Radiated Immunity Analysis of a Clock-Driven Digital State-Machine PCB.

Sammanfattning

This study presents a theoretical and experimental investigation of radiated electromagnetic immunity in a clock-driven digital state-machine on a printed circuit board. The objective was to study how radiated electromagnetic fields may couple into PCB conductors and whether such coupling can disturb the functional operation of a simple sequential digital circuit.A custom PCB was built using a D flip-flop state machine with XOR feedback, two clock sources, and LED indicators for the output. The state machine was chosen because its output follows a set pattern, so any errors caused by interference would be directly observable. The clock input was considered the most sensitive part since all state transitions happen at the clock edge. The feedback path from the XOR to the D input was also important because it sets the next state for the flip-flop.Radiated immunity testing was performed in an anechoic chamber from 80 MHz to 6 GHz at field strengths up to 10 V/m. The test included two frequency ranges, horizontal and vertical field polarisations, and two PCB orientations: lying flat and standing vertically. The functional behaviour of the circuit was monitored through the LED output sequence.The experimental work was supported by theoretical susceptibility calculations. Wavelength comparison, quarter-wave resonance estimation, induced-voltage estimation, loop-area effects, and logic-threshold comparison were used to estimate the expected coupling into the PCB. The calculations showed that several PCB dimensions and signal paths had estimated quarter-wave frequencies within the tested frequency range. However, the induced-voltage estimates indicated that the disturbance at the most sensitive logic nodes was unlikely to reliably exceed the logic noise margin under realistic coupling conditions.No visible errors were observed during the radiated immunity tests. The LED sequence remained stable for all tested frequency ranges, field polarisations, and PCB orientations. The experimental result was consistent with the theoretical estimates, indicating that the applied field strength was not sufficient to induce a disturbance capable of causing false triggering or incorrect state transitions. The study concludes that PCB-level radiated immunity behaviour must be interpreted through both physical layout features and circuit-level logic thresholds.