Koh, I'm afraid your formula won't work. The attenuation provided by the ferrite bead is directly dependant on the system source and load impedance at the particular frequencies, which won't generally be known. Without knowing the system impedances, you can't calculate directly what the insertion loss change will be when the bead impedance is changed, even though the system impedance doesn't change.
However, you can do some worse case studies by assuming a range of source and load impedances. For example, the measurement standard of 50 ohms provides a reduction of 0.93dB for the worse case change of 67ohms to 50 ohms at 68MHz. Beads provide less attenuation at higher source and load impedances, so if you had a 1000 ohms system, the change would reduce to just 0.14dB. However, if the system impedance were as low as 10 ohms, which is probably unlikely, the reduction would increase to 2.16dB. Even this approach assumes resistive source and loads, and a resistive bead, but is at least a guide. Regards, Jeff ------------------------------------------------------------- Dr Jeff Chambers Westbay Technology Ltd Suppliers of EMC Design Software Tel: +44 1229 869 108 Fax: +44 1229 869 108 http://www.westbay.ndirect.co.uk/westbay1.htm j.chamb...@ndirect.co.uk Main St Baycliff Ulverston Cumbria LA12 9RN England ------------------------------------------------------------- -----Original Message----- From: Koh Nai Ghee <koh...@cyberway.com.sg> To: Maxwell, Chris <chr...@gnlp.com>; 'Ralph Cameron' <ral...@igs.net>; Tony J. O'Hara <tonyoh...@compuserve.com>; Paolo Roncone <paolo.ronc...@compuprint.it> Cc: EMC-PSTC <emc-p...@majordomo.ieee.org> List-Post: emc-pstc@listserv.ieee.org Date: 21 September 2000 18:14 Subject: Re: Component Qualification > >Chris & other, >Thanks for the reply. > >Chris has given a great alternate method. I would like further add on to this >test method/approach. > >Those second source components (ferrite or oscillator) that we get of course >have to meet the PCB footprint as well as meeting the primary specification. > >For ferrite, let's now say the second source component has the correct impedance >at 100MHz and current rating. >However, the impedance curve of ferrite from different source are mostly of >different response curve. It would be difficult to judge whether does it >degrades the final product emission level. > >In view of this, I might add that we can make use of the final product scanning >results for a guide to make a final judgement. >For example, if the final product has operating frequencies of 34MHz, the worst >three case frequencies are >68MHz, 102MHz & 136MHz with passing margin of 4dB, 2dB & 6dB margin. >Comparison of the ferrite bead impedance at these frequencies yield the >following reading, >Freq 1st source 2nd source >68 67 ohm 50 ohm >102 98 ohm 96 ohm >136 80 ohm 85 ohm >How can we have a simple calculation to state that this 2nd ferrite is OK on the >product? >I would like to hear your view of such formula >dB change = 20 log (1st impedance) - 20 log (2nd impedance) > >At 68MHz, dB change = 20 log 67 - 20 log 50 = 2.54 dB >With this 2.54dB, the final product is still passing with 4-2.54 = 1.45dB. >Same for 102MHz yield 1.82dB and 136MHz yield 6.53dB passing. >Would this be the worst case scenario that will occur? >How's the group view on such approach? > >Regards >Koh > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: jim_bac...@mail.monarch.com Michael Garretson: pstc_ad...@garretson.org For policy questions, send mail to: Richard Nute: ri...@ieee.org
