Choosing a ferrite bead and a common-mode choke

Adding a ferrite bead or choke “to see what happens” is cheap, but if the impedance at the problem frequency is low it does nothing, and a resonance can even make things worse.

Selection criteria

• Impedance at the problem frequency: read the value from the datasheet impedance-versus-frequency curve at your frequency. Do not choose on the single 100 MHz figure.
• Check whether the resistive part dominates. At high frequencies a bead absorbs energy as heat when its resistive part is large.
• DC bias: inductance and impedance fall as current flows. Check the curve near the rated current.
• Self-resonant frequency (SRF): above it the part becomes capacitive and loses effectiveness.
• Saturation: a common-mode choke can saturate with large differential current. Check rated current and differential-mode (leakage) inductance.

Things to watch

• A bead in series with a power line can form an LC resonance with nearby capacitors. Calculate the resonant frequency (calculator: LC filter).
• Beads also affect signal quality. On high-speed signals check bandwidth first.
• Placement matters. Put it close to the noise source or connector with a short ground path.

Field notes

• Parts such as ferrites are effective at reducing a problem band. However, the noise often did not disappear but moved to another band.
• So apply a ferrite while watching the measurement and re-check the whole band afterwards.