Electron-Coupled Oscillators

Author: J.B. Hoag

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.

Fig. 29 A. Electron-coupled oscillator circuits

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 its frequency. We see in this circuit that the oscillator operates very much in the usual manner, but that the load circuit is " electron coupled " rather than direct, capacity, or magnetically coupled to it.

At the right of Fig. 29 A, an untuned output circuit is shown. In this case, the output voltage and power are lower, lacking the " build up " which a tuned circuit gives, but the plate circuit has better isolation from the oscillator, and hence the frequency stability is greater. Ordinary tetrodes and pentodes can be used in these oscillators. With a pentode, the suppressor grid should be grounded and not connected to the cathode, in order to give additional internal shielding of the load circuit from the oscillator circuit.

Granting that the electron-coupled feature just described largely solves the problem of stabilizing frequency against load variations, we still have left the other possible influencing factors for consideration. First in importance among these is the Q of the tank circuit of the oscillator. This must be as high as possible. It can best be obtained by making the L/C ratio of the circuit as low as possible. The resistance of the circuit is to be made low by winding the coil with large wire and by using condensers in which the dielectric losses in the insulation are low. The effective Q of the circuit is increased by using a high value of grid-leak resistance and by using the least possible feedback which will maintain stable oscillations. Frequency stability will be greatest when the ratio of the plate to the screen voltage is about three to one. The plate supply should be free from ripple and may well be voltage-stabilized.

In order to maintain frequency stability, constant strength, and freedom from harmonics, oscillators of the type just described should not be built to deliver large amounts of power. They should be followed by power-amplifying equipment.

Mechanical vibration, which causes frequency instability, can be avoided by care in the constructional details as, for example, the use of short leads, heavy chassis and panel materials.

It will be noticed that in Fig. 29 A, the cathode is above ground potential for r.f. The chance for 60-cycle hum from the filament heating supply is reduced by means of the bypass condensers indicated.