Reference
80E00 ElectricalSampling Modules User Manual 31
Figure 15 shows a typical waveform froma Tektronix CSA8200 mainframe
equipped with an 80E04 TDR/sampling module. Inthis case, the instrument is
connectedthrough a 50 Ωcoaxial cable to a 75 Ωdevice under test. The incident
step is about 2 divisions in amplitude, andthe reflection from the device under
test is about 0.4 division high. These numbersequate to a reflection coefficient
of 0.2ρ(0.4 divisions divided by 2 divisions). Plugging the known 50 Ωlevel
and the reflectioncoefficient into the above equation yields the 75 Ωvalue:
ZL=ZOᏋ1+
1−Ꮠ
ZL=50Ꮛ1+0.2
1–0.2 Ꮠ=75 Ω
Notice that theinstrument automatically performs this calculation and displays
the impedance (Ω) or reflection coefficient(ρ) for each curso r and the difference
between the two cursors.
75Ωline50Ωline 75 Ωline50Ωline
Figure15: TDRstep and reflection (50 Ωline terminated in 75 Ω)