Positive-Grid Oscillators

Author: J.B. Hoag

A type of oscillator was discovered in 1920 by Barkhausen and Kurz, which is variously known by the name of B-K, retarding field, or positive-grid oscillator. It differs from the usual oscillator in that the grid, instead of being negative, is positive; while the plate, instead of being positive, is at the potential of the filament or somewhat negative thereto. A majority of the electrons from the filament are accelerated toward the grid by its positive potential. They pass through its meshes into the retarding field between the grid and the plate and stop just before reaching the plate. Reversing their direction, they are accelerated back toward the grid. As before, a majority pass through the meshes and enter the retarding field between the filament and grid. Again they stop, just in front of the filament, and, replenished by a few new electrons from the filament, return toward the grid. Thus the cloud of electrons oscillates back and forth between the filament and the plate, picking up a few new electrons from the filament and losing a few to the grid each cycle.

An explanation of the processes whereby the oscillating electrons extract energy from the batteries and deliver it to associated high frequency circuits is not at all easy to give. The exact mathematical treatment is very complicated and does not assist greatly in the practical operation of the oscillators. Suffice it to say that, due to the pendulations of the electrons back and forth about the grid, high frequency oscillations are produced in a tuned circuit connected across the grid and the plate. A circuit of this type is shown in Fig. 38 A.

Fig. 38 A. A Barkhausen-Kurz oscillator circuit

The period of the oscillations which are generated is equal to the time taken by the electrons for one complete excursion inside the tube. Because the electrons travel very fast, this time is exceedingly small; hence the frequency of the radio waves is very high. The resonant circuits should, therefore, be of the linear-circuit type, as shown in Fig. 38 A. The intensity of the oscillations can be increased by using a double-ended tube, as in Fig. 38 B, where there are two wires from the plate and two from the grid. These are taken out on opposite sides of the glass envelope.

Fig. 38 B. Double tuning of a B-K oscillator

Peculiar non-harmonic oscillations are sometimes generated in B-K tubes. They have been found to be due to parasitic oscillations whose frequencies correspond to the natural electrical periods of the electrode structures inside the tubes, such as the filament lead wires and so on. These are, generally, of comparatively long wave-lengths.

The wave-lengths radiated from these oscillators are very short, ranging from a few meters down to approximately 10 cms. If the grid is equidistant from the filament and plate, and if the plate is at the same potential as the filament, then the following simple equation can be used to predict the wave-length λ (cms.) of the oscillations:

where D is the inside diameter of the cylindrical plate, in cms., and E_g is the potential of the grid, in volts.

Referring now to Fig. 38 A, let us measure the wave-length produced as the condenser across the two parallel wires is moved outward from the tube.

Fig. 38 C. Effect of moving condenser C of Fig. 38 A along the parallel wires

The curve in Fig. 38 C shows that for a short distance, marked B-K, the wave-length generated is independent of the external tuning circuit. Suddenly, however, the wave-length drops to a smaller value, after which it rises along a line, marked G-M, to its former value. In the G-M condition, the external circuit obviously influences the oscillations inside the tube so that they take place differently than they did by the pure B-K method. This operating condition was first discovered in 1922 by Gill and Morell (hence the initials G-M). It is found in practice that the G-M oscillations are not only shorter than the B-K type but also are very much more intense. Their wave-length, however, is subject to changes in the external system, whereas the B-K oscillations are determined exclusively by the voltages upon the given tube.