The specific function of shield is in mobile phone?

Generally, there are two shields in the cell phone cover RF and BB two parts respectively 1, there is interference to prevent external radiation 2, shielding the outside world the radiated interference of RF shield primarily, shielding interference is second function, the BB and the shield are the two function of function. Shield to prevent external radiation and radiation interference. But according to the circuit and layout is different, have different tendency by shield effect. Can't just say shield is to prevent the radiation or exposed to radiation. 1. Such as radio frequency (rf) if is zero intermediate frequency circuit, it may be the local oscillator leakage, shield to prevent external radiation on one hand, also can prevent outside interference signal to pay such as harmonic intermodulation interference demodulation. 2. If is a baseband circuit, such as the kernel is a high speed circuit, or baseband inside some inductance booster element, shield to prevent external electromagnetic radiation on the influence of the rf circuit, to prevent rf interference also affect the normal work of the baseband circuit of the device.