1334 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1956 



for a 2,600-volt helix it is about 5 db lower. The noise figure for 25 gauss 

 at the cathode remains constant, however. 



(2) There appears to be a threshold level of about 15-ma beam current 

 below which the peak does not occur. Between 15 and 25 ma the peak 

 increases. Above 25 ma it is roughly constant in magnitude. 



(3) The peak can be considerably reduced by intercepting some of 

 the edge electrons before they reach the helix region. 



For this discussion it has been necessary to extend the concept of 

 noise figure to the case of non-linear operation of the TWT. Essentially 

 this noise figure is defined by the means we use to determine it. A block 

 diagram of the equipment is shown in Fig. 40. The outputs of a calibrated 

 broad band noise source and a signal oscillator are combined and used 

 for the input to the TWT under test. The noise output from the TWT is 

 passed through a filter tuned about 100 mc away from the signal so as to 

 reject the carrier. It is then detected by a receiver tuned to the filter 

 frequency. The noise figure is measured by turning the noise source off 

 and on, noting the change in receiver output level and calculating the 

 noise figure in the conventional manner. This procedure reduces to an 

 ordinary noise figure measurement in the absence of input signal. 



There are other ways that could be used to measure noise figure of a 

 non-linear amplifier. A method more closely related to the use of the 



50 



45 



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UJ 



o 



^40 



UJ 



cc 



35 



111 

 in 

 o 



z 



30 



-20 -15 -10 -5 5 10 15 



MAGNETIC FLUX DENSITY AT THE CATHODE IN GAUSS 



20 



Fig. 39 — Noise figure as a function of magnetic flux density at the cathode 

 for several values of rf power output. The flux density was varied by using an in- 

 ductive heater through which ac current was passed. The present ]\I1789 uses 19 

 gauss at the cathode, all of which is obtained from the focusing magnet — the 

 heater now being non-inductive. 



