Fluke 8842a Manual Range, Triggering, Continuous Trigger Mode, External Trigger Mode

Operating Instructions 2

OPERATING FEATURES

2-16. MANUAL RANGE

In manual range, the 8842A remains fixed in the selected range until you select another range or press AUTO. If you select a range which is not valid for the present function, or select a function which is not valid for the present range, the 8842A selects the nearest valid range. For example, if the 8842A is in the VDC function and you press the 20 MΩ button, the 8842A selects the 1000V range.

The range buttons have no effect in the mA AC functions, since all measurements in these functions are made in the 2000 mA range.

2-17. Triggering

Triggering causes the 8842A to execute a measurement cycle and display the result. During each measurement cycle, the instrument samples the input a number of times and then averages the samples to compute a reading. The number of samples averaged for each reading depends upon the reading rate.

Each time a reading is triggered, the rate annunciator (S, M, or F) blinks off. In the fast reading rate, the F annunciator flashes so rapidly it appears to be almost constant.

How the 8842A is triggered depends on whether the continuous trigger mode or external trigger mode is selected. Pressing the EX TRIG (external trigger) button toggles the 8842A between the two modes.

2-18. CONTINUOUS TRIGGER MODE

In the continuous trigger mode, readings are triggered by a continuous, internal trigger. The rate of the trigger is set by the RATE button.

2-19. EXTERNAL TRIGGER MODE

In the external trigger mode, readings are triggered by pressing the TRIG button. If the IEEE-488 Interface option is installed, readings can also be triggered by remote commands or by using the rear panel external trigger (EXT TRIG) connector. (See the Options and Accessories section.)

In the external trigger mode, pressing any front panel button blanks the numeric field on the display until a new measurement is triggered. This ensures that all readings correspond to the instrument configuration indicated by the display annunciators. The blanking also occurs in the continuous trigger mode, but usually isn’t noticed because new measurements are triggered automatically.

The TRIG button does not trigger readings in the continuous trigger mode.However, it does blank the last reading to acknowledge a button was pressed.

2-20. Reading Rates and Noise Rejection

The RATE button allows you to optimize either measurement speed or noise rejection. The 8842A uses both analog and digital filtering to allow measurements in the presence of unwanted environmental noise (especially line-related noise). However, since filtering introduces a delay in response to a change in the input signal, there is an inherent trade- off between noise rejection and measurement speed.

The instrument has three reading rates: slow (S) and medium (M), with a 5-1/2 digit display, and fast (F), with a 4-1/2 digit display. To provide optimum combinations of measurement speed and noise rejection, the RATE button allows control of both the internal trigger rate and the degree of filtering. The same degree of filtering is used in both the continuous and external trigger modes. In the 20 mV, 20Ω, and 200 mA dc ranges, use of slow (S) filter provides maximum noise rejection.

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