GB815411A - Improvements in or relating to saw tooth wave generators - Google Patents

Abstract

815,411. Sawtooth wave-generating circuits. MARCONI'S WIRELESS TELEGRAPH CO. Ltd. Feb. 3,1958 [April 23, 1957], No. 12911/57. Class 40 (6). A sawtooth current wave generator comprises a first inductance connected in series with a capacitance across a normally open switch which when closed substantially discharges the capacitor, a charging circuit for the capacitor comprising a second inductance in series with a source of current, and a circuit for limiting the voltage across the first inductance to a predetermined value when the switch is open, the arrangement being such that when the switch is periodically closed. sawtooth current waves of the frequency of the switch closure are produced in the first inductance. Fig. 2 shows an inductor 2, which may be the deflection coils of a cathode-ray tube, connected in series with a capacitor 1 across a normally non-conductive transistor. At the end of the current sweep in the inductor 2 a pulse is applied to the input terminals 6 to cause the transistor to conduct and the capacitor 1 to discharge resonantly through the inductor. When the capacitor is substantially discharged (i.e. at the end of a half-cycle of oscillation) the pulse is terminated and the transistor becomes non-conductive. The current through the inductor then begins to decrease and the voltage across it reverses to a value which is determined by a self-biased limiter circuit 9, 10, 11. The constant voltage which accordingly exists across inductor 2 causes the current in the inductor to decrease linearly, thus producing the scan. During this period the capacitor 1 is recharged through an inductor 8, the circuit 8, 1 being resonant at half the sawtooth frequency so that the capacitor charges to twice the voltage of the supply. The position of the condenser 1 and inductor 2 may be interchanged (Fig. 1, not shown) and the left-hand terminal of inductor 8 may be connected to the lower end of capacitor 10 so that the capacitor is in series with the supply source 7 (Fig. 3, not shown). In Fig. 5 the limiting circuit 9, 10 is coupled to the coil 2 through a transformer having a ratio such that the desired discharge current of the capacitor is equal to that taken by the circuit so that the resistor 11 may be omitted. The positions of inductor 8 and the transformer limiting winding may be interchanged (Fig. 4, not shown) or the turns ratio of the transformer may be such that the limiting circuit may be connected across the battery instead of the condenser so that the current through the limiting circuit is returned to the battery (Fig. 6, not shown). The deflection coil is coupled to a further winding 22 of the transformer and the circuit of this winding may include a D.C. supply for positioning the trace on the tube. It is stated that the transistors may be replaced by valves.