Interchannel Interference Due to 

 Klystron Pulling 



By H. E. CURTIS and S. O. RICE 



(Manuscript received August 26, 1956) 



A source of interchannel interference in certain multichannel FM systems 

 is the so-called ^'frequency pulling effect.^' This effect, which occurs in 

 systems using a klystron oscillator, is produced hy an impedance mis- 

 match between the antenna and the transmission line feeding it. In this 

 paper expressions are developed for the magyiilude of the interference when 

 the speech load is simidated hy random noise. 



INTRODUCTION 



In a recent paper^ the problem of interchannel interference produced 

 by echoes in an FM system was treated. The mathematical development 

 in that paper can be used to calculate the distortion that arises when a 

 Klystron oscillator is connected to an antenna through a transmission 

 line of appreciable length. 



In the system we study, the composite signal wave (the "baseband 

 signal") from a group of carrier telephone channels in frequency division 

 multiplex is applied to the repeller of a Klystron and thereby modulates 

 the frequency of the Klystron output wave. If the antenna does not 

 match the transmission line perfectly, the output frequency is altered 

 slightly by an amount proportional to the mismatch. 



This effect, known as "pulling," results in intermodulation between 

 the individual telephone channels. In this study, the composite signal 

 will be simulated by a random noise signal of appropriate bandwidth 

 and power. It is assumed that some particular message channel is idle; 

 i.e., there is no noise energy in the corresponding frequency band (which 

 is relatively narrow in comparison with the bandwidth of the composite 

 signal). If the system were perfect, no power would be received in this 

 idle channel at the output of the FM detector. In the following work, 

 the intermodulation noise falling into this channel because of the "pull- 

 ing effect" will be computed. This leads to "Lewin's integral," so called, 

 which is tabulated herein. 



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