Electron-Coupled Oscillators

One of the outstanding problems in the design of vacuum-tube oscillators is that of keeping the frequency constant despite mechanical vibrations, temperature changes, voltage variations in the supply lines, and changes in the amount of power taken out of the circuit by the load. The effects of variable loading are greatly reduced by the use of electron coupling. Let us imagine that in the Hartley

circuit of Fig. 14 B the actual metal plate is replaced by a grid of wires. The stream of electrons reaching this electrode increases and decreases in number at the frequency of the oscillating circuit in the usual manner. But, with a grid of wires rather than a solid metal plate, a part of this stream of fluctuating electrons passes through the " plate " and can be used in a later portion of the tube. It is as though we had a " virtual" cathode, emitting electrons, whose number varied from maximum to minimum periodically at a high rate. Of course it is impossible to heat and cool an actual cathode at high frequencies. Yet the combination of the Hartley circuit with a grid which acts as the " plate " serves the same purpose. In the circuit on the left of Fig. 29 A, the fluctuating electron current, which has passed through the grids, reaches the solid plate and passes on to the tuned circuit LC. When the periodic fluctuations of the electron stream correspond to the natural frequency of the tuned circuit, they serve to supplant the losses in this resonant or tank circuit and keep it in oscillation. An important feature of this arrangement is that the upper grid is operated at ground r. f. potential, and hence acts as a shield between the output or load circuit coupled to LC and the oscillating circuit itself. In this way, variations in the load current are prevented from reacting upon the oscillator circuit and changing...

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