An Introduction to RF Shielding (Part 3): Ferrites

A look at cylindrical tubes and circular toroids as a way of reducing noise on cables.

800x300_RF-Shielding (1)

Ferrites are ceramic materials (made of iron oxide + a mix of manganese or nickel with zinc) that have unique magnetic properties that are used to suppress Electro-Magnetic Interference (EMI) and Electro-Magnetic Compatibility (EMC) in circuits. The ferrite ceramic can be formed into different shapes such as plates, cylindrical tubes, and circular toroids to fit a specific application. We will be looking at cylindrical tubes and circular toroids as a way of reducing noise on cables.

Typical Ferrite Impedence vs Frequency Plot

Typical Ferrite Impedance vs Frequency Plot

Basic Theory of Operation

The ferrite data sheet needs to be used the obtain the appropriate operating parameters at the desired frequencies that needs to be controlled.

Ferrites are designed to work on specific frequency bands, so that the datasheet is an important tool to use before installing in a circuit. Additionally, the ferrite has a saturation magnetization that depends on the current and temperature that can reduces the impedance (Z).

1 Turn, 2 Turns, 3 Turns…

Passing a wire through the ferrite inner core will increase the attenuation (impedance (Z), resistance (R), and reactance (X)).

By convention, toroid turns are counted as the number of wires going through the center of the core, wires on the outside are not counted.

The inductance increase is proportional to the square of the turns of wire in the core.

An equivalent circuit that can be derived from the impedance vs frequency plot.

The ferrite core with two turns (N=2) will have four times (N² = 4) the inductance of a comparable core with only one turn of wire.

The three-turn (N=3) ferrite core will have nine times (N2 = 9) the inductance of the one wire case.

Wire bundles in cables can also be used with ferrites to eliminate common mode signals.


Two Turns going through the Center of the Ferrite Core	Three Turns going through Center of Ferrite Core	Wire Bundle Closeup Around Toroid Closeup

An equivalent circuit that can be derived from the impedance vs frequency plot.

With the operating point impedance determined, we can calculate an attenuation of an interfering signal (Vin) that the load (Zl) will experience.


Simplified Equivalent Circuit	Impedance to Attenuation

Single Turn going through Three Ferrites

Three Wire Bundle with 12 Turns around Circular Toroid - Copy

Three Wire Bundle with 12 Turns around Circular Toroid

All in a Row…

The attenuation can also be increased by stringing the ferrites in series when the cable size does not allow for multiple loops. The effect of increasing in the number of ferrite core is additive, while at the same time lowering the overall frequency bandwidth.

The increase of the inductance is linear; one turn = L, two turns = 2*L, and three turns = 3*L

The wire bundles can be treated the same way as single wires, common mode noise which is present on all lines simultaneous can be reduced using the circular toroid.

Example:

We measure that an Ethernet cable has some noise at 60 MHz, and we would like to attenuate by 12dB. From data sheets or measurements, we find that the Ethernet cable has an impedance of 300 ohms at the desired frequency. Using the sample graph shown above, the impedance (Z) of the ferrite is 240 ohms at 60 MHz.

Re-arranging the attenuation conversion equation and solving for Zsc gives:

The source (Zs) and load (Zl) impedance are set equal for maximum power transfer (by design), to give:

The ferrite impedance (Zsc) and the load (Zl) impedance form a voltage divider that attenuates the undesired signal. The ferrite impedance needs to be increased by either passing the ethernet cable through the ferrite core multiple times or adding ferrites in series along the cable. In the linear case (ferrites in series) at least 7 to 8 ferrites are required to achieve the desired impedance (7*240 ohms = 1680 ohms: 11.59dB, or 8*240 ohms = 1920 ohms: 12.46dB).

When the Ethernet cable can be wound through a circular toroid, only 3 turns are required to achieve the desired attenuation (N =3²*240 = 2160 ohms:13.25 dB), with the advantage of only using one part.

KEMET ESD-SR-100 Split Ring Ferrite

What can be used

A split ring ferrite can be installed after a cable assembly is found to be causing EMI or EMC.

The split ring core needs to be properly clamped together when it is used in order to ensure the ferrite magnetization is not reduced due to the air gap between the two sections of the ferrite.

What it Looks like in Practice

Large circular toroids are available for cables with bulky adapters and connectors, the coil's inner diameter is large enough to allow the ends of the cable to pass through the inner ring. Flat ribbon cables can also be wound inside the toroid to improve shielding performance.