Learn the basics to control conducted emissions. Make your electronics more robust and less expensive to develop. Minimize retest and redesign by implementing the right EMI filters and packaging up front.
First, what are conducted emissions?
Conducted emissions are AC signals on electrical wiring that are put there by equipment connected to the wiring. Caused by circuitry that switches or oscillates, conducted emissions manifest as undesired noise superimposed on the desired signal or power waveform.
A conducted emission, abbreviated “CE,” is one of four phenomena in the field of electromagnetic interference. The other three are: conducted susceptibility, radiated emissions, and radiated susceptibility.
Controlling CE is essential for electromagnetic compatibility. All regulatory agencies restrict allowable power line CE levels and a few agencies also limit CE on signal cables.
Power line CE
Conducted emissions limit are imposed on power lines primarily to protect other equipment that shares the power lines.
Ripple caused by emissions from multiple devices on the line can at times be additive, so CE limits are usually much lower than conducted susceptibility (CS) test levels for the same devices.
Signal cable CE
A few industry specifications, RTCA/DO-160 for example, regulate allowable signal line conducted emissions. Usually, CE limits on signal lines apply to the entire cable bundle and are intended to control low-frequency radiation produced by cabling harnesses.
Even if your signal cables are exempt from CE limits, you must control conducted emissions on those cables to prevent them from generating excessive radiated emissions (RE).
How does a circuit generate conducted emissions?
When active circuits operate, they do so by switching logic gates or varying current or voltage at various points in the circuit. Fluctuations in power bus current or voltage translate directly to increased CE. Likewise, current or voltage changes on signal lines generate cable harness CE.
Take, for example, the circuit below. Noise voltage waveform Vs induces noise current at the Source terminals. The time-varying waveform at the Source terminals is, by definition, the circuit’s conducted emissions. The EMI filter attenuates the ac components of the Source waveform so that, if done properly, the waveform at the Load Circuit terminals is sufficiently small to meet CE requirements.