1322 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1956 



-20 



-15 



5 10 



INPUT IN DBM 



Fig. 27 — Output power as a function of input power at various beam currents. 

 Tliese curves were all taken with the helix voltage adjusted to give the maximum 

 gain at low signal levels. At low beam currents (<20 ma) there is insufficient gain 

 between the attenuator and the output so that at these currents the attenuator 

 section is limiting the power output. This accounts for some of the difference in 

 shape of the curves near maximum output. 



tor and introducing small changes in electrode voltages. The modulation 

 sensitivity of the helix is about two degrees per volt and that of the 

 accelerator about 0.1 degree per volt with the TWT operating under 

 nominal conditions. 



Significance of AM-to-PM Conversion 



Let us return briefly to a discussion of some consequences of AM-to- 

 PM conversion. As an example, we will consider the case of a low -index 

 FM signal. Assume the frequency deviation is ±5 mc peak to peak. 

 This gives a phase deviation of ±0.5 radian for a 10 mc modulating 

 signal. These values are typical of what might be found in a radio relay 

 system. Let us also assume that there is a residual amplitude modulation 

 of one db (about 13 per cent) in this signal and suppose further that the 

 signal is amplified by a TWT having a value of AM-to-PM conversion of 

 10 degrees per db. The phase modulation thus created in the TWT can 

 either add to or subtract from that of the original FM signal, thus chang- 

 ing its modulation index. At low modulation signal frequencies the phase 

 deviation of the FM signal will be large compared to that of the PM 

 interference and the interference will be of little consequence. At high 

 modulation signal frequencies the phase deviation of the original FM 

 and of the interfering PM signals will be comparable and the interference 



