Scaling Circuits

Author: J.B. Hoag

It is sometimes desired to count a succession of impulses occurring more rapidly than can be followed by even the most delicately constructed mechanical counter. Then one may use a modification of the inverter circuit so that the mechanical counter is operated by every other input impulse. Such circuits are consequently called scale-of-two circuits. Furthermore, by cascading a number of scale-of-two circuits, scale-of-four, -eight, -sixteen, and -thirty-two counters have been built. In the last case the mechanical counter's reading is to be multiplied by thirty-two to give the actual input number of impulses.

Fig. 30 K. A scale-of-two counter. (From E. & N. P.)

Referring now to Fig. 30 K, at the start the gas-filled triode, 1, is conducting while 2 is shut off. The plate current of 1, flowing through r₁, has produced an ir drop which charged C positively on the bottom plate and negatively on the top plate. An impulse at the input, which makes A positive, does not alter the plate current of 1, but does make the grid of 2 less negative, so that, if 2 were nearly ready to strike before the pulse came in, it will be turned on by the impulse. When 2 is turned on, the mechanical counter M is operated and an ir drop is produced across r₂, with positive at the top and negative at the bottom, which is the reverse of that across r₁. For a moment then, the drop across r₂ drives a current through r₁ into C, producing a total voltage across r₁ which is so great that, subtracted from the battery voltage, the plate voltage across 1 falls to zero or even to a negative value. Thus, by this indirect action, the process of starting tube 2 and reversing the polarity of condenser C, automatically shuts off tube 1.

A second impulse, positive on A, starts tube 1 and, in so doing, shuts off tube 2, resetting meter M. Thus the meter counts every other impulse.

When several scale-of-two circuits are connected in cascade and the counting rate becomes very high, the necessary de-ionization time of the first stage gas-filled tubes limits the frequency response of the unit. In addition, gas-filled tubes are notably subject to temperature variations. Hence it has been considered desirable to devise a scale-of-two circuit, using vacuum rather than gas-filled tubes. A circuit of this type¹ is described in the next section.

1	See also Review of Scientific Instruments, 8, 414 (1937).