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:
|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.
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.
|Figure 2: This example of a Trigger Setup dialog shows the type of settings|
available for triggering on a Teledyne LeCroy HDO4054.
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.