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. 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.