1336 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1956 



value may be considerably higher than the original average noise level. 

 As is the case with the noise figure in the M1789, the growing noise 

 current has been found to be very sensitive to magnetic field at the 

 cathode. By allowing sufficient field to link the cathode, the growing 

 noise current can be eliminated leaving the normal noise current standing 

 wave pattern on the beam. This phenomenon is not peculiar to the 

 M1789 gun. It has been observed by various workers at MIT^ and else- 

 where on other guns producing beams with comparable current densities. 

 A satisfactory explanation for it has not, at the time of this writing, been 

 arrived at. It seems safe to say, however, that the growing noise current 

 on the beam is the source of the high noise figures obtained in the M1789 

 when the cathode is completely shielded from the magnetic field. 



4.5 Inter modulation 



It has been found that certain intermodulation effects in the Ml 789 

 can be predicted from a knowledge of the compression and AM-to-PM 

 conversion. Alternatively, these effects can be used to determine com- 

 pression and AM-to-PM conversion. The procedure to be described has 

 the advantage of being simple to implement as compared with the phase 

 bridge arrangement of Fig. 24. 



UNIT VECTOR--. .^ — ■ 

 ROTATING AT / 



ANGULAR / 



VELOCITY 1^ 



2 7rAf \ 



UNIT 

 AMPLITUDE 



-Af — >| 



FREQUENCY 



(a) 



(b) 



4^ 4* 



4^ 



AM 

 VECTORS 



PM 

 VECTORS 



(C) 



Fig. 41 



(a) Spectrum of input signal to amplifier. 



(b) Vector diagram of two input signals and the resultant signal (R) in a frame 

 of reference rotating at an angular velocity 2irAf . Dotted line is the locus of the re- 

 sultant signal. 



(c) The rotating vector of the proceeding diagram can be broken down into a set 

 of two vectors representing amplitude modulation and a set of two vectors rep- 

 resenting frequency or phase modulation. 



