to be shielded and filtered to knock down the noise both conducted through traces and radiated around the board. This can get very complicated very quickly, so it is important to follow good PCB and system design rules for reducing the potential for Electromagnetic Interference (EMI). In our experience, switching regulators and motors have the biggest impact in our customer’s designs, so take extra care if your design uses these components.
External interference is typically other transmitters on the same frequency in the same environment at the same time. This generally results in corrupted packets and reduces range since the two sides need to be closer to successfully pass messages. This is somewhat like trying to talk to your friend at a loud concert. Since it is rare that the installer can affect other transmitting systems in the area, having methods to repeat messages and receive acknowledgements as well as the ability to have many frequencies (or channels) to transmit on helps reduce the impact of external interference.