I ran some NEC simulations and got some interesting results. Knowing just the magnitude of the ferrite bead impedance doesn't seem to be enough. One can get the same impedance magnitude with different R and X values. I was able to move the bead effectiveness (simulated signal strength) around by a few dB by changing R and X while holding |Z| constant. So, I would say that substituting a bead with a higher |Z|, or even the same |Z|, but with a different mix of R and X could increase emissions (or lower emissions). Dave Cuthbert Micron Technology
From: [email protected] [mailto:[email protected]]On Behalf Of Grasso, Charles Sent: Friday, December 19, 2003 11:28 AM To: 'Chris Maxwell'; 'Ken Javor'; 'Price, Ed'; '[email protected]' Subject: RE: Measuring a ferrite performance Well Chris - There is a long story attached to this requirement. Let me summarize: 1. We manufacture a significant number of settopboxes for satellite TV. I hesitate to put actual numbers in public - lets just say a sh*t load. Cost therefore is paramount...as you can appreciate. Mulitple vendors of components are therefore essential.EMI testing can get prohibitive in terms of time and cost so .. a). We take our baseline unit and will compare the freq response of the "golden bead" to an alternate. b). As long as the alternate matches or is better than the "golden bead" no brains required and no emissions test required. c) If the desired bead has inferior performance then a decision can be made to either schedule a test or nix the vendor. Makes sense ? I hope so..:-) Best Regards Charles Grasso Senior Compliance Engineer Echostar Communications Corp. Tel: 303-706-5467 Fax: 303-799-6222 Cell: 303-204-2974 Email: [email protected]; <mailto:[email protected]; > Email Alternate: [email protected] From: Chris Maxwell [mailto:[email protected]] Sent: Friday, December 19, 2003 7:29 AM To: Ken Javor; Price, Ed; Grasso, Charles; [email protected] Subject: RE: Measuring a ferrite performance As usual, this thread has been very educational, the mere asking of the question has unleashed many answers, from which I have gained in my own knowledge. The most practical question that I would ask is: Why the h___ would your component engineering group want you to measure the impedance of a ferrite? There can only be two reasons: either they can't get a graph of impedance from the manufacturer, or they don't trust it. If either of these are true; then I wouldn't buy the ferrite in the first place. I hope that this isn't a "slippery slope" issue for you. Otherwise, your component engineering group may be asking for V-I curves whenever you want to qualify a new resistor :-). All that said, I can appreciate the fact that you may want to perform this measurement just to learn how it's done. If you do perform the measurement, I would recommend one little detail. I would recommend that you first find a "known good" ferrite complete with a manufacturer's impedance graph. Compare your measurements to the graph just as a sanity check. Happy holidays Chris Maxwell Design Engineer Nettest Inc. Utica, NY [email protected]
