Triggering

The oscilloscope trigger is the mechanism that decides when to start drawing each new trace. Without a working trigger, successive traces begin at random phases of the signal and the display is an incoherent blur. With the trigger correctly configured, each trace begins at exactly the same point in the waveform cycle, and successive traces overlay each other precisely — producing the clear, stable image that makes measurements possible.

You were introduced to the basics of triggering in Module 12B. This lesson goes deeper: explaining how edge triggering actually works, what trigger hysteresis does, how trigger coupling filters the source signal, what holdoff does for complex waveforms, and how advanced trigger types like pulse width and glitch triggering let you isolate specific events from a complex signal. Understanding triggering thoroughly separates competent oscilloscope users from beginners who accept whatever the Auto button produces.

What Triggering Does

Consider a 1 kHz sine wave. The wave completes 1000 cycles every second. The oscilloscope acquires a new waveform capture many times per second. Without a trigger, each capture starts at a random phase in the sine wave cycle. The display shows many slightly different traces overlaid on each other — a “rolling” or blurred display. Even if you could freeze a single capture, it would show the waveform offset by some unpredictable amount from one acquisition to the next.

The trigger solves this by insisting that every capture starts at exactly the same point in the waveform. When the trigger condition is satisfied, the oscilloscope arms and begins recording. The result is that every capture starts at the same voltage level on the same slope — and when displayed, successive traces overlay perfectly to produce a sharp, stationary image.

This also means that triggering is not just about aesthetics. It is about measurement accuracy. If you want to measure the time between two events in a waveform, the first event must be at a known, consistent position on the screen. That consistent position is provided by the trigger.

Edge Triggering: Level, Slope, and Hysteresis

Edge triggering is the default and most-used trigger type. The oscilloscope monitors the trigger source signal and fires when it crosses a specified voltage level in a specified direction.

Trigger Level

The trigger level is the voltage threshold the signal must cross to fire the trigger. It is set with the trigger level knob and displayed as a small indicator (usually a triangle or arrow on the right edge of the graticule) showing where the level is relative to the waveform.

For a symmetric sine wave centered at 0 V, setting the trigger level to 0 V means the trigger fires each time the sine wave crosses zero. Since the sine wave crosses zero twice per cycle (once going positive, once going negative), you must also specify the slope to determine which crossing fires the trigger.

If the trigger level is set above the signal’s maximum or below its minimum, the signal never reaches the threshold and the trigger never fires. The most common beginner error is adjusting the trigger level outside the signal’s amplitude range and then wondering why the display is blank. The fix is simple: move the trigger level back into the range of the signal.

Trigger Slope

Slope selects whether the trigger fires on a rising edge (signal crossing the level going upward) or a falling edge (signal crossing the level going downward). For a sine wave, rising edge at 0 V triggers at the positive zero crossing; falling edge at 0 V triggers at the negative zero crossing. Both show a stable display, but they show different phases of the same waveform: the rising-edge trigger places the zero crossing at the trigger point with the waveform rising, while the falling-edge trigger places it with the waveform falling.

Trigger Hysteresis

Hysteresis is a small voltage band around the trigger level within which the trigger circuit ignores transitions. It exists to prevent false triggering on a noisy signal. Without hysteresis, a signal with noise riding on top of it would fire the trigger repeatedly as the noise caused the combined signal to jitter back and forth across the trigger level, producing multiple triggers per intended cycle.

With hysteresis, the trigger fires only when the signal crosses the upper edge of the hysteresis band in the trigger direction. It then requires the signal to move below the lower edge before re-arming. A typical hysteresis band might be 20–50 mV. This width filters out noise below that amplitude without affecting the triggering on the intended signal transition.

Most oscilloscopes set hysteresis automatically. Some offer a “noise reject” trigger coupling option that increases the hysteresis when dealing with especially noisy signals.

Edge triggering on a sine wave. The trigger level (horizontal dashed line) defines the voltage threshold. Rising-edge trigger fires when the signal crosses the level upward (left arrow); falling-edge trigger fires on the downward crossing (right arrow). The shaded band around the level represents hysteresis: the trigger fires only when the signal crosses the upper band edge and re-arms only after the signal drops below the lower band edge, preventing false triggers from small noise on the signal.

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Trigger Coupling

Trigger coupling is independent of the channel input coupling. It filters the signal that the trigger circuit sees, without affecting the signal displayed on screen. This separation allows you to display a signal with DC coupling while triggering on it with, say, high-frequency rejection — the display is unaffected but the trigger ignores HF noise.

The available trigger coupling modes are:

In ham radio work, HF Reject trigger coupling is particularly useful. When monitoring a 60 Hz power supply ripple waveform in the presence of RF pickup, the RF noise on the trigger source causes erratic triggering. HF Reject attenuates everything above ~50 kHz, keeping the 60 Hz trigger signal clean while the RF component is invisible to the trigger circuit. The display still shows the full signal including any RF noise; only the trigger sees the filtered version.

Trigger Holdoff

Holdoff is a user-adjustable time after a trigger event during which the trigger circuit is disabled and will not re-arm. The scope continues to acquire signal data during the holdoff period, but it cannot fire a new trigger until the holdoff time has expired. Holdoff solves the problem of triggering on complex waveforms that contain multiple valid trigger events per apparent cycle.

Consider a burst waveform: a group of high-frequency oscillations, then a pause, then another group. Each individual oscillation within the group contains a valid rising-edge trigger event. Without holdoff, the scope triggers on each individual oscillation within the burst, and the display shows a jumbled combination of different-phase captures. The display looks noisy and unstable, even though the burst waveform itself is perfectly regular.

