Coupling can occur through conduction via an unwanted path (a so-called “sneak circuit”), through induction (as in a transformer), and radiated or through-the-air coupling. Conductive coupling occurs when the coupling path between the source and the receptor is formed by direct contact. Direct contact may be caused by a transmission line, wire, cable, PCB trace or metal enclosure. Conducted noise can appear in a common or differential mode on two conductors. Inductive coupling occurs where the source and receptor are separated by a short distance. Inductive coupling can be due to electrical induction or magnetic induction. Electrical induction results from capacitive coupling while magnetic induction is caused by inductive coupling. Capacitive coupling occurs when a varying electric field exists between two adjacent conductors, inducing a change in voltage across the gap. Magnetic coupling occurs when a varying magnetic field exists between two parallel conductors, inducing a change in voltage along the receiving conductor. Inductive coupling is rare relative to conductive or radiated coupling.  Radiated coupling occurs when source and receptor (victim) act as radio antennas. The source radiates an electromagnetic wave which propagates across the open space between the source and the victim and is received by the victim.