EMC Question of the Week: July 22, 2024
Radiation from electrically small loops is proportional to loop area, but when it comes to meeting EM emissions requirements, radiation from electrically small circuits is usually
- negligible
- proportional to loop circumference
- inversely proportional to trace width
- proportional to frequency
Answer
The best answer is “a.” Loops and circuits whose maximum dimensions are much smaller than a quarter wavelength are very inefficient radiation sources. For example, to get a 5-cm by 1-cm loop to produce enough radiated emissions to exceed the FCC or CISPR 32 Class B emissions limit would require about 1.7 amperes of current at 100 MHz. Even if the loop had no resistance, its inductance would require the signal voltage to be more than 50 volts. At lower frequencies, the current and voltage requirements would be even higher (e.g., 19 amperes and 180 volts at 30 MHz).
While it's possible to design a circuit with sufficient loop area and current so that its differential-mode emissions exceed requirements, this rarely happens in real designs. Circuit traces routed above or below a solid plane can't realistically generate enough differential-mode emissions to be a concern. Without a return plane, loop areas in high-frequency circuits are generally small in order to control the inductance well enough to make the circuit functional. And even if you do manage to make a loop large enough to radiate above the limit, chances are that the dominant source of radiated emissions will be the common-mode currents induced by the circuit in the rest of the structure.
Yes, it's important to minimize the loop area of high-frequency circuits. Not because these circuits radiate, but because coupling from these circuits to larger structures that CAN radiate tends to increase as the loop area of the circuit increases.
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