Tektronix 80E01, User Manual

Page: 43 / 68

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 0.2
ρ
(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
line 50
Ω
line
75
Ω
line 50
Ω
line
Figure 15: TDR step and reflection (50
Ω
line terminated in 75
Ω
)