By setting the holdoff to be slightly longer than the burst duration, the scope triggers only on the first oscillation of each burst. After triggering, the holdoff period covers the rest of that burst and the following pause. The scope re-arms just as the next burst begins, triggering on the first oscillation of the next burst — and the display shows a stable, consistent picture of the burst pattern.

Trigger holdoff applied to a burst waveform. Without holdoff (top), each oscillation within the burst is a valid trigger event, and the scope triggers at random points within the burst — producing an unstable, unreadable display. With holdoff set slightly longer than the burst duration (bottom), the scope triggers only on the first oscillation of each burst, producing a stable display showing the complete burst pattern.

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Holdoff is also useful for triggering on digital packet data where the data bits contain many edges but the packet begins with a specific preamble. Setting holdoff slightly shorter than the packet period ensures the scope only triggers on the start of a new packet rather than on mid-packet data transitions.

Holdoff is adjusted in the trigger menu on most DSOs, typically in a range from the minimum sweep time up to several seconds. Start with the minimum holdoff and increase it until the display stabilizes.

Trigger Position: Pre-Trigger and Post-Trigger

The trigger position controls where in the acquisition record the trigger event falls. In the default (center) position, the trigger event appears at the horizontal center of the display. The left half of the screen shows what happened before the trigger (pre-trigger data), and the right half shows what happened after (post-trigger data).

Moving the trigger position to the right side of the screen increases the amount of pre-trigger data visible. The scope continually fills its acquisition memory as the signal runs, keeping the most recent samples available. When the trigger fires, the scope continues acquiring post-trigger samples and then displays the full record centered on the trigger event. With the trigger position set far right, nearly all of the display window is pre-trigger, showing what happened in the lead-up to the trigger event.

Pre-trigger capture is valuable in troubleshooting. If you are hunting for the cause of a system reset, you set the trigger to fire on the reset signal and set the trigger position to the right side. The scope then shows you what the signals looked like immediately before the reset — the events that caused it rather than just the reset itself.

Moving the trigger position to the left side increases post-trigger data, which is useful when you want to see the sequence of events following the trigger — for example, capturing the startup sequence of a microcontroller by triggering on power-on.

Advanced Trigger Types

Edge triggering handles most ham radio measurements. Several additional trigger types address specific situations that edge triggering cannot handle cleanly:

Pulse Width Trigger

A pulse width trigger fires only when a pulse is narrower than, wider than, or within a specified time range. This is invaluable for isolating anomalous pulses from a repetitive digital signal. In a serial data stream, all valid data pulses have widths within a defined range. A glitch — a spurious short pulse — would have a width outside that range. Setting a pulse width trigger to fire on pulses narrower than the minimum valid pulse width isolates glitches automatically, ignoring all the valid data and only capturing the faults.

In ham radio work, pulse width triggering is useful for CW keying analysis. A well-shaped CW element at 20 WPM has a dot of approximately 60 ms and a dash of 180 ms. If you set a pulse width trigger to fire on any element shorter than 40 ms, you will catch mis-keyed elements, contact bounce in a straight key, or timing faults in an electronic keyer.

Runt Trigger

A runt trigger fires on pulses that cross one voltage threshold but fail to cross a second, higher threshold. In digital logic, a valid logic high must exceed V_IH (the input high threshold). A “runt” pulse rises but only partially — it crosses the lower threshold but not the upper threshold. This is the signature of a marginal logic signal, a slow-rising signal that never reaches a valid high level, or a signal that has deteriorated due to a driver fault or excessive load capacitance.

Glitch Trigger

A glitch trigger is a short-duration version of pulse width triggering. It fires on pulses narrower than a user-specified minimum width. Glitch triggers are optimized to detect short transients — typically in the nanosecond range — that are too brief to be caught by edge triggering. They work in conjunction with peak detect acquisition mode for the highest probability of catching rare, fast transients.

Timeout Trigger

A timeout trigger fires when a signal stays at the same level for longer than a specified time. This detects “stuck at” faults in digital circuits: a signal that should be pulsing regularly but has stopped transitioning. In radio equipment, it can detect a transmitter that has failed to release (a stuck PTT, for example) or a clock signal that has stopped oscillating.

Pattern Trigger

A pattern trigger fires when a specified combination of logic states is present simultaneously across multiple channels. It is primarily a digital/logic analyzer function, and it is the main reason to use an MSO (mixed signal oscilloscope) in complex digital circuit troubleshooting. Pattern triggers can isolate specific register states, address values on a bus, or combinations of control signals that occur only during a particular fault condition.

External Trigger

Most oscilloscopes have a dedicated External Trigger (EXT) input connector, separate from the channel inputs. When the trigger source is set to EXT, the trigger circuit monitors the EXT input rather than one of the measurement channels.

External triggering is used when:

• All input channels are occupied with signal measurements and none is available for triggering
• You want to synchronize the display to a master reference signal that is separate from the signals being displayed
• The trigger signal is at a very different level or frequency from the measured signal and would interfere with channel measurements if used as a channel trigger

A practical ham radio example: you want to observe the audio output from a receiver while synchronizing the display to the station’s 1-second timer pulse. Both channels display the audio signal (CH1 and CH2 for comparison), and the EXT trigger input receives the timer pulse to lock the display to a 1-second cycle.

Trigger Strategies for Ham Radio Work

The following trigger configurations address common ham radio measurement scenarios: