EMC Question of the Week: December 21, 2020

On a circuit board where R1 is a resistor, C1 is a capacitor, and L1 is an inductor, B1 is most likely a

  1. ferrite bead
  2. varistor
  3. diode
  4. decoupling capacitor


The correct answer is "d". The B indicates a bypass capacitor, which is another name for a power bus decoupling capacitor. Some boards use BC to indicate bypass capacitors, while many boards simply use C. Bypass capacitors are typically no different than capacitors used for other purposes in a board design. However, labeling them differently makes it easier to locate them during a design review.

Circuit board with poorly connected decoupling capacitors

The board in the figure on the right has decoupling capacitors labeled this way. We can quickly see that all of the visible decoupling capacitors are poorly connected. Note that all of them connect to a ground fill rather than connecting to the ground plane. This results in a much higher connection inductance than necessary. (To convince yourself of this, trace the path from the power pin through the decoupling capacitor to the ground pin.)

Connecting high-frequency components to a ground fill on an outer layer instead of directly to the ground plane is a common circuit board design mistake.

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