EMC Question of the Week: February 13, 2023

LED driver on a circuit board driving an LED through a twisted wire pair cable

An LED mounted in the wall of a plastic enclosure is connected to a circuit board through a 30-cm unshielded twisted wire pair. The LED driver produces a single-ended, 3.3-V, 50-MHz, PWM signal. The radiated emissions are over the limit at 250 MHz by almost 50 dB. Which "fix" is likely to bring the product into compliance?

  1. increase the transition time
  2. put ferrite beads on both wires
  3. put a ferrite core on the wire pair
  4. replace the wire pair with a coaxial cable


The best answer is “d.” The primary source of the emissions in this example is the fact that an unbalanced driver on the circuit board drove a balanced cable. This created a common-mode voltage that drove the wire pair relative to the board ground. Replacing the wire pair with a coaxial cable maintains the imbalance from the source all the way to the LED and no common-mode voltage drives the cable relative to the board. 

Since the emissions are occurring at the 5th harmonic, increasing the transition time has little impact on the amplitude of the signal at 250 MHz. Ferrite beads on the signal wires may provide some resistance at 250 MHz, but any resistance that would provide 50 dB of attenuation would surely prevent the LED from turning on. 

A ferrite core on the wire pair near the circuit board would damp the resonance and reduce the common-mode current; but 50 dB of attenuation (a factor of 300) would require approximately 36 x 300 = 11 kΩ of common-mode resistance at 250 MHz. This is not achievable with any reasonable ferrite core. 

Of the choices provided, the coax is the only workable option. Another option would be to shield the wire pair. This doesn't prevent the common-mode voltage from driving the pair, but it returns the common-mode current to the circuit board on the inside of the cable shield, preventing the radiated emissions.

In many cases, the best option would be to drive the LED with a much lower frequency. If the frequency were a few hundred hertz, the common-mode voltage driving the cable would be of little consequence.

Note: An astute observer might point out that an LED mounted in the wall of a plastic enclosure is balanced. This is ok because the LED is small. A common-mode voltage that drives the LED relative to the cable does not produce any significant common-mode current.

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