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16 May 2022

Oscilloscope Basics: External, Line and Fast Edge "Triggers"

An oscilloscope trigger synchronizes the oscilloscope timebase to the input signal so that the displayed trace is stable. In digital storage oscilloscopes, while the digitizer runs continuously converting analog voltage/current inputs to digital values, it is the trigger event that defines the “acquisition window,” marking the point where data is stored to acquisition memory, locking the signal data for display, measurement and further processing. 

Figure 1: The trigger setup showing the possible choices for the trigger source.

Triggers are set to fire based on the state of a trigger source waveform. What are commonly known as External, Line and Fast Edge "triggers" are not really different trigger types, per se, but alternative trigger sources.  Figure 1 shows the typical setup options for an Edge trigger, the most commonly used trigger type.  With Edge triggering, the oscilloscope is triggered when the source waveform crosses a user-defined threshold level and slope.  Usually, the source will be analog input channel C1-Cn. However, three other sources can be used to initiate an Edge trigger: an Ext(ernal) input, the Line (mains) power and, on some oscilloscopes, the built-in Fast Edge signal. 

External Trigger

"External triggers" are most often Edge triggers looking for the selected slope and level in an externally generated pulse that is input through the Ext. In port on the front of the oscilloscope. In Pattern triggers, the external input may be examined for a High/Low state as part of the overall triggering pattern.

A major advantage of using an external trigger source is that it frees a channel for measurements that may be difficult if triggering on a probed channel—for example, if the signals on the active channels are all occurring at different frequencies, such as when you are going to measure several stages in a counter. If you trigger on the first stage, all the other stages will be unstable when they are probed. However, if you trigger on the external input using the last counter stage or the carry output, any counter stage that you probe will be displayed stably.   

Because the Ext. In port does not have an attenuation switch to handle different signal ranges, as do the analog channels, on some oscilloscopes, you must choose to trigger using either the full range Ext source or the attenuated Ext/10 source.

Line Trigger

Figure 2: The Line trigger setup for a 60 Hz,
3-phase measurement uses the oscilloscope
power source as the trigger source.
"Line triggers" are Edge triggers that look for the 50% level of the selected slope on the AC power line (mains) connected to the oscilloscope.  

A Line trigger is useful when measuring power supplies, inverters and rotating machinery that are Line frequency dependent. Figure 2 shows an example of the Line trigger setup for a 3-phase 60 Hz power measurement.

Fast Edge Trigger

Some oscilloscopes generate a Fast Edge signal that can be output from the Aux Out port, or, on some legacy models, from a dedicated Fast Edge output.  It is a square wave with a nominal rise time of < 300 ps (the actual number depends on the oscilloscope model) and bandwidth in excess of 1 GHz. The Fast Edge signal is usually connected from Aux Out to a channel and used as a reference for deskewing high-bandwidth probes. To learn how this is done, see the tutorial, “Probe Calibration.”

However, the signal can also be used as a trigger source. "Fast Edge triggers" are Edge triggers that look for the 50% level of the selected slope on the oscilloscope's internally generated Fast Edge signal. There is no need to connect this signal to another channel when using it to trigger, rather than to deskew.

Figure 3: Using the Fast Edge pulse for measuring
the frequency response of a low-pass filter.
Frequency response measurements require a trigger source signal that has a flat spectrum. The Fast Edge pulse has a bandwidth in excess of 400 MHz, which makes it a useful trigger source for frequency response measurements. Figure 3 shows the Fast Edge pulse being used both as a trigger source and as a step source to measure a 37.6 MHz low-pass filter.

By simultaneously utilizing the Fast Edge as a step source input to channel C1, it is possible to derive the impulse response using the oscilloscope’s derivative function.  This can then be applied to the Fast Fourier Transform (FFT) function to obtain the frequency response. The FFT of the differentiated edge in the bottom left trace shows that the Fast Edge is spectrally ‘flat’ over the entire 100 MHz displayed range. The filter output, after differentiation, results in the displayed impulse response in the bottom right trace. To learn more about making frequency response measurements using step sources, see the application note, “Frequency Response Measurements.”

Alternative trigger sources free analog channels to help you maximize the number of signals you can simultaneously input and measure, but also have unique characteristics that make them useful in defining the acquisition window for different measurements.

See also:

Oscilloscope Basics: Stabilizing Waveform Display, Pt. 1

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