Figure 9.2: Horizontal sweep voltage
In order to display the time variation of a potential difference which is applied across the vertical deflection plates, the beam can be deflected horizontally by an internally-generated voltage which increases linearly with time. Figure 9.2 indicates the ``sawtooth'' voltage which, when applied to the horizontal deflection plates, sweeps the beam horizontally across the screen at constant velocity, returns the beam to its initial position and repeats the sweep, etc.
Figure 9.2: Horizontal sweep voltage
The process is somewhat similar to writing on paper. To write, one ``wiggles'' the pen up and down while moving the hand horizontally across the paper. Here, the time-varying voltages applied to the vertical plates ``wiggles'' the electron beam up and down while the linear sweep voltage moves the beam horizontally with constant velocity. The beam leaves its written trace on the fluorescent tube screen. Because of the persistence of vision (approximately 0.05 s) and because of the screen fluorescence, we see a plot of the potential as a function of time displayed on the screen.
Figure 9.3: The image on the screen is formed by combining
the constant horizontal sweep with a vertical deflection
proportional to some time-dependent input voltage
which you want to observe.
Usually one observes the repeating pattern of a periodically varying voltage by appropriate adjustment of the sweep time and synchronization of the start of successive sweeps. Oscilloscope controls allow one to determine when the sweep initiates (in other words, when the sweep is ``triggered''). See the description of the horizontal sweep trigger controls in the oscilloscope instructions which follow.