Ferrites have been successfully used to suppress EMI/RFI for many decades now. It is an effective and economical way of complying to FCC rules. The first step in addressing the EMI.RFI problem is to isolate the source of unwanted signals. If possible, reduce this source of signal to an acceptable level. Ferrites are then used to reduce or eliminate the interference on wirings and cables.
Ferrite suppressors are available in many different configurations. Ferrites and toroids are common configurations. Split cores, multi aperture cores, round and flat cable suppressor cores are for the computer industries. Tapes beads on leads, radial beads, surface mount beads in accordance to EIA Specifications are available for automated manufacturing environments. Multi-hole suppressor plates are for connectors, and PC beads or multi-line suppressor beads are for circuit board mounting. The multi-line beads can be used as multiple signal turn beads or for better suppression, as multi turn beads.
The selection of ferrite material for an application is determined by the frequencies of interferences. The following table summarizes the types of materials to use:
The core size and shape are determined by the space available on the device or board, mechanical consideration, and the desired impedance at the interference frequency. These are estimated from the Impedance vs. Frequency charts with the following equation:
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