Usage:
This section provides a brief description of triggering and the time delay features of the Fluke 90 Series ScopeMeters. The descriptions discuss the effects of these functions of the scope traces on the screen.
Time Delay
The Fluke 90 Series ScopeMeters offer a display feature called Time Delay. The time delay allows the user to view the signals at a defined point in time before or after the trigger occurs. This feature allows the horizontal time scale to be set small to get good resolution and still be able to view the desired waveform segment on the screen.
The time delay value indicates the number of horizontal divisions the displayed traces are delayed from the trigger event. The Fluke 90 Series can be used to show the traces anywhere from 20 divisions (2 screens) BEFORE the trigger event to 640 divisions (64 screens) AFTER the trigger event. The displayed traces are shifted an integer number of horizontal time divisions before or after the trigger. An increase in the time delay number scrolls the waveforms to the left. A decrease in the time delay number scrolls the waveforms to the right. Figure 2 shows the same signal with a +0 and a +1 Time Delay. The location of the trigger event moves the same as the waveforms as the Time Delay value is changed.
The horizontal time scaling defines the amount of time delayed from the trigger event for a given TIME DELAY value. For example, if the time scale is set at 10 mS/DIV and the TIME DELAY is at +6, the left edge of the scope screen is 60 mS after the trigger event.
Figure 2 Effect of Time Delay on the position scope traces (Dot indicates same point on both traces): Signal with +0 Time Delay (top trace) Signal with +1 Time Delay (bottom trace)
If the time scale is set at 5 mS/DIV and the TIME DELAY is at +6, the left edge of the scope screen is 30 mS after the trigger event.
This indicates that different TIME DELAY values are needed to keep the left edge of the screen at the same time after the trigger.
External Triggering
Triggering Definition
Triggering generates a synchronized and stable display from a repetitive signal. The trigger signal can be connected to any of the display inputs or the external trigger input. The trigger signal controls the position of screen data and the update (sweeping) of the scope display. When the trigger signal crosses a preselected slope and voltage level, the scope sweeps the display. After sweeping the display, the scope waits for the next trigger event before updating the screen display. External triggering uses the signal on the external trigger input. This signal, however, is not displayed on the scope screen.
The trigger slope and trigger level determine the point when the trigger signal will generate the trigger event. The trigger slope indicates whether the slope of the signal should be plus (positive) or minus (negative) when the trigger event is generated (see Figure 3). The trigger level determines the voltage value at which the trigger event is generated. When the trigger signal passes the trigger level with the chosen slope, the trigger event is generated. For this procedure, the triggering is set to 2V, -SLOPE on the cylinder 1 combustion signal. The trigger event will be generated when the signal voltage passes 2 V on a negative-going slope.
Figure 3 The trace segments from 1 to 2 and from 5 to 6 are examples of positive slopes. The trace segment from 3 to 4 is an example of a negative slope.
External Triggering Signal Location
The external trigger signal is not displayed on the scope screen. This technique allows both display inputs to be used for other signals while using a different signal to control the display stability. The scope locates the trigger event at the left edge of the scope screen (refer to Figure 4), regardless of the horizontal time scale. This procedure uses the Cylinder 1 Combustion Buffer signal as the trigger source. This signal provides only one trigger event between all the cylinder firings. It provides a time reference point for all the cylinder firings.
Figure 4 External Trigger Signal from Cylinder 1 Combustion Buffer (top trace), Odd Buffer Signal (middle trace), and Even Buffer Signal (bottom trace)