NEW 2010 Circular Interconnects Catalog - All sections - 12-C3

AmphenolAerospaceImpedance Matching Formula(Your System to a 50 Ohm System)Options High SpeedEMI Filter Printed5015 83723 III 26482 38999OthersFiber OpticsTransient Circuit Board26500 Pyle Crimp RearContacts Release Matrix Matrix Pyle Matrix 2 SJT I II IIIThe following formula and example are offered in order todeter mine the expected filter performance in an impedancesystem other than 50 ohms.With the attenuation expressed in 50 ohms and the transferimpedance curve shown in Figure 1 below, a designer can relatethe expressed attenuation to the input and output imped ance ofhis circuit.Example:(1) Noise is 40dB above specification level at 100 MHz(2) Input and output impedance are 10 and 100 ohmsrespectively(3) Amphenol ® VHF 7000 pf filter has a 65 dB minimumattenuation at 100 MHz and +25°CFormula (Taken from Figure 1):1.4 x 10 ohm = transfer impedancefor 65 dB in a 50 ohm systemAttenuation - dB14012010080604020Attenuation vs Transfer Impedance in 50 Ohm System010-410-310-210-110-0101Transfer Impedance - Z 12 OhmsFigure 1Atten (dB) = 20 log 101 +Z S= source impedanceZ L= load impedanceZ 12= transfer impedanceAtten = filter performance in a system other than 50 ohmsAtten (dB) = 20 log 101 +Attenuation = 56.3dBZ SZ LZ 12(Z S+ Z L)10(100)1.4 x 10 –2 (10 + 100)In this case, the 7000 pf VHF filter will give 56.3 dB which is16.3dB below the desired reduction in noise (40dB) as statedin the above problem.284Contact Amphenol Aerospace for more information at 800-678-0141 • www.amphenol-aerospace.com