Magnetic Sweep Circuits

A simple television receiver sweep circuit is shown in Fig. 260.

Fig. 260. Television sweep circuit.

Pulse voltage applied to the tetrode grid appears across the transformer primary winding inverted. Current in the lower part of the transformer primary has the shape shown in Fig. 257. This is the current wave shape in the transformer secondary and deflection coil (termed the yoke). An autotransformer extension of the transformer primary winding is used to transform the pulse voltage backswing shown in Fig. 257 to a high value. This voltage is actually much larger than Fig. 257 indicates, and needs only 3:1 step-up to furnish 7 to 14 kv. It is then rectified and applied to the accelerating anode of the oscilloscope. In this way, a separate high-voltage supply is avoided. A damper diode is used to convert the backswing current into useful current during the next sweep interval. Backswing current reaches its negative peak at the end of retrace period τ_r. As indicated by the dotted oscillation at the left of Fig. 261, this current would continue to oscillate for some time if left undamped. With the damper diode circuit, this current never oscillates but instead charges the diode R-C network, which slowly discharges into the yoke. Before damper current reaches zero, the tetrode starts to conduct. Because of winding capacitance, the tetrode tube current is not initially linear. It is offset by exponential decay of damper tube current. Yoke current then proceeds in a linear manner, following the dotted line in the transition from damper to tetrode current, as in Fig. 261.

Fig. 261. Deflection yoke current and voltage.

With the large consumer demand for television receivers, there has been an incentive for improved efficiency of the basic sweep circuit. Partly this has been accomplished by ingenious schemes for improved linearity with lower transformer Q, and partly by using the plate input resistance of the tetrode for the damper diode bias resistance. Thus otherwise wasted power is put to a useful purpose.⁽¹⁾

(1)	See "Television Deflection Circuits," by A. W. Friend, RCA Rev., March, 1947, p. 98; also "Magnetic Deflection Circuits for Cathode-Ray Tubes," by O. H. Schade, RCA Rev., September, 1947, p. 506.