Observing Waveforms

Oscilloscope Triggering

Observing Waveforms:

Oscilloscope Triggering

Introduction

In our First Touch Oscilloscope Project, we looked at signals which changed as a result of our direct intervention—by manually connecting and disconnecting the power supply from our circuit. This was intended to give us a physical feeling of the basics of the oscilloscope display. For the most part, the signals we are interested in will be changing very quickly— usually more quickly than we can register with our naked eye. This can make it incredibly difficult to correlate the signal's behavior with any particular “time”. Oscilloscope triggering allows us to assign a “zero time” to a particular feature on the signal. That feature gets placed on the same point on the plot window every time the oscilloscope screen updates; if the signal repeats itself based on this feature, the oscilloscope will display the same section of the signal every time the screen updates, making the signal appear to be unchanging.

Note

If we are trying to view a 10kHz sinusoid, we will probably want our horizontal scale to be on the order of 100μs/division. This means that the plot screen contains only one-thousandth of a second of data. There is no possible way that we can follow that update speed. If we trigger the update of the screen to occur at a consistent point on the signal ever time the signal is “re-drawn”, the wave form displayed to the screen will appear to be unchanging.

As an example, the wave shape displayed in Fig. 1 shows a section of an infinitely long waveform, repeating itself indefinitely.

Figure 1. Example waveform.

We would like to display this waveform using an oscilloscope with a small enough time base, allowing us to observe significant levels of detail on the waveform. If we simply display successive sections of the waveform at the desired time base, our plot will consist of a set of “frames” of the waveform, which will be displayed sequentially to the screen —potentially at a very high rate. Fig. 2 shows a potential set of frames which may be displayed. The frames in Fig. 2 show more-or-less random “snapshots” of the wave form; if the rate of display is high, the resulting display will simply be a jumbled mess that is unlikely to convey any useful information.

Figure 2. Untriggered, repeated acquisitions.

The data are easier to understand if the acquisition of the “frames” of Fig. 2 is based on a trigger point. The trigger point corresponds to a particular feature on the waveform which defines the point at which the “frame” of data displayed to the screen begins. Fig. 3 shows a set of “frames” which are based on trigger points. Successive display of these frames results in what appears to be a single “picture” of the waveform.

Figure 3. Triggered, repeated acquisitions.

A typical trigger point is defined by a voltage level and a slope condition. The slope condition defines whether the trigger point is set at a point where the voltage level is increasing (called a rising edge) or where the voltage level is decreasing (called a falling edge). The trigger points of Fig. 3 are based on a rising edge trigger at the indicated voltage level.


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