Radio Broadcast (Nov. 1925-Apr 1926)

352 RADIO BROADCAST JANUARY, 1926 Represents Tube Capacity C Is a Sharply Tuned Circuit B Is Average in Selectivity A Is Broad FREQUENCY FIG. 2 The circuit above depends, for selectivity, upon the resistance of the tuned portion, represented by the coil L and the variable condenser shunting it. If the resistance is high, the tuning response will be broad as in the curve A. As the resistance is decreased the selectivity becomes sharper, as depicted on curves B, and C. Also, as the resistance of the circuit is decreased, more current will flow in the grid and plate circuits and oscillations will be produced due to the coupling between these two circuits by the inherent capacity of the tube in question. The well known neutrodyne system is shown in Fig. 3. Another system worthy of comparison is that developed by Walter Van B. Roberts. It is shown in Fig. 4. Here, any potential set up in the plate coil P is set up also in the plate coil N, but in opposite relation to that flowing in P. Then through the capacity C, which balances out the tube capacity, this potential is applied to the grid of the tube, effectively preventing any possibility of oscillation because it is equal in potential, and opposite in phase, to that oscillation, but keep the circuit as sharply tuned to that particular frequency as before, then a distinct and worthwhile advantage would result. The various systems for stopping this oscillation are known as neutralization methods. The word itself carries a world of meaning, and implies an equalization or neutralization of the capacity of the tube which is the coupling agent producing the oscillation. In effect, neutralization is the setting up of an equal and opposing voltage which, due to its opposition, prevents unwanted oscillations from taking place in the grid-plate circuit of the tube Neutralizing Capacity FIG. 4 The Roberts system of neutralization. The coil in series with the neutralizing capacity is connected counter to the plate coil, and produces an effect on the grid, through the neutralizing capacity, equal and opposite to that produced by the plate coil acting through the grid-plate capacity of the tube FIG. 3 Professor L. A. Hazeltine is responsible for the neutralization, or balancing-out system shown above. A goodly portion of all the manufactured receivers sold during the past few years incorporated this system using licenses granted under the neutrodyne patents which might be fed back to the grid of the tube by the coil P through the tube capacity. Yet, even this method is not possible of adjustment independent of frequency. Absorption systems, still another way, never were regarded as truly a satisfactory neutralization method, and were more correctly termed "losser" systems. OSCILLATION CONTROL METHODS ONE of the most simple and common methods is to add sufficient resistance to the grid or plate circuit to prevent the possibility of oscillation. Considering the efforts that have been made to reduce resistance in coils and condensers, and the value attributed to such efforts, the fallacy of again deliberately introducing such losses into a circuit is evident. Were it not for the popular delusions, how much more simple it would be to use high-loss coils and condensers in the first place. Eddy current losses result from placing condensers within the field 1st r.f. 2nd r.f. 3rd R.F , 1 1 ST A.F 2 NO A.F FIG. 5 In the "Counterphase" circuit shown here, the tendency of the radio frequency stages to oscillate is prevented by the separate circuits comprising the inductances M-R and the condensers Cn. It is necessary to adjust the condensers Cn, the variable part of this circuit to suit the tube employed. On the high frequency end of the tuning scale, the tendency of a circuit to oscillate is greater than at the other end of the scale, the lower frequencies, so a panel adjustment is provided in the dual resistance control to compensate for these changes, thereby obtaining maximum efficiency on all frequencies within the tuning range