Synchronization

Author: J.B. Hoag

Let us suppose that a linear sweep-circuit has been applied to the horizontal deflecting plates of a cathode-ray tube in such a manner that the total sweep requires approximately 1/60 of a second. Suppose, also, that a 60-cycle alternating voltage is applied to the vertical deflecting plates. Then, as the beam is moved horizontally, it is simultaneously deflected vertically in proportion to the amplitude of the alternating voltage, and traces a typical sinusoidal wave-form on the screen. If the horizontal sweep-time (plus the negligible fly-back time) is exactly 1/60 of a second, then the spot of light will retrace its path along the sinusoidal curve every cycle and a stationary pattern will be produced. This is a condition of great advantage for both visual and photographic study of the individual parts of the cycle. Also, in the case of the double sweep-circuit, the necessity for exact synchronization of the horizontal and vertical sweeps has been pointed out. Without special devices it is difficult to adjust the sweep time to an exact multiple of the signal frequency. Then the pattern appears to move across the screen, either from left to right, or vice versa. In order to stabilize a moving wave-form, a synchronizing device is added to the sweep-circuit. As shown in Fig. 22 H, this is accomplished by the injection of a small fraction of the signal's voltage into the grid circuit of the thyratron tube, in series with the fixed C-bias. Then, as the tube is near its point of striking, a small voltage from the " sync " circuit is added and trips the tube just at the peak of the signal voltage. The magnitude of this " lock-in " voltage need be only a small fraction of the fixed C-bias. Much more elaborate circuits have been devised for synchronization of the horizontal and vertical sweeps used in television. They are necessary for the proper starting time of each frame in synchronism with the starting time of the same frame at the transmitter.