It is essential to understand the potential interference sources a device will be up against in the real world. Interference is the presence of unwanted noise or signals that increases the difficulty of receiving a signal. Man-made noise is everywhere today; alternators, generators, and many other devices produce unintentional RF radiation. Noise can also be internal to the product. Switching power supplies, PWM generators and high speed clocks all produce noise that can affect the radio inside the same product. If the product is going to use any of these then it is best to consider measures to handle the noise early in the design process.