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Show VACUUM TUBE USED AS AN OSCILLATOR Transforms Direct Current Energy Ener-gy Into Undamped Alternating Current of Any Frequency. As an oselllutor, a three-electrode vacuum tube hus tlie property of transforming the direct current energy ener-gy supplied to the plate circuit into an undamped alternating current of any desired frequency. By connecting connect-ing the tube functioning as an oscillator oscil-lator In the proper electrical circuit, alternating currents have been obtained ob-tained ranging ' frequency from one-luil one-luil T cycle per second to l."i(),()(K),(XW cycles per second. In fact, u vacuum tube used as an oscillator Is about the only source of undamped alternating current at radio frequency available for the amateur. Undamped or continuous wave telegraphy tel-egraphy and radio telephony are made possible only by use of the vacuum tubes. When the three-electrode vacuum tubo was functioning as an amplifier It was explained just bow operating the grid at a midpoint of the characteristic char-acteristic curve csused the plate current cur-rent to vary synchronously with the grid potential. Not only did the plate the tuned circuit, between the points X and Y, and both are increasing and decreasing together, that Is In phase, the oscillations In the tuned circuit will be sustained or undamped. The amplitude of the oscillations will neither Increase nor decrease. Should tbo potential variations In the plate circuit be greater than the potential operating In the tuned circuit, between the points X and Y, then the oscillations oscil-lations in the tuned circuit will Increase In-crease In amplitude until the tube becomes be-comes saturated. At this point the upper and lower bends of the characteristic charac-teristic curve keep the amplitude of the plate circuit oscillations from further increasing as the amplitude of the grid circuit osi-IlluUons are increased. T 3 i-rJ 1 r I current vary synchronously with the grid potential, but if the plate circuit were tuned the potential variation In the external plate circuit was an amplified reproduction of the change in grid circuit potential. ' In Fig. XXXVI is shown a three-electrode three-electrode vacuum tube with an alternator alter-nator furnishing an alternating difference dif-ference of potential to the grid circuit cir-cuit at T-T. The plate circuit of the tube is connected to a circuit containing con-taining an inductance L und a capacity capac-ity C, which together form a tuned circuit, between the points X and Y. By adjusting the value of the variable condenser, C, the natural period of the tuned circuit between X and Y can be made the same as the frequency fre-quency of the alternator furnishing the alternating grid potential applied to the grid circuit at T-T. When this condition is reached, the tuned circuit between X and Y becomes an infinite impedance in the external plate circuit. cir-cuit. The potential variations be- j tween X and Y are now maximum and j are exact amplified reproductions of j the potential variations applied to the j grid circuit by the alternator. When a three-electrode vacuum i tube is used as a source of undamped j alternating current, it is not excited ! by an alternator connected to the grid j circuit. Instead some of the output i of the plate circuit Is coupled back to the grid circuit, thus making the i tube self-exciting; Fig. XXXVII shows j how this is accomplished. j In place of the alternator that was ! connected across the grid circuit at T-T to externally excite the tube by i applying an alternating current to the grid circuit, there is an inductance 1. j The two coils forming the inductance I I and L are placed close together so j that the potential variations that ex- 1st between X and Y wdll be trans- j . xxxm Terrell Irom ij o me gnu circuit through the medium of the coil 1, which lias replaced the alternator. Any disturbance in the circuits electrically such as the tuning of cither the A or B battery will cause a change between the points X and Y. This change will be transferred from the plate circuit to the grid circuit cir-cuit by the magnetic coupling existing exist-ing between the two coils L and 1. The frequency at which the tube will oscillate will depend on the values of I, and I, while the amplitude of the oscillator will be determined by the degree of coupling that exists between the two coils L and 1. The grid potential variations impressed im-pressed acmss the terminals T-T by the coil 1 have the same effect in the plate circuit as a potential K (when K is the amplification factor of the tube) times us great applied directly to the plate circuit. When the plate circuit potential variations are just equal to the potential variations in |
