Coupling Paths & Victim Circuits
RF couples in through cables acting as antennas and gets demodulated at nonlinear junctions. Learn to find the victim and the entry path.
The dominant RF entry path is the cable: at the test frequencies a cable is an efficient antenna, so it collects RF and delivers common-mode current into the product. That current flows through the board's ground system and develops voltages that reach victim circuits — exactly the reciprocal of how cables radiate in the emissions course.
RF itself is often out of band, but any nonlinearity — a diode, an ESD-protection cell, a transistor base/gate, an op-amp input — rectifies the modulated carrier, producing a DC shift plus the 1 kHz envelope right in the victim's operating band. That is why an 80% AM signal at 300 MHz can appear as noise on a slow sensor line.
Susceptibility is frequency-selective, so note which frequencies fail — that hints at a resonant cable length or a specific victim. On the bench, inject with a hand probe/antenna while monitoring the suspect signal to localize the entry point and the vulnerable band, using the delta discipline from the Pre-Compliance course.