Display of Electrical signals by Oscilloscope:
The are two basic ways of display of Electrical signals by Oscilloscope—the X-Y mode and the triggered-sweep mode. The X-Y mode displays the graph of the variation of two external signals—one versus the other. The triggered-sweep mode displays the variation of only one external signal as it varies with time. The triggered-sweep mode is by far the most commonly used of the two modes.
When the oscilloscope is being used in the X-Y mode, an external signal applied to the horizontal deflection plates controls the deflection of the electron beam to the right and left (X-direction). Simultaneously an external signal applied to the vertical deflection plates moves the electron beam up and down (Y-direction). If both applied voltage signals change slowly with time, the display on the screen will be a slowly moving spot of light. If both signals are periodic, the moving spot will trace the same path on the screen over and over again.
If the frequencies of the periodic electrical signals by oscilloscope are high enough, the repeating trace will appear to be a steady pattern painted by solid lines of light on the screen. Frequencies of 50 Hz or more will cause the trace to appear as a non-flickering solid line due to persistence of vision and the persistence of the phosphor.
In the triggered-sweep mode, the time base generates a sweep waveform at accurately timed intervals. The sweep waveform (Fig. 51.16) causes the electron beam to be swept horizontally across the screen at a uniform rate. Simultaneously, the vertical input signal deflects the electron beam up and down. Because the horizontal position of the beam is proportional to time, the resulting display is an image of the vertical input signal as it varies with time.
In order for this image to be a steady pattern on the screen, the sweep waveform must be started at exactly the same point on the vertical input waveform as the previous sweep waveform was started. In addition, for the pattern to appear to be a solid line, the sweep waveform must be repeated at least 50 times per second.