Sources of Radiated Emissions
Emissions come from five broad categories of source: digital circuits, power converters, mechanical/electromechanical parts, the PCB layout itself, and — very often the biggest offender — cables attached to the product. This module goes beyond naming the categories: for each one, you'll see the actual physical mechanism — the current loop, the switching edge, the arc, the ground split — that turns ordinary circuit behavior into a radiated field.
Every product's total emissions are the sum of contributions from several distinct source categories. Identifying which category dominates a failing frequency is usually the first step in troubleshooting (Module 09 covers this troubleshooting process in depth).
FPGA, CPU, GPU, DDR memory, Ethernet PHY, PCIe, USB, HDMI.
Buck converters, boost converters, flyback converters, motor drives, inverters.
Motors, relays, solenoids, contactors.
Long traces, loop areas, split ground, floating copper, poor return paths.
USB cable, Ethernet cable, HDMI cable, power cable, sensor cable.
Every digital signal transition (a clock edge, a data bit changing state) pushes current out along a trace and pulls an equal return current back through the ground plane beneath it. Together, the outgoing trace and its return path form a small current loop. That loop, driven by a fast-changing current (high dI/dt), behaves exactly like a small loop antenna — efficiently radiating energy at the clock frequency and its harmonics.
Realistic view: an IC drives a signal trace (copper, top layer) to a connector pad. The return current rides in the solid ground plane directly beneath the trace, hugging its path — the thin strip between the two is the actual radiating loop.
Buck, boost, and flyback converters all work by switching a transistor on and off tens of thousands to millions of times per second, chopping a DC voltage into a square wave. The small loop formed by the switching transistor, the diode (or synchronous rectifier), and the input capacitor — often called the "hot loop" — carries a current that reverses direction on every switching edge. A square wave is rich in harmonics by nature (a Fourier series of odd harmonics for an ideal square wave), so this hot loop radiates a comb of frequencies at multiples of the switching frequency.
Realistic view: an actual buck converter's hot loop — the MOSFET (TO-220), the flyback/catch diode, the input bulk capacitor, and the output inductor — wired on a real power board. Watch the glowing loop pulse on every switching edge, continuously looping like a GIF.
Circuit Schematic: Current Flow Through a Buck Converter
A buck converter has two distinct current paths, and the circuit alternates between them on every switching cycle. When the switch is ON, current flows from the input capacitor through the switch and inductor to the load — this is the high dI/dt "hot loop" path. When the switch turns OFF, the inductor's stored energy keeps current flowing, but now it freewheels through the diode instead of the switch, closing a different loop. Watch the animation switch between the two current paths.
Good Layout vs. Bad Layout: Same Circuit, Different Emissions
The schematic above is electrically identical in both boards below — same switch, same diode, same inductor, same switching frequency. The only difference is layout and component choice. On the left, the hot-loop components are placed tightly together with short, direct copper. On the right, the same components are spread out (often because "there was more room over there") and connected with long, leaded (through-hole) parts instead of compact SMD packages — both of which increase loop area and parasitic inductance. The radiated field scales with loop area, so the badly laid-out board radiates dramatically more at the same current and switching frequency.
Relays, solenoids, and contactors emit very differently from digital or power sources. As mechanical contacts open or close, the current interrupting across a small air gap can cause an electrical arc — a brief, chaotic discharge that contains energy spread across a very wide frequency range rather than concentrated at a single clock or switching frequency. This is why mechanical sources tend to be broadband and impulsive rather than narrowband and periodic.
Realistic view: an actual automotive/industrial relay — coil, bobbin, spring-loaded armature, and contact set — shown mid-cycle. Watch the armature pull in and the contacts spark as they close.
A digital signal's return current always takes the path of least inductance, which in a well-designed board is the ground copper directly beneath the signal trace — keeping the loop area (and therefore the radiated field) tiny. A split or slotted ground plane forces that return current to detour around the gap, dramatically increasing the effective loop area for exactly the same signal current. Floating (unconnected) copper regions make things worse in a different way: they couple capacitively to nearby traces, pick up energy, and re-radiate it like an unintentional parasitic antenna.
Realistic view: a two-layer board with a copper ground plane (bottom layer, shown as a solid fill) that has been slotted for an unrelated routing reason. The top-layer signal trace crosses directly over the slot — watch the return current get forced into the long detour around it instead of following the trace.
The most common radiator is often the cable, not the PCB. A cable acts as an efficient antenna whenever common-mode currents from the board couple onto it. Unlike the differential signal current (which flows out one conductor and back the other, largely canceling its radiated field), common-mode current flows in the same direction on every conductor in the cable at once — with nothing to cancel against, even a few millivolts of common-mode voltage can drive a cable to radiate far more effectively than the compact traces on a well-laid-out board, because the cable is often close to a resonant length (recall the λ/10 and λ/4 rules from Module 02).
Realistic view: an AC motor (housing, cooling fins, shaft, mounting flange) fed by a shielded motor cable from a drive controller. Watch the common-mode current travel the length of the cable and the field radiate outward from the whole cable and motor frame — not just from the drive electronics.
Module 3 Quiz
Select the best answer for each question. Instant feedback is provided after each response.