Pulse Generators

Author: J.B. Hoag

The blocking circuit of Fig. 30 D may be used to generate a succession of sharp pulses if the time constant RC of the output circuit is made small (R = 10,000 ohms, C = 0.001 µfd.). The voltage changes across R₂ of this circuit are shown at (a) in Fig. 30 E.

Fig. 30 E. Output waveforms from the circuit of Fig. 30 D as RC is changed

When the plate current of the 6SJ7 tube suddenly drops to zero, the upper end of R₂ rises in potential where it remains while the tube is blocked. The sudden positive pulse on the left plate of C draws electrons to its right-hand plate at time 1 (Fig. 30 E). During the time the tube is blocked, these electrons leak off of C through R, at a rate depending upon the product RC (the time constant). The smaller the RC product, the faster C discharges through R, and vice versa. This flow of current through R gives a potential drop across R of the same shape as that of the discharging current. This is indicated at (b) in Fig. 30 E for the case when RC is small; at (c) when RC is of medium value and at (d) when it is large. If desired, an additional rectifier can be used after the circuit of Fig. 30 D to eliminate the negative or the positive pulses. Also, if short square-topped pulses are desired, the sharp-pointed pulses may be amplified and applied to a " limiter " tube so that their peaks are squared off.

Instead of producing pulses by applying square waves to an RC circuit, as just described, a second method may be used where a sinusoidal e.m.f. is applied to the grid of a tube biased way beyond cutoff, so that only the tips of the applied wave cause plate current to flow. This is illustrated at (a) in Fig. 30 F.

Fig. 30 F. The biasing method of producing sharp pulses

With this biasing method, pulses of from 100- to 500-microseconds duration can be produced, spaced at small or large time intervals T one from the other. A circuit for this purpose is shown in Fig. 30 G, where the tube at the left is biased well beyond cutoff.

Fig. 30 G. Biased-tube method of producing pulses

It will be noted that the resistor R is connected at P instead of at +B, as in the usual amplifier circuit. Then, when a pulse of current flows through R and R₂,the added voltage drop across R₂ assists in sharpening the pulse.

In another circuit, the pulses may be sharpened to only a few microseconds width by applying the output of a full-wave rectified sine wave (through a phase-inverting tube) to a highly biased amplifier tube, as indicated at (b) in Fig. 30 F.