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. Clip-on ferrites work in the tens-to-hundreds of MHz — they do nothing useful at a buck converter's switching frequency, only at the fast-edge harmonics above it. On the current Cinematic build they are not fitted by default: they were dropped with the external XL4015 buck on 2026-09-24. They remain the first remedy if noise correlated with VTX power shows up.

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 useful only where the bead's impedance is large compared to the circuit impedance around it — a manufacturer's impedance-versus-frequency curve, not a generic dB figure, tells you whether a bead helps at a given 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 VTX on the Cinematic build is powered from the Matek H7A3-SLIM's onboard, switchable 9 V BEC — a switching regulator whose switching frequency Matek does not publish. What any buck converter puts on its output wire splits into two parts:

• The switching fundamental and its low harmonics (hundreds of kHz to a few MHz for typical small bucks). Clip-on ferrites have almost no impedance here. This part is handled by capacitance at the load and by keeping the wire short — not by beads.
• Fast-edge harmonics reaching into the tens and hundreds of MHz. This is where clip-on ferrites work, and this is the part that can radiate from the wire toward the GPS (1.575 GHz front end, raised noise floor) and the receiver.

If beads are needed, the correct placement is at the source end — on the VTX power lead as close as possible to the FC's 9 V pad. Beads near the VTX end leave the whole run between FC and beads free to radiate.

Reference¶

Ferrite bead specification¶

A single 22–24 AWG wire is thinner than the clamp's rated cable range and sits loose — secure the stack (step 5 below).

Where ferrite beads are applied on libdrone¶

Applications if noise problems are observed: - VTX power lead at the FC 9 V pad (fast-edge harmonics of the BEC) - 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 source as possible — for the VTX lead, right next to the FC 9 V pad.
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¶

Deciding whether beads are needed at all, and whether they helped, is the same A/B test — the switchable 9 V BEC makes it easy:

1. Outdoors, props off, GPS locked. Note satellite count, HDOP, RC link quality and compass heading with the VTX rail off.
2. Switch the 9 V rail on (VTX transmitting). Wait a minute. Note the same values.
3. A clear drop in satellites or rise in HDOP, a link-quality drop, or a heading shift that follows VTX power is the symptom beads address. No difference: don't fit them.
4. After fitting beads, repeat 1–3 and compare.

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 high-frequency hash on a wire, fitted by a builder rather than designed onto a PCB with simulation tools, the ferrite bead's resonance-free behaviour is preferable: it never amplifies. Low-frequency ripple is a different problem and belongs to capacitance at the load (→ capacitor-placement-emc).

Why beads were dropped as a default fitting¶

Earlier builds stacked clip-on beads at the output of an external XL4015 buck, aimed at its 180 kHz switching frequency. A clip-on Ni-Zn ferrite is specified in the tens-to-hundreds of MHz; at 180 kHz its impedance is negligible, so that fitting never did what it was described as doing. With the XL4015 gone and the VTX fed from the FC's onboard BEC, beads became a measured remedy (the A/B test above) instead of a default part.

Connections¶

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