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Reference
80E00 Electrical Sampling Modules User Manual
31
Figure 15 shows a typical waveform from a Tektronix CSA8200 mainframe
equipped with an 80E04 TDR/sampling module. In this case, the instrument is
connected through a 50 Ω coaxial cable to a 75 Ω device under test. The incident
step is about 2 divisions in amplitude, and the reflection from the device under
test is about 0.4 division high. These numbers equate to a reflection coefficient
of 0.2ρ (0.4 divisions divided by 2 divisions). Plugging the known 50 Ω level
and the reflection coefficient into the above equation yields the 75 Ω value:
Z
L
= Z
O
Ꮛ
1 + ◽
1 − ◽
Ꮠ
Z
L
= 50
Ꮛ
1 + 0. 2
1–0.2
Ꮠ
= 75 Ω
Notice that the instrument automatically performs this calculation and displays
the impedance (Ω) or reflection coefficient (ρ) for each cursor and the difference
between the two cursors.
75 Ω line50 Ω line
75 Ω line50 Ω line
Figure 15: TDR step and reflection (50 Ω line terminated in 75 Ω)