New Understandings of the Use of Ferrites in the Prevention and Suppression of RF Interference to Audio Systems
Building on the work of Muncy,  the author has shown that radio-frequency current on cable shields is often coupled to audio systems by two mechanisms – “the pin 1 problem” and shield-current-induced noise (SCIN). [2, 3, 4, 5] An improved equivalent circuit for a ferrite choke is developed that addresses both dimensional resonance within ferrites and the self resonance of inductors formed using those materials, then compared with measured data. Field tests show that chokes formed by passing signal cables through ferrite cores can significantly reduce current-coupled interference over the range of 500 kHz to 1,000 MHz. Guidelines are presented for diagnosing the causes of EMI from sources as diverse as AM broadcast transmitters and cell phones. Solutions are presented for use in new products and for RFI suppression in field installations.
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