Ferrite beads

Summary¶

A ferrite bead is a frequency-selective resistor. At DC and low frequencies it presents near-zero impedance — power passes through normally. At high frequencies it presents high impedance — noise is attenuated. Unlike a capacitor, it does not resonate and does not affect DC efficiency. On libdrone, three to four clip-on ferrite clamps stacked on the VTX power wire at the XL4015 buck converter output attenuate the 180 kHz switching frequency and its harmonics before they can propagate along the wire and radiate or couple into the GPS or receiver.

Concept¶

How a ferrite bead works¶

Ferrite is a magnetic ceramic material — iron oxide combined with other metal oxides. When a wire passes through a ferrite core, the ferrite increases the inductance of that wire by a factor determined by the ferrite's permeability. For DC and low frequencies (where the ferrite's core loss is low), the added inductance is small and the impedance is minimal — power passes through with negligible loss.

At high frequencies, the ferrite's core loss increases dramatically. The bead now acts like a resistor in series with the wire at those frequencies — it absorbs the high-frequency energy and converts it to heat. The heat is negligible (milliwatts for typical drone applications). The attenuation is substantial — a single ferrite bead can provide 20–40 dB of attenuation at its target frequency.

Unlike an LC filter (inductor + capacitor), the ferrite bead does not resonate. An LC filter has a resonance peak where it actually amplifies the signal before cutting off above resonance. A ferrite bead is purely resistive at high frequencies — no resonance, no amplification, just monotonically increasing attenuation above its corner frequency.

Why clip-on and why stack 3–4¶

Solid ferrite cores (where the wire passes through a pre-formed toroid) provide the highest impedance per unit length. Clip-on ferrite clamps (clamshell type) have a small air gap at the clip joint, which slightly reduces permeability compared to a solid core.

Stacking 3–4 clip-on beads compensates for this reduced permeability: the total impedance scales approximately linearly with the number of beads. Three to four clip-on beads achieve impedance comparable to one or two solid toroids at the target frequency.

Clip-on beads are preferred for drone use because: - They can be added after wiring is complete — no rethreading required - They are repositionable if the first placement is not optimal - They grip the wire by friction — less susceptible to vibration loosening than a wire threaded through a toroid and held only by the wire tension

Application: VTX power wire¶

The XL4015 buck converter switches at 180 kHz, generating: - 180 kHz fundamental - 360 kHz second harmonic - 540 kHz third harmonic - ... continuing up into the MHz range

These frequencies propagate along the VTX power wire by conduction, then radiate from the wire as an antenna. Ferrite beads placed on the wire near the converter output intercept this conducted noise before it travels further.

The correct placement is at the converter output — as close to the source as possible. Placing the beads near the VTX end of the wire allows the noise to propagate and radiate along the unprotected portion of the wire between converter and beads.

Reference¶

Ferrite bead specification¶

Recommended: TDK ZCAT-series or equivalent Ni-Zn clamshell type. Avoid Mn-Zn ferrite (effective at lower frequencies, < 1 MHz is different material).

Where ferrite beads are applied on libdrone¶

Additional applications if noise problems are observed: - FC 5V supply wire from ESC BEC (attenuates any residual 48 kHz ESC noise) - GPS supply wire (protects GPS receiver from conducted supply noise)

Procedure¶

Installing clip-on ferrite beads¶

1. Cut the VTX power wire to final length before adding beads — beads are added at a fixed point and cannot be slid along a wire once connectors are on.
2. Open the clamp. Place the wire in the groove. Snap the clamp shut firmly until it clicks. The wire should be centred in the bore.
3. Add the next bead immediately adjacent to the first. Stack 3–4 total.
4. Verify the stack is as close to the XL4015 output as possible — within 30 mm of the converter output pads.
5. Optionally secure the stack with a cable tie or a small piece of heatshrink over the stack to prevent the beads from sliding along the wire in flight.

Verifying effectiveness¶

After a maiden flight, review the Blackbox gyroscope spectrum. If the GPS is reporting correct positions and the receiver link is stable, the ferrite beads are doing their job. If compass heading drifts or shows noise correlated with VTX power state (VTX on vs VTX off), add another bead or check placement.

Rationale¶

Why ferrite beads rather than an LC filter¶

An LC low-pass filter (inductor + capacitor) provides steeper roll-off and lower ripple, but has a resonance frequency where it amplifies rather than attenuates. If any conducted noise hits near the LC filter's resonance frequency, the filter makes things worse. For drone EMC where the noise frequencies are fixed (180 kHz from XL4015) and the filter is placed by a builder rather than a PCB designer with access to simulation tools, the ferrite bead's resonance-free behaviour is preferable. The bead always attenuates at the target frequency — never amplifies.

Connections¶

requires: - emc-noise-sources related: - capacitor-placement-emc - power-signal-separation - voltage-regulation leads_to: - gps-antenna-placement - conformal-coating