Keysight Infiniium 8000 Series, Service Manual

Page: 55 / 162

Chapter 3: Testing Performance
To verify threshold accuracy
55
High- to- Low Transitions
1
Set the DC Signal Source voltage to the "Initial V1 Setting" as shown in the
table below. The DC Signal Source voltage is referred to as "V1" throughout this
procedure.
Table 1: Oscilloscope Threshold Voltage Setting and Initial V1 Setting
Note: In this test, complete steps 2, 3 and 4 for the first threshold voltage setting in
the table above. Then set the threshold voltage to the next value in the table and repeat
steps 2, 3 and 4 for this threshold voltage setting. Finally, set the threshold voltage to
the last value in the table and repeat steps 2, 3 and 4 for the last threshold voltage setting.
2
Check the logic display at the bottom of the screen to ensure that all bits are
logic 1. If this is not the case, check the following:
• Read the DMM to determine if the input voltage is correctly set to the "Initial V1
Setting" value shown in Table 1. Adjust the DC Signal Source voltage if necessary.
• Check the oscilloscope’s Digital set up to ensure that the logic threshold voltage is
set to the "Oscilloscope Threshold Voltage Setting" value shown in Table 1, and ensure
that the display of all bits is enabled.
• Ensure that the oscilloscope is in Run mode.
• Ensure that the oscilloscope trigger system is properly configured.
• If the oscilloscope is set up correctly and the input voltage is accurately set to the
initial V1 setting and at least one bit is displayed as 0, then either the oscilloscope
is out of calibration or it is malfunctioning. If the oscilloscope has been powered on
for at least 30 minutes, re- calibrate the oscilloscope and retry this set up.
3
Perform the coarse threshold voltage search (to determine the measured
threshold voltage within ±50mV.) You will repeatedly adjust the voltage until
all bits change from logic 1 to logic 0. Record the voltage you applied just before
all bits switched.
a Repeat the following steps until all displayed bits are logic 0:
• Reduce V1 by 50mV and examine the logic display.
• If all bits are 0, record the initial value of V1 (the value applied just before all
bits switched) as the V2 value in Table 2 and proceed to the Fine Threshold
Voltage search.
• If at least one bit is still 1, reset the input voltage to the initial value of V1
shown in Table 1. This is done so that when V1 is reduced, a significant voltage
transition occurs.
Oscilloscope Threshold Voltage Setting Initial V1 Setting
+5V 5.35V
0V 200mV
-5V -4.65V