1318 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1956 



decrease in output power). We will present some additional data on this 

 effect in Section 4.3. 



Distortion of the Modulation Envelope 



The curves of Figs. 23(a) and (b) tell what happens when a single 

 frequency carrier signal is passed through the TWT. In addition we 

 would like to know the effect on modulation which may be present on 

 the signal. In particular, it is desirable to know the compression of the 

 envelope of an AM signal and the amount of phase modulation generated 

 in the output signal as a result of amplitude modulation of the input 

 signal, (an effect commonly known as A]\I-to-PM conversion). As a 

 measure of compression of an AM signal the quantity per cent com- 

 pression will be used. This is defined as 



% Compression 



AV,/V,_ 



100 



where Vo is the voltage of the output wave, Vi is the voltage of the 

 input wave, and AYo is the change in output voltage for a small change 

 AVi in the input voltage. When AF/F is small it can be expressed in db 

 as 8.68 AF/F = AF/F in db. From this it follows that 



% Compression 



1 =- > ni do 



APi 



100 



where APo is the change in output power for a change APi in input power, 

 and the two powers are measured on a db scale. When the per cent 

 compression is zero the TWT is operating as a linear amplifier; when it 

 is 100 per cent the TWT is operating as a limiter. 



From the above expression it may appear that the per cent compres- 

 sion could be determined directly from the slopes of the input-output 

 curves. This would be the case were it not for fading effects. Since there 

 is fading, however, the slope for rapid input level changes is different at 

 high levels from the slope of the static curves. Thus it is necessary to 

 determine compression from the resulting effect on an AM signal. 



The electrical length of a TWT operated in the non-linear region is 1 1 > 

 some extent dependent on the input level. Therefore, an AM signal ap- 

 plied to the input of the TWT will produce phase modulation (PM) of 

 the output signal. This effect ma}^ be of particular concern when a TWT 

 operating at high output levels is used to amplify a low-index FM signal. 

 If such a signal contains residual amplitude modulation, the TWT 

 generates phase modulation with phase deviation proportional to the 

 input amplitude variation. Under certain circumstances this can cause 



