Single Trigger; External Trigger - Fluke 867B Service Manual

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The upper comparator, controlled by DAC2, recognizes signals crossing the more
positive of two thresholds. The DAC2 output passes through filter R67 and C79; the
filtered DAC2 output can be observed at TP16.
•
The lower comparator, controlled by DAC1, recognizes signals crossing the more
negative threshold. The DAC1 output passes through filter R80 and C33; the filtered
DAC1 output can be observed at TP20.
Use the following procedure to check either of the DACs:
1. Connect a high impedance voltmeter to one of the two test points mentioned above.
2. Vary the trigger level associated with the DAC you are monitoring and look for a
corresponding change in DAC level. In order for the trigger output to be set high, a
signal must cross both thresholds; therefore, the hysteresis of the dual threshold
trigger is adjustable. Trigger slope can be adjusted by pressing t.

Single Trigger

The single trigger circuit uses a single level comparator with built-in hysteresis. The
single trigger circuit and the dual trigger comparator use the same internal input signal
(TP19.)
The U1 DAC generates the required single threshold voltage for the comparator; this
voltage can range from -3.2V to 3.2V. This range is derived from the DAC output
current, converted to a -3.2V to 0V signal by the DAC amplifier (U30), C35, and
associated feedback resistor (U1), and level shifted by a CT amplifier (U30), R84, R58,
and R129.
Check the Single Trigger circuitry with the following procedure:
1. Rotate the selector to Component Test.
2. Check for a 3.2V peak-to-peak sine wave between Ë and â.
3. Rotate the selector to Set Up.
4. Select Single Trigger on the Set Up screen.
5. Rotate the selector to AC Volts.
6. Monitor U30-4 and adjust the single trigger threshold level (press u and v.) .
The voltage on U30-4 should change as the trigger level is adjusted. Trigger slope
can be selected by pressing t.

External Trigger

The external trigger input uses the single level trigger comparator. External trigger
inputs, which can range from 0V to 5V, are applied to the divide-by-3 circuit composed
of R75, C36, R59, R157, and C100. Divider output is then routed to the single level
trigger comparator in U30-37. External trigger threshold voltage is generated in the U1
DAC.
Use a high impedance voltmeter for viewing divider operation.
Note
Theory of Operation
Waveform Triggering
2-40.
2-41.
2-21
2