Effectively short high frequency signals, providing noise suppression. They are often attached to a cable or lead and appear as a lump or "goober" which is sometimes called the FCC lump because RF noise suppression is necessary for FCC certification. Ferrite beads can provide suppression of RF EMI caused by signals in the MHz to GHz range. The advantage of ferrite is that since it does not actually contact the wire electrically, it introduces no capacitance or other serious effects which could be introduced by a more conventional filter.
Best Material for given Frequency ranges
| Frequency | Ferrite Bead Material |
Resistivity | Impedance | Permeability | |
| 1Mhz - 40Mhz | MnZn (Manganese/Zinc) | Low | High | High | |
| 20-200Mhz | MgZn (Magnesium/Zinc) or NiZn (Nickel/Zinc) | High | High | Medium | |
| 200Mhz - | NiZn (Nickel/Zinc) | High | Medium? | High |
To achieve maximum EMI filtering performance in a given application, select a ferrite with an inner diameter most closely matching the outer diameter of the wire or wire bundle to be filtered. However, passing 2 turns of the wire conductor through the ferrite core can substantially increase the effectiveness of the ferrite so you may also wish to select one that is double the size of the cable. More than 2 turns may cause the ferrite to reach DC saturation, effectively reducing its permeability and it's effectiveness in suppressing EMI.
In general, you need at least 50 Ohms Impedance at the frequency being suppressed after the DC saturation level has been reached. Note that although you may find a listed value for "maximum direct current" in the data sheet for the ferrite, that value is typically listing the point at which the device will be damaged or destroyed, and has no relation to the saturation level.
DC saturation is caused by the non-changing magnetic field which is built up by the current flowing through the conductor. The strength of this field can be calculated from the following equation:
| H | = |
0.4(Pi)(N)(I) (PL) |
Where (PI) is the constant Pi (3.1415926...), N is the number of turns of cable through the ferrite, I is the current in the cable (totaled for all wires) in amperes, and PL is the length of the magnetic path in centimeters. Some information on the effect of this field on the permeability of the ferrite may (may) be available from the datasheet, but in most cases, the correct part must be selected by repeated test and measurement.
In other words, order one of each of a range of ferrites of the appropriate type (from the table above) and keep trying until you find the one that works. If none work, look to other noise reduction techniques.
See also:
Cell Phone Wired Headset Safety
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