Model 4200A-SCS Parameter Analyzer Reference Manual
Section 5: Pulse measure and pulse generator units
4200A-901-01 Rev. C / February 2017
5-61
DUT resistance determines pulse voltage across DUT
Each output channel of the pulse card is effectively a voltage source with a series 50 Ω resistance.
Since the pulse card, like many other pulse sources, does not have any measurement capability, the
actual voltage across the device under test (DUT) is directly related to the DUT resistance.
There are many output effects that are part of using a pulse generator. This section covers the most
common issue, which is the voltage across the DUT not meeting initial expectations.
There is more than one way to explain the effect of impedance on the DUT voltage. The approach
used below relies on DC concepts and explains the settled portion of the pulse and does not require
knowledge or use of RF concepts. RF concepts are necessary to explain time-based effects, such as
rise time, overshoot, ringing, and reflection.
Figure 197: Schematic of pulse channel and connected DUT
As shown in the figure, the voltage across the DUT, V
DUT
, is directly related to the resistance R
DUT
.
The discussion here is simplified, including resistive impedances only.
The 50 Ω output resistance of the pulse channel and the DUT resistance effectively make a voltage
divider. Ohm’s law is the only concept required to determine I
DUT
and V
DUT
in this simple non-dynamic
approach. Calculate the current, and then use the current to calculate the voltage across the DUT.
Example 1: Ideal situation (50 Ω DUT)
The ideal situation (50 Ω DUT) is shown in the figure below.
R
DUT
50 Ω
Pulse V
high
5 V
Pulse V
low
0 V
Pulse load
50 Ω
Figure 198: 5 V pulse i
nto a 50 Ω DUT load
