EMI Catalog - Spectrum Control

Application GuidelinesInsertion Loss MeasurementInsertion loss (IL) is a measure of the effectiveness of afilter. It is defined as the ratio of the voltage (E1) acrossthe circuit load without the filter and the voltage (E2)across the load with the filter. Since insertion loss isdependent on the source and load impedance in whichthe filter is to be used, IL measurements are definedfor a matched 50 ohm system. The insertion loss ismeasured in decibels (dB) and defined as follows:IL (dB) = 20 logE1E2Circuit Impedance vs. Insertion LossIn practical circuit applications the source and loadimpedances may be quite different from 50 ohms. If theseimpedances are known, Spectrum engineering can provideinformation on the expected Insertion Loss or an estimatecan be made using the following formula:1IL (dB) = 20 log +Where Zs = Source impedance in ohmsZl = Load impedance in ohmsZt = Transfer impedance in 50 ohm systemExample:1. System source and load impedances are100 ohms and 600 ohms respectively.2. Selected filter has insertion loss of 50 dBat 100 MHz in a 50 ohm system.3. From the IL vs Transfer Impedance curve(right) the transfer impedance is 0.08 ohms.14. IL = 20 log += 20 log 1072= 61 dBZs ZlZt (Zs + Zl)100 x 6000.08 (100+600)Insertion Loss (dB)7060504030C Filter1.4uF.56uF.1uF.27uF.01uF.027uF.056uF20102700pF5000pF00.01 0.1 1 10 100 1000Frequency (MHz)706050403020Pi Filter5000pF.065uF1500pF.025uF1000pF100 0.1 1 10 100 1000Frequency (MHz)10080604020120100VariousCircuitConfigurationsPi FilterT FilterDouble T FilterL FilterC Filter00.01 0.1 1 10 100 1000Frequency (MHz)Transfer Impedance GraphInsertion Loss (dB)8060402010 -410 -3 10 -2 10 -1 10 -0 10 1Transfer Impedance — ZΩ(For 50Ω Source and 50 Ω Load)SPECTRUM CONTROL INC. • 8031 Avonia Rd. • Fairview, PA 16415 • Phone: 814-474-2207 • Fax: 814-474-2208 • Web site: www.spectrumcontrol.comSPECTRUM CONTROL GmbH • Hansastrasse 6 • 91126 Schwabach, Germany • Phone: (49)-9122-795-0 • Fax: (49)-9122-795-5813