HP 54620A User's And Service Manual page 110

Example
Consider a situation where you have a squarewave that swings from 0 V to
+60 V; thus, it is too large for the logic inputs or the external trigger input.
However, by using a 10:1 divider probe, such as are in the HP 54620A/C, you
can safely apply the squarewave to the external trigger input. You can then
set the threshold for the external trigger to +3.0 V (60 divided by 2 divided
by 10). Then, set an edge trigger for the rising or falling edge as desired.
Signal Margin on External Trigger
The above example uses a 10:1 divider probe because of the large swing of the
analog signal. But, consider the situation where the analog signal swing is 0 to
4 V. The 10:1 divider will reduce this to a 400 mV swing. However, the minimum
input swing required by the analyzer is 500 mV about the threshold. Thus, the
analyzer input will not reliably recognize the signal transitions. The solution is to
use a 1:1 probe. See chapter 7, "Performance Characteristics," for more
information on the input characteristics of the analyzer.
Figure 32
Using the External Trigger Input as an Edge Trigger
Making Analyzer Measurements
To use the external trigger input
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