Figure 3 92: Step Generator With A Shorted Output; Figure 3 93: Step Generation With A 50 W Load - Tektronix 11801C User Manual

TDR
Operation Into a Short Circuit
Initially, the diode switch is conducting -10 mA. Since the step generator
output is initially shorted, the resistance to ground is 0 W and the output
voltage is 0 V.
When the diode switch opens (reverse biased), apparent resistance to
ground at the acquisition point (and at the channel connector) is 25 W,
because the internal termination resistance is 50 W and the connector
impedance is 50 W. The voltage at the acquisition point rises to +250 mV.
The transition propagates to the short in the device under test and is nega
tively reflected back to the acquisition point; cancelling the transition. The
time displayed from the first transition to the second transition is the propa
gation time from the acquisition point to the short in the device under test
and back. See Figure 3 92.
250 mV
0 V

Figure 3 92: Step Generator with a Shorted Output

Operation Into a 50 W Load
Initially, the diode switch is conducting. Since the step generator output is
connected to a 50 W load, the resistance to ground at the acquisition point is
25 W (because of the internal 50 W impedance). The 10 mA current source
places -250 mV at the acquisition point.
0 V
-250 mV

Figure 3 93: Step Generation with a 50 W Load

When the diode switch opens (reverse biased), the return path to ground is
broken and the acquisition point rises to 0 V. The matched impedance
allows the acquisition point to remain at 0 V. See Figure 3 93.
Operation Into an Open Circuit
Initially, the diode switch is conducting. Since the step generator output is
open, the resistance to ground at the acquisition point is 50 W (because of
the internal 50 W impedance). The -10 mA current source places -500 mV
at the acquisition point, as a starting condition.
3 192
Reference