Tektronix 8.00E+03 User Manual
Reference
18
80E00 Electrical Sampling Modules User Manual
Read the following topics; they provide details that can help you set up and take
effective TDR measurements.
effective TDR measurements.
TDR Step Generation. Both channels in the 80E04 TDR/sampling module have a
selectable polarity step generator which gives both channels measurement
capabilities. You can use the outputs of both generators to perform differential
and common-mode TDR measurements.
selectable polarity step generator which gives both channels measurement
capabilities. You can use the outputs of both generators to perform differential
and common-mode TDR measurements.
The step generator circuitry consists, fundamentally, of a polarity-selectable
current source and a diode switch. Initially, before the step, the diode switch is
biased to conduct current to the output. When the diode switch opens, the step
occurs. A DC current source assures that the baseline level stays close to zero
volts. Figure 7, a simplified diagram, shows the switch and the current source.
current source and a diode switch. Initially, before the step, the diode switch is
biased to conduct current to the output. When the diode switch opens, the step
occurs. A DC current source assures that the baseline level stays close to zero
volts. Figure 7, a simplified diagram, shows the switch and the current source.
Acquisition point to
main instrument
50
Ω
10 mA
DUT
10 mA
Figure 7: Simplified schematic diagram of step generator - positive polarity
The following sections and Figures 8--10 describe the operation with a short
circuit, an open circuit, and a 50
circuit, an open circuit, and a 50
Ω load, with a positive step source.
Operation Into a Short. Initially, the diode switch is conducting --10 mA. Since
the step-generator output is initially shorted, the resistance to ground is 0
the step-generator output is initially shorted, the resistance to ground is 0
Ω.
When the diode switch opens (reverse-biased), apparent resistance to ground at
the acquisition point (and at the channel connector) is 25
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 incident amplitude E
i
.
The transition propagates to the short in the Device Under Test (DUT) and is
negatively reflected back to the acquisition point, E
negatively reflected back to the acquisition point, E
r
= --250 mV reflected,
causing the voltage at the acquisition point to drop back to 0 V. The time
displayed from the first transition to the second transition is the round trip
displayed from the first transition to the second transition is the round trip
Keys to Using