Conformity Magazine - June 2008 - (Page 55)

Core Selection The designer knows that the core must accept a #22 wire, and fit into a space of 10 mm by 5 mm. The inside diameter will be specified as .8 mm. Solving for the estimated impedance and its components, first a bead with an outside diameter of 10 mm and a height of 5 mm is chosen: at 100 kHz #22 awg wire. Referring to Figure 7, Impedance per unit Lo for three ferrite materials, or Figure 8, complex permeability for the same three materials, an 850 μi material is chosen4. Using the graph of Figure 9, Z/Lo for the medium permeability material, at 25 MHz, is 350 × 108 ohm/H. Solving for the estimated impedance: Z= 350 108 × .0461 × log10 (5/.8) × 10 × 10-8 Since: Z= ωLo μ and Toroidal Lo= .0461 N2 log10 (OD/ID) Ht 10-8 (H) Then: Z= ωLo (2500.38) = (6.28 × 105) × .0461 × log10 (10/.8) × 5 × (2500.38) × 10-8= 3.97 ohms Where: Rs= Lo ωμs" = .069 ohms XL = Lo ωμs'= 3.97 ohms at 900 kHz Z= 33.3 ohms, Rs =14.48 ohms, XL =30.0 ohms Then a bead with an outside diameter of 5 mm and a length of 10 mm is selected: at 100 kHz Z= ωLo (2500.38) = (6.28 × 105) × .0461 × log10 (5/.8) × 10 × (2500.38) × 10-8 = 5.76 ohms Where: Rs = Lo ωμs" = .100 ohms XL = Lo ωμs' = 5.76 ohms at 900 kHz Z = 48.1 ohms, Rs = 20.9 ohms, XL = 43.3 ohms In this instance, as in most, maximum impedance is achieved by using a smaller OD with the longer length. If the ID were larger (for instance 4 mm), the reverse should have been true. This same approach can be used if graphs of Impedance per unit Lo and phase angle vs. frequency are supplied. Figures 9, 10 and 11 are representative of such curves for the same three materials used throughout this article. Example The designer wants to guarantee maximum impedance for the frequency range of 25 MHz to 100 MHz. The available board space is again 10 mm by 5 mm and the core must accept a Z=128.4 ohm Φ = 30 degrees XL = Z sin Φ = 126.8 ohms Rs = Z cos Φ = 19.81 ohms Figure 8 EMI Shielded Vents & Filters Honeycomb Vents, Fan Vents, Quiet Vents & Air Filters Cross Cell honeycomb offers the highest level of EMI shielding effectiveness Lightweight extruded aluminum frames for increased strength Many standard EMI gaskets can be added for optimal frame to enclosure shielding Optional dust screens and captive fasteners available Free Samples at www.tech-etch.com/shield ISO 9001:2000 REGISTERED TECH-ETCH, INC., 45 Aldrin Road, Plymouth, MA 02360 JUnE 2008 Conformity 55 http://www.tech-etch.com/shield http://www.tech-etch.com/shield

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Conformity - June 2008 Contents Editor's Note 700 MHz Auction Raises a Record $20 Billion FCC Modifies PCS and AWS Power Limit Rules for Broadband Wireless Commission Designates Spectrum Test Bed FCC Releases Report on Wireless Competition Commission Proposes $5 Million Fine for Slamming Using EDX for Non-Destructive Detection of Lead in Consumer Products ESD Open Forum Challenges in Testing - Improving Election Security and Accuracy Part 1 Challenges in Testing - Human Body Model: The Hidden Challenges High-Speed Signal Integrity Considerations for ESD Components Focus On...EMC Components Buyer's Guide FCC Levies $2.6 Million Fine for Junk Faxes FDA Updates Guidance on its Product Review Process EU Issues Updated Energy Star Regulations EU Repeals Directive on Veterinary Electro-Medical Equipment New Product Announcements Updated Standards List for the EU's PPE Directive CPSC Announces New Effort to Keep Out Hazardous Products CPSC Actions in the News IEC Standards Update UL Standards Update Product Reviews Telcordia Standards Update From Our "You Can't Make This Stuff Up" Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity - June 2008

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