Fluke 863 - Single Trigger; External Trigger

Theory of Operation
Waveform Triggering
2
2-21
• 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 2-40.
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 2-41.
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.
Note
Use a high impedance voltmeter for viewing divider operation.