Oscilloscope Trigger / Triggering Tutorial

- the oscilloscope trigger function is one of the key elements of any oscilloscope, enabling a stable image to be viewed on the screen.

The oscilloscope trigger facility is one of the key functions within the oscilloscope.

Triggering enables the scope, whether digital or analogue, to display a steady image on the screen that can be viewed by the user / engineer investigating a particular circuit.

Without a trigger, or other form of synchronisation, it would not be possible to display a steady signal on the screen.

Scope triggering basics

As described, the trigger is used to enable the scope to display changing waveforms to be displayed on the screen of the scope in a steady fashion.

This is achieved by starting the time-base sweep at a selected point on the signal. As a result, triggering allows the display of periodic signals such as sine waves and square waves, as well as non-periodic signals such as single pulses, or pulses that do not recur at a fixed rate.

Many oscilloscopes have a variety of trigger controls that enable the required waveform to be presented in the manner required. These operate in a variety of ways and many are described below.

Trigger level & trigger slope

The trigger level and trigger slope are the two basic trigger controls on any oscilloscope whether digital or analogue.

The trigger level detects when a certain voltage level has been reached and at this point sets the time-base in operation to sweep across the screen. In effect, the trigger level is like a comparator which switches the time-base to start when a voltage level has been reached.

The trigger slope, as the name indicates, determines whether the time-base sweep is triggered on a positive or negative going edge or slope.

A view of an oscilloscope screen showing a waveform triggered with a positive going edge

A view of an oscilloscope screen showing a waveform triggered with a negative going edge

As can be seen, the setting of both the oscilloscope trigger levels and slope determine the image that is seen on the screen. Careful adjustment of both controls ensures that the required portion of the waveform is seen.

In addition to the trigger level and +/- slope triggering, there is also likely to be an option to select whether AC or DC coupling is used.

Trigger sources

There are several ways in which oscilloscopes can obtain their trigger points. These may be labelled on the front panel and the correct selection needs to be made for the application and signal being viewed.

  • Signal channel:   This is the most usual method for triggering an oscilloscope trace. However for multiple trace scopes, it is possible to trigger of the different channels. The most common is the A channel, but those with two channels may also be able to trigger of the B channel as well, therefore it is necessary to check and ensure the correct one is selected.
  • External:   On most scopes there is the possibility to trigger from an external source. This can be very convenient when the triggering needs to be synchronised to an external signal, possibly one operating the circuit under test for example. The trigger level and + / - controls still apply.
  • Line:   This is useful for looking at power line related signals. It can be used for investigating power related interference or looking at the signals themselves.
  • Video:   This was used for looking at analogue video signals. The scope extracted the video sync pulses and used these to trigger the scope.

Trigger holdoff

One facility that is incorporated on virtually all modern scopes is referred to as the hold-off control. First introduced in the early 1970s, the control adds delay after a trace has finished before the next trace or sweep starts.

In analogue operation terms, once the scope sweep is completed, the beam is blanked and the scope returns the sweep voltage back to the starting point. In this way the trace moves back to the left hand side of the screen in what is termed as the 'retrace'. After the sweep and retrace are completed the trigger circuit is ready for another trigger impulse to start the sweep again.

Although achieved rather differently in digital scopes, the same overall operation is seen.

An example of how the oscilloscope trigger hold-off function can be used

While the sweep and retrace are in operation, the trigger circuit will ignore any further trigger pulses that may arrive and it is 'held-off' until the sweep and the retrace are complete. Then, the trigger circuit is re-armed and will respond to any impulses that may trigger the sweep again.

The Trigger Hold-off control provides for an additional, user-defined delay to the re-arming of the trigger circuit, beyond the end of the sweep/retrace period. This gives the user some control over how rapidly, or how often, the oscilloscope can be triggered and it can often add clarity to the display seen on the scope.

Trigger auto facility

One of the disadvantages of an oscilloscope trigger is that if the trigger level has not been set for the incoming signal, or there is no incoming signal, then no trace will be seen. This can be an issue when wanting to set the trace position before applying the signal, or if the trace needs to be found.

To overcome this an 'auto trigger' facility is normally included on the scope. This auto-trigger will start the sweep if no signal is present a timer within the scope triggers the sweep. In this way the trace will not disappear if no signal is present.

Often, for general use, the scope may be left in the auto-trigger mode. However for more exacting investigations, once the trace has been located the 'Normal' mode is often the required option. In this mode, the sweep will not start until the trigger has been activated.

Advanced triggering options

With the advent of digital scopes, there are many possibilities for advanced triggering options. These can all be used to help locate and display waveforms that may require more complex triggering options. However with the software in digital scopes this can now be achieved whereas it would not have been possible with analogue scopes.

  • A & B triggering:   Although many scopes offer triggering from the A and B channels, some digital scopes offer more complex triggering options for A and B channels. For example they may offer logic qualification to control when to look for various events. Others may have a form of delayed triggering a set time after a previous trigger event.
  • Serial pattern triggering:   This form of trigger looks at a serial data stream and triggers after a given serial pattern is seen.
  • Search and mark :   This form of trigger scans for a multiple event types before triggering. Individual marks can be added to portions of the sweep to highlight areas.
  • Trigger correction:   It is sometimes necessary to be correct for trigger delays in very fast systems. As the trigger and signal paths have different times delays there is an inherent time difference between the trigger position and the data that is acquired. This can result in jitter on the display or in skew. To overcome this, a trigger correction system is employed that compensates for the delay differences between the trigger and data acquisition paths. When used in this mode, the trigger point can be used as a reference point for the measurement.

The oscilloscope triggering system forms one of the key elements of the overall test instrument. With the complexity of equipment increasing, this also results in a rising level of waveform complexity, for which the more sophisticated trigger systems are required.

By Ian Poole

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