ips (PIAs, conductivity and permeability, and of frequency. Reflection losses ACIAs, etc) will maintain their initialized set up state indefi- decrease with increasing frequency for the E-field (electric) and nitely. They need to be re-initialised periodically. increase for the H-field (magnetic). In the near field, closer than the distance between source and barrier also affects the reflec- 23.13 Shielding tion loss. The re-reflection loss B is insignificant in most cases where A is If despite the best circuit design practices, a circuit still radiates greater than 10dB. A itself depends on the barrier thickness and its unacceptable amounts of noise or is susceptible to incoming radi- absorption coefficient. The inverse of the absorption coefficient is ated interference, the next step is to shield it. This involves placing called the 'skin depth' (8). Skin depth is the measure of a magnetic a conductive surface around the critical parts of the circuit SO that phenomenon that tends to confine ac current to the surface of a the electromagnetic field which couples to it is attenuated by a conductor. The skin depth reduces as frequency, permeability and combination of reflection and absorption. The shield can be an all conductivity increase, and fields are attenuated by 8.7dB (1/e) for metal enclosure if protection down to low frequencies is needed, but every skin depth of penetration, as in Equation 23.8 where u, is if only high frequency (>30MHz) shielding is needed then a thin relative permeability (1 for air and copper), and is relative conductive coating deposited on plastic is adequate. conductivity (1 for copper). 23.13.1 Shielding effectiveness = 6.61 centimetres (23.8) How well a shield attenuates an incident field is determined by its Thus at high frequencies, absorption loss becomes the dominant shielding effectiveness, which is the ratio of the field at a given point term. Figure 23.25 shows the combined reflection and absorption before and after the shield is in place. Shielding effectiveness of losses 