116 BELL SYSTEM TECHNICAL JOURNAL 



tarding field type to designate the arrangement of the electrode po- 

 tentials. The relative ease with which such oscillations can be ob- 

 tained at frequencies above 300 megacycles by the use of conventional 

 tubes, and the widespread interest in this frequency range for com- 

 munication purposes, have led to the appearance of a large number of 

 papers on the experimental and theoretical aspects of such operation. 

 With the positive grid oscillator there are found to exist preferred 

 frequencies of operation fixed by the electrode spacings and the applied 

 electrode potentials. For the lowest preferred frequency mode of 

 oscillation the relationship is such that the period of one complete 

 oscillation is approximately equal to the total transit time of an 

 electron which fails to strike the grid on its first transit, is retarded and 

 finally turned back by the plate potential, and again missing the grid, 

 returns to the cathode. Under these conditions the relationship, 



Eg 



—5- = constant 



is found to hold approximately, where Eg is the applied grid potential, 

 n is the frequency, and the constant is a function of the tube geometry. 

 Other high-frequency modes of oscillation can be obtained. One of 

 these is particularly easy to excite if the grid of the tube is in the form 

 of a simple helix. The important role played by the electron transit 

 time in determining the frequency of the positive grid oscillator con- 

 trasts sharply with the minor role it plays in determining the frequency 

 of the negative grid oscillator. 



For maximum output it is necessary to adjust the tuning of the 

 external circuit to correspond to the preferred frequency fixed by the 

 applied electrode potentials. The relative dependence of the fre- 

 quency upon the circuit tuning and on the applied electrode potentials 

 varies greatly with the design of the tube. In any case the improper 

 adjustment of either parameter results in a marked decrease in output. 

 In general it appears that the better the tube design and the higher 

 the operating efficiency the greater will be the dependence of fre- 

 quency upon circuit tuning and the less will be its dependence upon 

 the applied electrode potentials. 



The most efficient operation of the positive grid oscillator is ob- 

 tained when the space current is limited by the cathode emission, as 

 contrasted with the most efficient operation of the negative grid 

 oscillator when the current is limited by space charge. Not only 

 must the space current be emission limited but it must have a fairly 

 critical value. This makes it necessary to adjust the cathode tem- 

 I)erature critically. Since the cathode emission characteristics are apt 



