Reference
18 80E00 ElectricalSampling Modules User Manual
Read the following topics;they provide details that can help you set up and take
effective TDR measurements.
TDRSt ep Generation.Both channels in the 80E04 TDR/sampling module have a
selectablepolarity step generator which gives both channels measurement
capabilities. Youcan use the outputs of both generators to perform differential
and common-mode TDR measurements.
The step generatorcircuitry consists, fundamentally, of a polarity-selectable
currentsource and a diode switch. Initially, before the step, the diode switch is
biased to conductcurrent to the output. When the diode switch opens, the step
occurs. A DC current source assures th at the baseline level stays close to zero
volts. Figure 7, a simplified diagram, shows the switch and the current source.
Acquisitionpoint to
maininstrument
50Ω
10mA
DUT
10mA
Figure7: Simplifiedschematic diagram of st ep generator - positive polarity
The following sectionsand Figures 8--10 describe the operationwith a short
circuit, an opencircuit, and a 50 Ωload, with a positive step source.
OperationInto a Shor t.Initially, the diode switch is conducting --10mA. Since
the step-generatoroutput is initially shorted, the resistance to ground is 0 Ω.
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 incident amplitude Ei.
The transition propagatesto the short in the Device Under Test(DUT) and is
negatively reflectedback to the acquisition point, Er= --250 mV reflected,
causing the voltageat the acquisition point to drop back to 0 V. The time
displayed fromthe first transition to the second transition is the round trip
Keys to Using