Tdr; Tdr Step Generation; Figure 3 91: Step Generator Simplified Schematic Diagram - Tektronix 11801C User Manual

TDR

This section describes how to perform time domain reflectometry (TDR)
measurements. TDR measurements can be made only with an SD 24 TDR/
Sampling Head or an SD 20 Loop Thru Sampling Head; none of the other
SD Series sampling heads provide TDR capabilities.

TDR Step Generation

Both channels in the SD 24 TDR/Sampling Head have a step generator,
which gives both channels TDR measurement capabilities. You can use the
outputs of both generators to perform differential and common mode TDR
measurements.
The SD 20 Loop Thru Sampling Head is capable of performing single ended
TDR when used with an external signal generator. This discussion pertains
mainly to the SD 24 TDR/Sampling Head.
The step generator circuitry consists fundamentally of an adjustable 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. Figure 3 91, a simplified diagram, shows the switch and the
current source.
Acquisition Point
DUT
50 W
10 mA

Figure 3 91: Step Generator Simplified Schematic Diagram

Because of the architecture of the step generator, the output voltage of the
step depends on the DC resistance to ground of the device under test. The
following sections describe the operation with a short circuit, an open circuit,
and a 50 W load.
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