In general, if the application can support serial resistance, a resistor will give you better –reproducible- results. The inductive part
of a ferrite bead may give resonances, and resistors are available in much wider ranges. Resistors especially SMD, have good RF properties if used on the proper PCB footprint. Values in general between 22 and 100 ohm. This is my experience with fast Ethernet and TTL level serial connections. I can imagine that laser trimmed resistors have worse characteristics, but as far as I understand the trimming process that is just in terms of increased inductivity a property that creates less of a problem for this application. The bead’s properties are not always specified completely, most are limited to a graph of (real and/or imaginary) impedance versus frequency. In some cases the inductive impedance is simply to high ( >500) to reduce emissions. The source impedance of CM currents is not that of a simple voltage or current source, but a unknown mixture of the 2, and blocking one CM current on one cable will increase others. The interference current are principally caused by charging and discharging (clock) of semiconductor internal and external capacitances (0-3 or 0-5V), and each charge discharge cycle releases an amount of energy (0.5 CV2 ) that HAS to be dissipated locally in real impedances. Too inductive beads will not dissipate , and simply reflect the energy. Blocking the currents (beads) will rise the voltage opening other ways out, shorting the voltage (caps) will rise the current flowing, both creating more emissions. Regards, Ing. Gert Gremmen From: Amund Westin [mailto:am...@westin-emission.no] Sent: Friday 5 February 2016 10:00 To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] SV: Ferrite bead vs. resistor A resistor will cause a voltage drop, and that could make problems for the functionality. Ferrite bead will not make a DC voltage drop. For higher frequencies (above 100MHz) the bead is more precise described (Z, X and R), while the impedance of a resistor is maybe not defined. #Amund Fra: Amund Westin [mailto:am...@westin-emission.no] Sendt: 5. februar 2016 09:38 Til: 'EMC-PSTC@LISTSERV.IEEE.ORG' <EMC-PSTC@LISTSERV.IEEE.ORG> Emne: Ferrite bead vs. resistor Had some EMI issues, caused by a LVDS line. By inserting CM chokes, the radiation seemed to lower significant. Inserting resistors as well (aprox 22 ohm) made it even better. I can’t check the eye curve, but 22ohm makes no problem with the EUT operation. But, should the series resistor be replaced by ferrite beads instead? Will it make any really differences? The LVDS clock is approx 40MHz and data rate 160Mbps. #Amund - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) <http://www.ieee-pses.org/list.html> List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <sdoug...@ieee.org> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher <j.bac...@ieee.org> David Heald <dhe...@gmail.com> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <sdoug...@ieee.org> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
