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BELL SYSTEM TECHNICAL JOURNAL 



of this standard. The comparison of detectors can be very readily 

 carried out by means of such a circuit as shown in Fig. 53. This 

 circuit makes use of a grid blocking condenser but could readily be 

 rearranged not to employ it. In use, switches 5 and K are operated 

 together in such manner that the receiver shunt is cut out when the 



Fig. 53 



receiver is connected to the tube of lower detecting power. By proper 

 adjustment of the shunt the two tubes can then be brought to apparent 

 equality, the difference being read from the calibration of the shunt. 



IX. Vacuum Tube Oscillators 



As pointed out in the section on amplifiers, it is easy to design feed- 

 back circuits which will sing, i.e., will generate continuous oscillations. 

 The necessary requirements which an oscillating circuit must meet 

 are two in number and are readily understood. Any small alternating 

 voltage when applied to the input generates a current in the output, 

 and by virtue of the feed-back a portion of this energy is returned to 

 the input. For continuous oscillations, the energy returned must be 

 in phase with the original input supply. Furthermore, letting e 

 represent the initial input voltage, the feed-back coupling must be 

 sufficient to return to the input a voltage greater than e. If it is less 

 than e the circuit will amplify but will not oscillate. 



The circuit requirements necessary for any given tube to return a 

 voltage greater than e may readily be stated in mathematical form, 

 but so far as the practical design of oscillators is concerned, this state- 

 ment has no particular value. The design of circuits is still very largely 

 an empirical matter, and the problem is not so much to make the 

 circuit oscillate as to make it oscillate with the proper frequency, 

 efficiency and output power. These requirements can usually best be 



