Control Conducted Emissions
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).
CE basics
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.
Time-domain waveform versus frequency domain spectrum
We tend to think of circuit operation in the time-domain. Usually, circuit design is concerned with voltage and current waveforms and digital logic states. Conducted emissions, however, are almost always specified in the frequency domain, as a current or voltage spectrum.
To understand conducted emissions, we have to convert from the time-domain to the frequency domain. When we calculate conducted emissions, we first translate the time domain waveform into its frequency domain spectrum using a Discrete Fourier Transform. The two plots below show a trapezoidal waveform and its corresponding spectrum.
The green line superimposed on the second plot is the conducted emissions limit. To comply with this limit, conducted emissions voltage shown in red, and expressed in units of decibels, must be reduced so that it is below the green limit line.
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To convert from units of Volts to units of dBmV, use this formula:
EMI filters are commonly used to reduce conducted emissions from power lines and signal lines.
EMI filters
Several simple passive EMI filter topologies are displayed below. The choice of topology and component values depends on the impedance of the Source and Load circuits and the amount of attenuation required.
For a brief introduction to EMI filters, see past posts on the EMI Analyst website:
“How to Calculate Filter Insertion Loss”
“EMI Filter Insertion Loss — How Circuit Impedance Affects Filter Performance”
“Improve EMC Testing Results with Realistic EMI Filter Modeling”
Sharpen your pencil. The math is complex.
Without analysis software, accurately calculating conducted emissions is an arduous process.
Performing CE calculations correctly requires treating the cables as transmission lines. Adding to the difficulty is the complex impedance of the end circuits and filter components, which vary with frequency. If you are analyzing a multi-conductor cable, such as a wire pair over a ground plane, forget about doing the calculations by hand.
Many factors affect emissions
There are so many design elements that affect conducted emissions, the only practical way to predict CE levels is with software like EMI Analyst.
As a minimum the CE analysis model must account for: · Cable length · Height above ground · Circuitry at both ends of the cable · Noise waveform characteristic · Parasitic capacitance between active circuits and chassis · Frequency range
· EMI filter insertion loss
A quick example
The graph below plots the voltage induced across the 100-ohm resistor in Figure 1. The plotted voltage level is the noise source voltage Vs, shown in Figure 2 and Figure 3, after being attenuated by the EMI filter. The analysis shows that with proper EMI filter design, compliance with the CE limit is readily achievable.
For reference, it took about an hour to model the Source waveform, EMI filter, cables, and Load circuit using EMI Analyst. Calculations took just a few seconds. Analysis properties are summarized below.
Do the analysis!
Without this analysis, the EMI filter would be either over-designed or under-designed. · Over-design means too much filtering, which usually means higher cost, more weight, and greater volume.
· Under-design means too little filtering, which results in EMC testing failure followed by redesign, retest, and of course much higher cost.
For more information about controlling conducted emissions, visit the EMI Software website and check out CE Analyst, the conducted emissions application of EMI Analyst™.
Originally published at www.emisoftware.com.
