In oscilloscope development, triggered sweep was a major innovation. It emerged as part of the great electronics boom in the post-World War II period. It came in handy for troubleshooting the large number of consumer TVs that appeared around the same time. TV technology was orders of magnitude more complex than that of the simple superheterodyne radio receiver. Consequently, service technicians needed to look at waveforms at various points in the chassis to locate the defective stage, circuit and component.
Without triggering, the oscilloscope could not properly display repetitive signals. The problem was that the screen had only finite width (especially in the early round CRT models). So a repetitive waveform would quickly have to return to the left side of the display to continue. The retrace could be blanked out as in a TV, but the problem remained that successive traces would not coincide, making for an unstable display. To see what this looks like, turn the appropriate knob in a modern oscilloscope so as to raise the triggering level higher than the peak of the waveform that is displayed. Notice that triggering is abruptly lost, and multiple superimposed traces are seen.

Radar — as developed in England during World War II to detect enemy aircraft — laid the groundwork for many advances in oscilloscope technology, including triggered sweep. Howard Vollum and his colleagues at Tektronix developed triggered sweep for the oscilloscope. In this new instrumentation, successive waveforms were aligned so they began at the left side of the display and were precisely superimposed to make a single sharp trace.
In the default mode, the scope triggers on the rising edge when the signal’s amplitude reaches a user-specified level. This triggering level may be adjusted by turning the triggering level knob, which generally is grouped with other triggering controls near the right side of the front panel. In the same area is the triggering menu button. When it is pressed, the triggering menu appears across the bottom of the display. The items in this menu are Type, Source, Coupling, Slope, Level and Mode (Auto and Holdoff).
When the soft key associated with any of these items is pressed, a submenu appears along the left or right side of the display. Activating Type, we see that any of the alternatives may be chosen using Multipurpose Knob a. They are Edge, Sequence (B Trigger), Pulse Width, Timeout, Runt, Logic, Setup and Hold, and Rise/Fall time. Scrolling down, we observe that triggering is lost in some triggering modes because they don’t work with the sine wave accessed from the AFG. That is because these modes (Pulse Width, Logic, Setup and Hold and Rise/Fall Time) are relevant to digital signals.
Pressing the soft key associated with the second menu item from the left, the Source submenu appears. Using Multipurpose Knob a, the operator can select the triggering source. Assuming the AFG sine wave signal is connected to Channel One, when any of the other channels, including digital, is selected as source, triggering (of course) is lost. When the ac line is selected as source, the display reflects the complex interplay of frequencies in the displayed waveform and the 60-Hz signal as sent by the utility.


The third menu item is Coupling. Pressing the soft key brings up the submenu. The default setting is dc Coupling. Others are ac Coupling, High Frequency Reject, Low Frequency Reject and Noise Reject. Edge and Sequence Coupling use DC Coupling, AC Coupling, Low Frequency Rejection, High Frequency Rejection and Noise Rejection. The other trigger modes make use of DC Coupling exclusively.
If we return to AFG, then go to Output Settings, we can add 50% noise to the sine wave. Then, returning to the triggering Menu and Coupling, press High Frequency Reject. Notice that a major portion of the noise goes away. That is because noise is a wide-spectrum signal with a large high-frequency component.


The next item in the triggering menu is Slope. Pressing the associated soft key, we see triggering may be set on the rising edge, falling edge or both edges of the signal. With the sine wave displayed, we see that as the operator chooses Rising or Falling Edge, the phase of the signal is shifted 180°. When both slopes are chosen, as may be expected, the two out-of-phase signals are superimposed in the display. These out-of-phase signals can be separately captured and used to produce Lissajous patterns when the oscilloscope is placed in the XY mode.
The next menu item is Triggering Level, which may be altered by turning Multipurpose Knob a. The level is shown as a horizontal line, which remains visible for two seconds. As the level is increased or decreased, the waveform shifts to the right or left so for any given level, the waveform is shown to precisely intersect the Y axis. This same effect may be realized by turning the level button in the triggering section adjacent to the keyboard.
When the triggering level is raised above the signal’s positive peak or lowered below the negative peak, triggering is lost and the waveform becomes unstable, as in the pre-world War II oscilloscope.
The final triggering menu item at the right below the display is labeled Mode, Normal and Holdoff. Pressing the soft key, the relevant menu appears at the right of the display. Auto, at the top, disables triggering so an unstable waveform is displayed, and Normal restores triggering for the default sine wave.
Holdoff is, as the name implies, a time interval during which triggering is suspended until both the specified amount of time elapses and a new triggering event commenses. (To be clear, both these conditions must be met.) The holdoff interval may be set by turning Multipurpose Knob a. An easier way is to enter a value in the numeric keypad and press the softkey adjacent to the desired units.
The purpose of holdoff is to prevent the instrument from triggering on a spurious event. If the signal is noisy, a rising anomaly in the waveform can constitute one of these unwanted triggering episodes. Another false trigger is seen in complex waveforms or when a burst (such as the color burst in a TV signal) causes the instrument to trigger at the wrong point along the timebase. A carefully chosen holdoff interval provides an excellent workaround when these anomalies interfere with an otherwise good display.
Triggering holdoff is often used when working with digital signals, which are often packetized and/or full of bursts. Interchip bus signals, including SPI and I2C are particularly subject to illicit triggering, as are RF signals such as Bluetooth, Zigbee, WLAN and cellular. These are characterized by short signal bursts that are deployed in the interest of power reduction and time division multiplexing. If you do much of this type of work, you will quickly become adept at triggering holdoff.
Triggering was first developed back in the time of the all-analog oscilloscope with a CRT display. Most of today’s oscilloscopes are digital. Triggering and holdoff are essential in both technologies. Triggering and acquisition are accomplished by totally different means, but the end results from the user’s point of view are similar.
Digital oscilloscopes sample the analog signal continuously, at a fast rate that must be sufficient to prevent aliasing. Generally, for a higher bandwidth instrument, a faster sampling rate is required.
The Nyquist Theorem states that if the sampling rate is at least twice the frequency of the signal to be sampled, all information in the analog signal will be captured. This is conditional on the signal having a Fourier Transform that is zero outside a finite frequency range. In other words, the signal’s bandwidth cannot be infinite.
In a digital oscilloscope, because the input signal is sampled continuously and put into memory, data that is both before and after triggering is available to the user. This capability did not exist in the old-world analog oscilloscope.
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