An oscilloscope measures time-dependent signals, such as sinusoidal oscillations. If a signal to be measured is connected to the oscilloscope, the device does not know anything about type and frequency of the signal. How is the oscilloscope supposed to display an unknown signal?
One possibility is to make a so-called "single acquisition" without (or with free-running) trigger. The signal is sampled once and shown only once. Now, if you want to display the signal constantly, you would have to press the "single acquisition" button constantly, which seems very impractical. In addition, the signal would start at a different location each time it is sampled, so that the signal shape is phase-shifted while being displayed.
A better possibility is archived by using a trigger. A trigger is actually a component that starts a switching operation. In oscilloscopes, the trigger synchronizes the display with the signal.
You can trigger to the signal itself or to an external signal (frequently EXT input on the oscilloscope). It is possible to react to rising or falling signals.
As shown in the figure above, the trigger level is represented by a small arrow at the left side of the oscilloscope. It defines timepoint "zero" for the selected signal level. The signal is displayed from the point in time when it reaches the trigger level. The trigger level therefore has the unit "Volt". The figure shows that the waveform shifts when you set the trigger level to a different value, since the signal level at the left edge of the display changes. Since the signal level at the left edge of the display (time 0) is different, the display "shifts" to the left. Additionally oscilloscopes can trigger falling or rising edges. Depending on the type of trigger the setting changes the zero point of the waveform. Trigger settings can be tested within our oscilloscope simulator .
If you have an oscilloscope with "Ext" input, an external signal can be used as trigger signal. Often this is a square-wave signal synchronous to the measuring signal shape, which is provided, for example, by frequency generators.