Reference
20 80E00 ElectricalSampling Modules User Manual
When the diodeswitch opens (reverse-biased), apparent resistance to ground at
the acquisition point (andat the channel connector) is 25 Ωbecause the internal
termination resistanceis 50 Ωin parallel with the connector impedance of 50 Ω.
The voltage at the acquisition point rises to +250 mV.
The transition propagatesto the open in the DUT and is positively reflected back
to the acquisition point, causingthe voltage at the acquisition point to rise to
+500 mV.At the acquisition point, the time displayed from the first step to the
second step is theround trip propagation time from the acquisition point to the
open in the DUT and back. See Figure 10.
BaselineCorrect ion.T he baseline of a current-source based step generator
normally shifts its DC level with loading. The use of a DC current source to
cancel the step source current maintains t he baseline level close to 0 V (see
Figure 7 on page 18).
Shapeof Reflections.The shape of a reflection reveals the nature and magnitude
of the load impedance,mismatch, or fault, even when the load impedance is not
a short, 50 Ω,or open. Figure 11 shows typical TDR displays and the load that
generatedthe reflection.