Multiple and Sub-Multiple Linear Sweep Frequencies

Author: J.B. Hoag

Next, let us suppose that a linear sweep is applied to the horizontal deflecting plates and that the total time for one trip across the screen is greater than that required for one cycle of the a.c. on the vertical plates. If it is exactly twice the 1/60 of a second of the a.c. applied to the vertical plates, two complete wave-forms will be seen in a stationary pattern on the screen. If, however, the spot moves across the screen in 1/120 of a second, the pattern will appear as in Fig. 22 M.

Fig. 22 M. The sweep time is one-half the period of the a.c. under study

If the sweep-frequency is still further increased so that the time represented from the left to the right of the screen is only one-quarter of the a.c. period, then the pattern will appear as in Fig. 22 N.

Fig. 22 N. The sweep time is one-quarter of the period of the a.c.

In case the sweep and alternating potentials have a frequency ratio of 3 to 2, the pattern will appear as in Fig. 22 0.

Fig. 22 O. The ratio of the sweep to a.c. frequency is 3 to 2

From this discussion it should be obvious that, by observation of the stationary patterns on the screen, it is possible to calibrate the sweep-frequency against a standard alternating frequency such as the 60-cycle line. Actually, this calibration proves to be satisfactory only when the ratio of the two frequencies is not too great. After the calibration of the sweep-frequency, the process may be reversed to determine an unknown frequency applied to the vertical deflecting plates. However, it is difficult to build sweep-circuits in which the frequency is always the same for a given adjustment of the circuit constants. In the case of the circuit of Fig. 22 H, the striking-potential of the thyra-tron varies slightly with the temperature of the tube.