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17 May 2013

Back to Basics: Triggering

At some point, it's likely you've had the experience of capturing a waveform on your oscilloscope only to see a wildly unstable trace displayed on the screen. Chances are that you hadn't adjusted the triggering correctly. Let's take a brief look at what triggering is and why it's important in an oscilloscope. Trigger modes determine when the oscilloscope acquires and what is displayed.

Triggering synchronizes the acquired waveform to a selected source so that the waveform display is stable. Trigger sources include the input channels, external trigger input, and line power. Some oscilloscopes also provide a built-in mode that simplifies capture of infrequent events on fast edge rates.

On most oscilloscopes, triggering is controlled from the instrument's trigger menu. Slope, polarity, and trigger threshold for each source can be set independently of each other. Trigger sources can be conditioned using AC or DC coupling, high pass/low pass filters, and high-frequency coupling.

Most modern digital oscilloscopes provide two broad classes of trigger types:

Stable runt trigger
Figure 1: Shown is a screen capture of a stable trigger on a runt
pulse. Horizonal cursors are turned on to show upper and
lower trigger amplitude points.
1) Simple triggers that sense particular characteristics of the input signal. Examples include edge triggers, which trigger on an edge of user-specified polarity, slope, and level on the channel chosen as the trigger source; and width triggers, which trigger on either a positive- or negative-going pulse when the pulse width is lesser or greater than a specified limit, or in or of a specified range.

2) More complex triggers that sense user-selected waveform characteristics such as period, logic levels, data patterns, or signal dropouts and triggers the scope on their occurrence. Complex triggers can be used to find glitches, runts, or abnormal slew rates (Figure 1).

Basically, edge triggers kick in when the trace achieves a user-specified voltage level on either the positive or negative slope. Width triggers will find those instances of pulses that are out of spec.

Trigger Setup dialog
Figure 2: This example of a Trigger Setup dialog shows the type of settings
available for triggering on a Teledyne LeCroy HDO4054.
Most oscilloscopes will provide a trigger setup dialog that is accessible either through drop-down menus or from a front-panel button (Figure 2). In this example, basic trigger types are shown on the right. Hitting the Smart triggers key invokes the Smart Type menu, which includes most complex trigger types. Setup options include signal source, coupling, and slope (positive or negative). The coupling types deserve special mention:

DC: All signal frequency components are coupled to the trigger circuit for high-frequency bursts.
AC: Signal is capacitively coupled, DC levels are rejected, and low frequencies are attenuated.
LFRej: Signal is coupled through a high-pass filter network, DC is rejected, and low frequencies are attenuated. This type of coupling is ideal for triggering on medium- and high-frequency signals.
HFRej: Signals are DC-coupled to the trigger circuit and a low-pass filter attenuates high frequencies, which is ideal for triggering on low frequencies.

Another triggering tool worth mentioning is trigger holdoff, which delays triggering until both the trigger condition and holdoff condition are met. These conditions can be defined in time or number of events. Holdoff by time helps obtain a stable trigger on waveforms with multiple trigger events. Holdoff is used where a waveform is periodic but has multiple features which meet the trigger requirements.