How much common current is too much?: Rule of Thumb #31
Passing an EMC certification test is hard. The biggest source of failures is radiation from common currents on external cables. It only takes about 5µA of common current to fail some EMC compliance tests. Watch out for this “common” failure mode.
Spoiler summary: It only takes 5µA of common current to fail an FCC part 15 Class B EMC compliance test.
Remember: before you start using rules of thumb, be sure to read the Rule of Thumb #0: Using rules of thumb wisely.
Passing an EMC certification test is hard. The most sensitive test, FCC part 15 class B, requires the maximum far field strength, 3 meters distant from your product, to be less than 100 µV/m at 88 MHz, with a 120 kHz bandwidth in the detector.
Just how much is this? If your product were a radio station, radiating in all directions, so that its far field strength at 3m distant was enough to fail this test, it would only have to radiate, in the 120 kHz bandwidth, about 10 nW of power. Not much.
In my experience, the most common source of radiated emissions is from common currents on external cables like power cables, or to peripherals. These common currents, with their return paths being literally the floor, or other adjacent metal, or even the chassis, produce the strongest far-field strengths.
Using a simple model for an antenna – a wire with a voltage source driving current in it, we can estimate just how much common current is required to fail an FCC test. Figure 1 shows an example of how common currents might be generated, and how they look somewhat like electric dipole antennas.
Figure 1 How an electric dipole can radiate in electronic products
This sort of antenna is called a Hertzian electric dipole. We can analytically calculate the far-field radiated strength from a length of wire with current at some frequency. The best reference which walks though this calculation is one of the late, great, Clayton Paul’s books, Introduction to Electromagnetic Compatibility, which is available as a free pdf download. See page 424.
The far-field strength from a Hertzian electric dipole, with a distance, r, of 3m, is:
If we consider the FCC class B test conditions, and set the far field to 100 µV/m at 100 MHz and a cable length of 1m, the maximum current before we fail an FCC compliance test is:
It only takes 5µA of common current on an external cable to fail an FCC EMC compliance test. Keep your common currents below this and you will have a much better chance of passing.
This is a really small amount of current and is why it is so hard to pass an EMC compliance test. If you know an EMC engineer, take him or her out to lunch and say thank you.
—Eric Bogatin is the Dean of Teledyne LeCroy’s Signal Integrity Academy and a well-known SI evangelist. Additional information on this and other signal integrity topics can be found at the Signal Integrity Academy, www.beTheSignal.com.
- Bogatin's Rules of Thumb
- Not all common currents are bad
- Using current probes to estimate E-fields
- EMC questions answered