May 1973 Popular Electronics
Table of Contents
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All oscilloscopes have some ability to trigger the display sweep on the waveform being measured. Some models provide more options than other to the user for controlling how the sweep is triggered - positive-going edge, negative-going edge, high or low repetition rate, frequency, specific voltage level, from the AC line, or even from an external source. Without control over the trigger source, repetitive waveforms would be "walking" across the screen, random signals and very long interval signals would be missed, and complex signals would be a garbled mess. This article gives an introduction on how oscilloscope triggering circuits are implemented and ways to use them.
This "Scope-Trace Quiz" from a 1965 issue of Popular Electronics will make a compliment to this article.
Understanding Your Triggered Sweep Scope
A Brief Explanation of Those Puzzling Trigger Controls
By Virgil A. Thomason
Fig. 1 - Basic trigger controls for typical o-scope. Particular markings may differ from one scope to another but the principle is the same.
Most technicians and electronics enthusiasts are familiar with the theory behind a triggered sweep scope. Others, however, still seem to be troubled by the special triggering controls used in this type of scope. Let's see if we can't clarify the uses of these sometimes baffling controls.
First, just what is a "trigger"? Remember that there are two different types of scope sweeps: recurrent and triggered. The recurrent sweep is always present on the CRT face and can be synchronized by a front-panel control appropriately marked. A triggered sweep is not recurring and usually is invisible until a trigger pulse comes along to start the sweep. In normal operation (before triggering), the invisible spot is at the left side of the CRT. The incoming trigger not only starts the trace, but also triggers an internal circuit that "unblanks" the beam for the duration of that sweep. Once the sweep has been initiated, a special "lock-out" circuit keeps any other trigger from affecting the sweep until that particular sweep is completed and the beam has returned to the left side of the CRT and is ready to accept another trigger. Therefore, any signal applied to the trigger circuit (usually from the vertical amplifier) will have no effect during the sweep time. This is what contributes so much to triggered sweep stability.
The controls for a typical scope triggering section are shown in Fig. 1. Your particular scope may have different names for these controls, but the principles are the same. Follow the signal flow shown in Fig. 2.
When the scope is to be used to view a signal, the first step is to estimate the input signal level and set the vertical amplifier attenuator controls accordingly. Although a scope, unlike a VOM, cannot normally be damaged by excessive off-scale operation, it is good practice to make this signal level estimate - keeping in mind that the ac signal may have a dc component. If the latter is the case, set the vertical amplifier input selector to AC to begin with. If the scope is left in the DC mode, the desired ac signal may be riding on enough dc to cause the display to be so far off scale that it can't be seen. If you know that only an ac signal will be present, then you can use the DC mode of the vertical input selector.
Fig. 2 - This diagram shows the usual signal flow through triggered scope.
The first trigger control encountered is the SOURCE selector. If you want to trigger the sweep at some point on the displayed waveform, use the INT (internal) position. This automatically picks up the signal from the vertical amplifier. The EXT (external) position allows the use of a trigger signal from outside the scope. If the displayed signal is related to the commercial power line frequency, then the LINE position will pick up a trigger signal from the power supply within the scope.
The next control is the trigger COUPLING selector. If you want to trigger at a particular dc level on the applied signal, use the DC position. To trigger from an ac signal use either AC position. If your scope has two AC positions, the one marked AC FAST uses a network that passes only the higher frequencies and is usually used to block any 60-Hz component that might be present on the triggering signal.
The SLOPE switch is used to pick a triggering point on either the positive or negative portion of the triggering waveform. The TRIGGERING LEVEL control is used to pick the actual point on either the positive- or negative-going portions at which you want the trigger to occur.
The TRIGGERING MODE switch usually has three positions. In FREE RUN, the sweep oscillator is made free-running, thus starting another sweep directly after the first is completed. This is similar to a conventional recurrent sweep scope. If your triggered sweep scope does not have this position, then the AUTO position may provide this feature, usually at some low frequency (in many cases, about 50 Hz). The selected trigger signal will override the 50 Hz to synchronize the sweep properly. In other words, the AUTO mode is the same as ac coupling, your choice of slope, and the exact center of the triggering level - provided the applied trigger is faster than 50 Hz. The NORMAL mode is used with the triggered sweep.
As a further aid to understanding these controls, try this little experiment. Use a filament transformer as the vertical input source to the scope, As a triggering source, you can use either the LINE position of the SOURCE switch or you can feed the secondary of the transformer to the horizontal input also and use the external input as the trigger source. Use AC coupling and place the SLOPE switch on the positive position. The level potentiometer can now be adjusted to start the sweep on any portion of the positive-going sine wave being displayed. Changing the SLOPE switch to the negative position will now enable you to pick almost any point on the negative half cycle as the starting point.
Posted February 26, 2018