Figure 8: Step Generator With A Shorted Output; Figure 9: Step Generation With A 50 Ω Load; Figure 10: Step Generation With An Open Circuit - Tektronix 80E01 Manual

80E00 Electrical Sampling Modules User Manual
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propagation time from the acquisition point to the short in the device under test
and back. See Figure 8.
250 mV
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Figure 8: Step generator with a shorted output

Operation Into a 50 Ω Load. Initially, the diode switch is conducting - -10 mA.
Since the step-generator output is connected to a 50 Ω load, the resistance to
ground at the acquisition point is 25 Ω (because of the internal 50 Ω impedance).
+250 mV
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Figure 9: Step generation with a 50 Ω load
When the diode switch opens (reverse-biased), apparent resistance to ground at
the acquisition point (and at the channel connector) is 25 Ω because the internal
termination resistance is 50 Ω in parallel with the connector impedance of 50 Ω.
The voltage at the acquisition point rises to +250 mV.
The transition propagates to the 50 Ω load and no reflection occurs.
Operation Into an Open. Initially, the diode switch is conducting - -10 mA. Since
the step-generator output is open, the resistance to ground at the acquisition point
is 50 Ω (because of the internal 50 Ω impedance).
+500 mV
+250 mV
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Figure 10: Step generation with an open circuit

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Reference
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