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Show GREAT DISCOVERY OF EDWIN H.ARMSTRONG Arrangement by Which Local Plate Current Is Fed Back to Grid Circuit. An ordinary three-electrode vacuum vacu-um tube used as a detector in a circuit cir-cuit where the input is connected to the grid and the output taken from the plate circuit will give amplification amplifica-tion of from 5 to 15. Adding to this amplifying action the stability of adjustment of the tube gave the tube preference over the crystal detector. coming slgn:i I until a point is reached where the energy fed hade from the plate circuit is just eipial to the losses in tile secondary circuit. An increase in the coupling between be-tween the two coils M and N will cause the tube to oscillate. By changing chang-ing the tuning of the secondary circuit, cir-cuit, the frequency of the tube oscillations oscil-lations will be slightly dlfereut from the frequency of the incoming oscillations oscil-lations due to the revived signals, and the beat note will be heard in the phones. The pitch of the beat note will be the arithmetic difference between the frequency of the incoming incom-ing oscillations and the oscillations furnished by the oscillating detector tube. The human ear is most sensitive to a note corresponding to 1,000 vibrations vibra-tions per second or electrically to a note produced by 500 cycles. Suppose the received signal was on a 200-meter wave length of 1,500,000 cycles. By adjusting ttie circuits so that the tube would oscillate at a frequency of 1,- T fjji p 1N I T F;9.H However, It was not until Edwin H. Armstrong mad! his great discovery of a circuit p.rrangement which fed back a portion of the large local plate current Into the circuit through which the incoming signals arrived, thereby greatly amplifying the strength of the nignal, that the vacuum tube reached its now recognized position of superiority superi-ority as a detector of radio signals. The circuit arrangement by which eneixy from the plate circuit of a thre -electrode vacuum tube is fed bae't to the grid circuit is known by a Aifferent name, though, on account of the different names used and the different circuit arrangements used to recompllsh the same results, amateurs re very apt to become confused. Some of the names given to the circuits which feed energy from the plate circuit cir-cuit to the grid circuit are "Feed Back," "Regenerative," "Tickler," 'Tuned Plate" and "Ultra-Audion." In Fig. F Is shown a three-electrode vacuum tube with an alternator alterna-tor furnishing an alternating difference differ-ence of potential to the grid circuit at T-T. The plate circuit of the tube Is connected to a circuit containing an Inductance L and a capacity C which together form a tuned circuit between the points X and Y. By adjusting ad-justing the value of the variable con- ? 5- c l I z. 1 I nr I denser (', the natural period of the ck-qtitt between X and Y can be made t6 have the same frequency electri-x-ftlty as the alternating current sup-piled sup-piled to the grid at T-T by the alternator. alter-nator. When this condition is reached the tuned circuit between the points X and Y becomes an infinite Inductance Induct-ance in the external plate circuit. The potential variations between X and Y are now a maximum and are exact amplified reproductions of the potential poten-tial variations applied to the grid circuit cir-cuit by the alternator. Going back to the simple type of standard loose coupler hookup for a vacuum tube, we will have a circuit arrangement as shown In Fig. G. Suppose Sup-pose that the principle of the circuit of Fig. F be applied to the circuit arrangement ar-rangement of Kig. G. The result will he Fig. H. In Fig. II the Inductance M has been placed in the secondary circuit and the inductance N lias been placed In the plate circuit. When the coils M and N. are widely separated Fig. II is essentially the same as Fig. G. An incoming radio signal Impresses a small difference of potential on the grid of the tube in Fig. II wtleh iu turn liberates a large amount of energy in the plate circuit. As the coll N is brought nearer the oil M some of this plate circuit energy ener-gy is transferred to the grid circuit. 'Khis reinforcement, electrically, of the incoming signal by energy transferred lYom the plate circuit to the grid circuit cir-cuit by the coupling between the tvo coils M and N, causes an increase in the energy liberal ed in llie plate circuit. cir-cuit. Coupling the two coils M and N closer and closer will cause greater great-er and greater amplification of the in- 500,000 or 1,499,500, the best noto would be 500 cycle pitch or a 1,000 vibration note. This beat method of reception is used to receive undamped or continuous contin-uous wave signals and is called the "heterodyne" method. When the same tube is used to perform the functions of a detector and an oscillator, the method is termed the "antodyne" method. |