A Second Type of Linear Sweep-Circuit

Author: J.B. Hoag

An entirely different type of sweep-circuit is shown in Fig. 22 I.

Fig. 22 I. A sweep circuit operating on the principle of blocking-action

Any one of a variety of oscillators could be used as well as the one shown. Oscillations of very high frequency, say 2 MHz, are produced by the usual processes. As the oscillations start, condenser C is charged (with negative on the grid side) by grid current flowing through resistor R. This C-bias continually increases because R is chosen of very high value. It is so high that it prohibits C from discharging during one, or even many, of the high-frequency oscillations. Finally, the C-bias becomes so great that the plate current, and hence the oscillations, are completely shut off. This is known as blocking action.

When the oscillations have ceased, the grid is no longer driven positive, there is no longer any current to charge condenser C, and it slowly discharges through R. When the voltage of the grid has risen to a sufficiently small negative value, a " spurt" of high-frequency oscillations takes place, the tube is again blocked, and the slow discharge of the condenser through the resistance is repeated. In practice, resistor R is replaced by a current-limiting tube of the type described in connection with Fig. 22 G. This causes the discharge of the condenser, C, Fig. 22 I, to occur at a constant rate. Hence the voltage drop across the condenser, applied to the deflecting plates of a cathode-ray tube, causes a linear sweep. The frequency of this sweep-circuit depends largely on the time for C to discharge through R. By changing the grid control of the constant-current tube, it becomes easy to change the sweep-frequency. With this circuit, higher frequencies can be attained than with those containing gas-filled tubes, because the de-ionizing factor is no longer present. With an oscillator operating at 5 MHz, essentially linear sweeps have been attained up to one-half million cycles per second. Special precautions are needed with this circuit to keep the oscillator's high frequency, and its harmonics, out of adjacent r.f. receiving apparatus.