An Approximately Linear Sweep-Circuit

Author: J.B. Hoag

For use with an oscilloscope, we desire a curve of the saw-toothed shape shown in Fig. 22 F, rather than that developed by the simple relaxation oscillator just described.

Fig. 22 F. A saw-toothed waveform which will give a linear sweep

Voltages of the type shown in Fig. 22 F, when applied to the horizontal deflecting plates of the cathode-ray tube, will produce a linear sweep ¹ of the electron beam across the screen. This means that as the voltage slowly rises at constant rate, the beam is deflected at constant velocity from the left side of the screen to the right. In other words, the time it takes in traveling a unit distance in the X direction at one point will be exactly equal to the time required to travel an equal distance at some point farther along the axis. Having traveled to the extreme right end of its path, the electron beam is suddenly flipped back to its starting position on the left of the screen just at the moment the voltage across the deflecting plate is suddenly reduced to zero by the sudden discharge of the condenser. In general, the fly-back time must be made as short as possible in comparison with the sweep-time, i.e., in Fig. 22 F, t₂ must be only a small fraction to t₁.

The voltages developed by the simple relaxation oscillator described above, and shown in Figs. 22 C, D, and E, would not give a uniform or linear sweep. The electron beam would be deflected rapidly at first, then at a slower and slower rate toward the other side of the screen, as the condenser approached its full charge. This non-linear feature is undesirable because it distorts the form of a wave applied to the Y deflecting plates. Hence we proceed to a description of certain modifications of the sweep-circuit which will cause it to develop a linear sweep.

If the voltage of battery E in Fig. 22 A is made very great in comparison with the striking voltage of the glow-tube, then the discharge of the condenser will occur very early along the curved path of Fig. 22 C, such as at point P. Since only a short portion of the curve is used, the voltage rise across the condenser takes place at almost a constant rate. There are, however, better methods of insuring the linearity of the sweep, which have the added advantage of ease of change in the time and amplitude of sweep.

1

When a certain quantity Y is plotted against another quantity X to yield a straight line on the graph, Y is said to vary linearly with X. This means that if X is changed by a certain amount, resulting in a certain change in Y, then twice the change in X will cause double the change in Y. In other words, Y is directly proportional to X. The graph of Y vs. X is a straight line; hence the term " linear."