Radio broadcast .. (1922-30)

Oscillations and the Vacuum Tube 515 FIG. 38 and which is responsible for the variations of grid potential. 61. ANALOGY FOR AN OSCILLATOR CIRCUIT A CLOCK is a good analogy for a vacuum tube oscillator circuit. The swinging of the pendulum corresponds to the alternating current in the oscillation circuit, and the natural frequency of the pendulum is pretty close to the frequency with which it will swing when the clock is running. Corresponding to the alternating current in the oscillation circuit operating the grid potential, we have the swinging of the pendulum operating some sort of escapement mechanism that controls the action of a spring or the falling of weights. And just as the plate current controlled by grid potential acts to keep the oscillating current going, so does the intermittent uncoiling of the spring or falling of the weigHts released by the escapement mechanism act to keep the pendulum swinging. 62. AN ILLUSTRATIVE OSCILLATION CIRCUIT WE WI LL build up an oscillator circuit by following our list of conditions one by one. The first condition is satisfied by the simple oscillation circuit of Fig. 36. The second condition is satisfied by Fig. 37 as the transformer type coupling between the coil L and the grid coil makes alternating current in L produce an alternating potential difference between grid and filament. The last condition is satisfied in Fig. 38 for the plate coil is assumed to be wound in such a direction that the plate current variations reinforce the current in the oscillation circuit. The flow of power is indicated by the arrows in Fig. 38. A small amount flows through the transformer coupling at the left side of the diagram to the grid and is the power input to the tube. Then if the lube amplifies power an increased amount flows through the transformer coupling at the right, back into the oscillation circuit. The difference between these two amounts of power just takes care of the rate at which energy is being dissipated in the oscillation circuit. This is the Meissner circuit. 63. OTHER OSCILLATION CIRCUITS FIG. 39 shows the Hartley circuit which is the same as the Meissner except that the couplings are "direct," that is, transformer type coupling has been replaced by "auto-transformer" type coupling. In Fig. 40 we have what is usually meant by the term "feed-back" circuit. (All oscillator or regenerative circuits are really feed-back circuits of some sort). The coupling to the grid is direct while the plate circuit is coupled to the oscillation circuit by mutual inductance or transformer coupling. Fig. 41 is exactly the reverse of the last arrangement. Fig. 42 is the Colpitts circuit, obtained by interchanging all capacities and inductances in the Hartley circuit. The only difference is that the capacity has to be split up into two condensers in order to connect the filament in the middle of it, and a direct current path has to be provided for electrons to get from plate back to filament. The alternating current however is kept out of this path by the high reactance of the choke coil. Fig. 43. This is the circuit used in receiving sets employing "plate-variometer" regeneration. Sometimes the tuning condenser is omitted. In any case, the circuit between the grid and filament functions as a simple inductance, and as there is no mutual inductance between the plate variometer and the grid coil, this circuit is really a Hartley circuit with no mutual inductance, the necessary capacity FIG. 39