10-503140-032 Datasheet, PDF (285/437 Pages) Amphenol Corporation – The Interconnection Leader

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Impedance Matching Formula
(Your System to a 50 Ohm System)
The following formula and example are offered in order to
deter­mine the expected filter performance in an impedance
system other than 50 ohms.
Atten (dB) = 20 log₁₀  1 +
ZZ
SL
Z (Z + Z )
12 S
L
With the attenuation expressed in 50 ohms and the transfer
impedance curve shown in Figure 1 below, a designer can relate
the expressed attenuation to the input and output imped­ance of
his circuit.
Z = source impedance
S
Z_L = load impedance
Z₁₂ = transfer impedance
Atten = filter performance in a system other than 50 ohms
Example:
(1) Noise is 40dB above specification level at 100 MHz
(2) Input and output impedance are 10 and 100 ohms
respectively
(3) Amphenol^® VHF 7000 pf filter has a 65 dB minimum
attenuation at 100 MHz and +25°C
Formula (Taken from Figure 1):
1.4 x 10 ohm = transfer impedance
for 65 dB in a 50 ohm system
10(100)
Atten (dB) = 20 log₁₀  1 + _{1.4 x 10}–2 _{(10 + 100)}
Attenuation = 56.3dB
In this case, the 7000 pf VHF filter will give 56.3 dB which is
16.3dB below the desired reduction in noise (40dB) as stated
in the above problem.
Attenuation vs Transfer Impedance in 50 Ohm System
140
120
100
80
60
40
20
0
10 ^-4
10^-3
10^-2
10^-1
10^-0
10¹
Transfer Impedance - Z₁₂ Ohms
Figure 1
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