To view a sign on the screen of the oscilloscope, it is necessary to deflect the CRT beam through a horizontal scan signal, so that any vertical signal variation can be observed. With the electron beam in a vertical distance kept constant, the voltage horizontal, from negative to zero and then positive, makes the beam move from the left side of the tube to the center and to the right. The exposure is a straight above the vertical center, corresponding to the applied voltage.
The voltage sweep is obtained from a waveform remains unlimited. This is necessary for observation of a signal of long duration. A limited scan time would not allow the visualization of the input signal on the screen. Repeating the scan, the exposure is generated over and over per second, producing a continuous exposure of the waveform to be analyzed. If a scan rate is too slow (set by controls of the oscilloscope time scale), the actual path of the beam across the face of the tube can be observed.
Applying just a vertical sinusoidal signal inputs (no horizontal sweep), resulting in a straight vertical line. If the sweep speed (frequency sinusoidal signal) is reduced, you can see the beam of electrons moving up and down along the straight line.
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