Specifications

Operating Manual SEFRAM 4451
M4451 A00
16
2.7.2 Impedance Matching
A mismatch, or different impedance in a transmission line, generates a reflection back along the line to the
source. The amplitude and polarity of the reflection are determined by the load impedance in relation to the
characteristic impedance of the cable. If the load impedance is higher than the characteristic impedance of
the line, the reflection will be of the same polarity as the applied signal. If it is lower, the reflection will be of
opposite polarity. These reflections add or subtract from the amplitude of the incident pulse causing
distortion and irregular pulse shapes.
Impedance-matching network that provides minimum attenuation
A simple resistive minimum attenuation impedance matching network that can be used to match the
instrument output into relatively low impedance is shown in the above figure. To match impedance with the
illustrated network, the following conditions must exist:
and
Therefore:
R1 R2 = Z1 Z2, and R1 Z1 = R2 (Z2-Z1)
or
and
( )
R
Z
R
R
Z
R
Z
1
2 2
1
22
1
+
+ +
=
RZZZ1221=−()
RZ
Z
ZZ
21
2
21
=
−
R
RZR
ZR
1
112
12
+
+
+