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The trigger makes repeating waveforms appear static

on the oscilloscope display by repeatedly displaying

the same portion of the input signal. Imagine the

jumble on the screen that would result if each sweep

started at a different place on the signal (see Figure

34).

Trigger Level and Slope

Your oscilloscope may have several different types of

triggers, such as edge, video, pulse, or logic. Edge

triggering is the basic and most common type.

For edge triggering, the trigger level and slope

controls provide the basic trigger point definition.

The trigger circuit acts as a comparator. You select

the slope and voltage level of one side of the

comparator. When the trigger signal matches your

settings, the oscilloscope generates a trigger.

• The slope control determines whether the trigger

point is on the rising or the falling edge of a

signal. A rising edge is a positive slope and a

falling edge is a negative slope.

• The level control determines where on the edge

the trigger point occurs.

Figure 35 shows the effect the trigger slope and level

settings have on how a waveform is displayed.

Trigger Sources

The oscilloscope does not necessarily have to trigger

on the signal being measured. Several sources can

trigger the sweep:

• Any input channel

• An external source other than the signal applied

to an input channel

• The power source signal

• A signal internally generated by the oscilloscope

Most of the time, you can leave the oscilloscope set

to trigger on the channel displayed. Many oscillo-

scopes provide a trigger output that delivers the

trigger signal to another instrument.

Note that the oscilloscope can use an alternate

trigger source whether displayed or not. So you have

to be careful not to unwittingly trigger on, for

example, channel 1 while displaying channel 2.

Trigger Modes

The trigger mode determines whether or not the

oscilloscope draws a waveform if it does not detect a

trigger. Common trigger modes include normal and

auto.

In normal mode, the oscilloscope only sweeps if the

input signal reaches the set trigger point; otherwise

(on an analog oscilloscope) the screen is blank or (on

a digitizing oscilloscope) frozen on the last acquired

waveform. Normal mode can be disorienting since

you may not see the signal at first if the level control

is not adjusted correctly.

Auto mode causes the oscilloscope to sweep, even

without a trigger. If no signal is present, a timer in

the oscilloscope triggers the sweep. This ensures that

the display will not disappear if the signal drops to

small voltages. It is also the best mode to use if you

are looking at many signals and do not want to

bother setting the trigger each time.

Figure 34. Untriggered display.

Figure 35. Positive and negative slope triggering.

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