ipment under test involve impulsive interference from a motor or switching circuit Mains being fed into the 0V rail of a microprocessor circuit because they input share the same earth return. Note that because of resonances, the receiver impedance of earthing and bonding conductors is high at frequen- cies for which their length is an odd multiple of quarter wave- lengths. Another coupling route is through the equipment power supply to or from the mains. The power supply forms the interface between Figure 23.21 Equivalent circuit of an LISN the mains and the internal operation of the equipment, and all 23/16 Safety and EMC 10kHz 100kHz Frequency 10MHz Figure 23.22 Impedance-frequency plot of the artificial mains network of Figure 23.21 Magnetic induction occurs when the magnetic flux produced by a (23.6) volts per metre changing current in the source circuit links with the victim circuit. The voltage induced in the victim circuit by a sinusoidal current In the far field the field strength falls off linearly with distance. at frequency f due to a mutual inductance M henries is given by The electric and magnetic field strengths are related by the imped- Equation 23.3. ance of free space, E/H = 3770. In the near field, a loop radiator will give a higher H (magnetic) field and this will fall off proportionally V=2rfl.M volts (23.3) to 1/d3 while the E field falls off as 1/d2. Conversely, a short rod will give a high E field, which will fall off as 1/d3 while the H field falls off as 1/d2. M is proportional. to the areas of the source and victim circuit loops, the distance between them, their relative orientation and the Once conductor lengths approach a quarter wavelength (one presence of any magnetic screening. It is normally hard to charac- metre at 75MHz) then they cannot be treated as 'electrically small' terise accurately. For example, short lengths of cable within the and they couple much more efficiently with ambient fields. same wiring loom have mutual inductances in the range of 0.1 HH to Magnetic induction 