From the view of EMC an EUT (Equipment Under Test) is a kind of antenna. This antenna
receives the RF field that is radiated by the transmitting antenna. As a result there are RF
currents and voltages induced into the EUT.
This state is valid for all conductive structures, that are connected to the EUT. At low frequencies (eg a few 10 MHz) this is mainly the external wiring harness. At higher frequencies also the circuit board could be concerned itself. Based on our EMC practice it is also know, that conductive structures in semiconductors could be affected by direct coupling, even at frequencies of a view 100 MHz.
These RF currents and voltages can cause interferences in the EUT, such as wrong analog values, undesired functions or RESETs.
However, the RF disturbances themselves are only seldom a reason for those interferences.
In most cases there is a RF rectification at non-linear junctions1).
Hence, the interferences are usually caused by the demodulated (i.e., low frequency) signals.
The diagnosis interface can now be used to read out these (interfered) values and states. In the EMC test set-up shown above the K line is used for this purpose.
1) Non-linear junctions could be simple p-n junctions, as they are used thousandfold in integrated circuits, and it is even the same, whether they are used as an overvoltage protection, an input amplifier, a logic stage or an output driver.