Ferrite beads are used for decoupling (keeping out unwanted
signals) on dc supply and some signal lines and provide attenuation
of selected frequency bands. The physical shape of a bead is
similar to a toroid, but the bead has a greater length to diameter
ratio and usually a greater outside to inside diameter ratio than
most toroid cores. Where the length to diameter ratio is even
greater the bead is often referred to as a sleeve. Different size/shape
beads of the same material have different degrees of suppression.
Refer to bead comparison table.
RF chokes are usually wound as a solenoid on a ferrite rod and can exhibit
relatively high Q values and produce an external field which can
have mutual coupling to other circuit elements. This is undesirable
in many circuits and can lead to instability (unwanted
oscillation). In contrast, the bead equivalent circuit looks like a
series resistor and inductor, has a very low value of Q, and is
self shielding. A plot of Z versus frequency for any particular bead will show
that at the lower frequencies XL and R are similar, but as the frequency
is increased the R dominates and XL drops away. This is the lower
of the range of suppression frequencies. The upper suppression frequency is a
property of the material, and the R value will eventually peak and also fall away.
A bead is often used on transistor base leads and
FET gate leads to provide stable operation of amplifier circuits by
lowering the Q and or suppressing unwanted high frequency feedback.
When used in conjunction with a bypass capacitor a bead can provide
extremely good decoupling.
The above diagram show the equivalent circuit of a bead (R and L) inserted between a source (ZS) and load (ZL).
A quick glance at the equivalent circuit shows that maximum attenuation due to the simple impedance divider will occur when ZS and ZL are low. For example, if ZS and ZL are 10 ohms and the ferrite impedance at a given frequency is 100 ohms, the total attenuation (versus without ferrite) isA = 20log10 [(10+10)/(10+100+10)] = -15.6dB
but if the circuit impedance is 200 ohms, the attenuation becomesA = 20log10 [(200+200)/(200+100+200)] = -2dB
For more detailed information on suppression see the excellent article on ferrite suppression.
Some beads have more than one hole, which allows more than one turn to pass through the core without sacrificing the shielding properties.
The ferrite material determines the range of frequencies for suppression purposes, and the physical size and shape of the bead determines the amount of attenuation. It can be seen from the tables that there is some overlap between the various types of materials, but in general the following table can provide a guide to the attenuation characteristics.
|75 or J||0.5 - 10 MHz|
|72, 73, 77, F||1 - 50 MHz|
|43, 44||20 - 400 MHz|
|311||1 - 500 MHz|
|61 or 64 2||>200 MHz|
Over the specified frequencies, the beads present very little reactance but a large resistive component. The data tables give some idea of the relative impedance factors for different sizes and shapes of beads. Beads can be added in series on a wire to achieve greater attenuation, since the attenuation is proportional to the length for a given type of bead. The 6-hole bead (FB-( )-5111) produces 6 to 7 times the impedance of a 101 size bead of the same material when the wire is passed through the holes in a 2½ turn configuration. Increasing the turns through a single hole bead also increases the attenuation, but decreases the shielding properties. When the conductor through the bead is carrying DC there will be a reduction in the impedance of the bead depending upon the ferrite material. The DC provides a biasing magnetising force (H) which results in a lowering of the effective impedance. Tables of various 1 hole beads, 6 hole beads, toroids, and split cores show impedance vs frequency data.
Some special types of beads are available for specific purposes. It is often only after terminating a cable with a connector that the need for some form of rf suppression becomes clear. Some examples are: rf pickup on the outer of a coax cable; radiation of rfi from a computer via an external cable. Two sizes of split beads are available for round cable. One size is for max. 0.25" dia. cable (e.g. RG58 coax) and the other is for max. 0.5" dia. cable (e.g. RG8 coax). The split bead allows the two halves to be placed around the conductor and held together with the cable clamp/housing which is included. Another form of split bead is made to fit over flat ribbon cable as used in many computer leads. This bead is made as a flat bar with a wide shallow cutout section, which allows two pieces to form a flat "split bead" at right angles to the cable. The two pieces can be held together with a cable tie at each end, or by including a nylon housing with the assembly.
A common problem with most rf generating equipment (transmitters, computers, dimmers, fluorescent lights, motors) is the radiated rf from the external leads attached to them. This is called rfi (rf interference) or emi (electromagnetic interference), and it can cause interference to the reception of TV or radio broadcasts, and even interfere with audio equipment. Apart from suppressing the source of the rfi, it is usually necessary to prevent the rf from entering the receiving equipment. In these cases it is sometimes better to use a large toroid core made from suitable material, so that several turns of cable can be passed through the core to improve the suppression properties. This approach is particularly useful where a mains lead or speaker lead is the pickup point of rfi.