TRAVELING WAVE TUBE FOR 6,000-MC RADIO RELAY 



1323 



can considerably change the net phase deviation of the overall signal. 

 For the example we are considering the frequency responses in Fig. 26 

 show what would be seen at the output FM terminal. Curves are given 

 both for the PM interference adding to and subtracting from the original 

 FM signal. We see that a gain-frequency slope of about 4 db over 10 

 mc is introduced by AM-to-PM conversion. To prevent such an effect, 

 a limiter should be used prior to the TWT in applications of this nature so 

 as to remove the offending AM from the input signal. 



The fact that compression and amplitude-to-phase conversion vary 

 with input level means that in addition to the first order distortion just 

 described, higher order distortions of the modulation envelope will 

 occur. If, for example, the input signal is amplitude modulated at fre- 

 quency /i , the output modulation envelope will contain amplitude and 

 phase modulation both at /i and at harmonics of /i . The amount of 

 higher order distortion can be estimated by expanding the compression 

 and amplitude-to-phase conversion curves as a function of power input 

 in a Taylor series about the operating point. Such an expansion shows 

 that the greater the slope of these curves the greater will be the higher 

 order distortions. 



2450 



2400 



l5LU 



io 



o>, 



2350 li.^ 



2300 



2250 



IU_| 



I< 



2200 



10 20 30 40 50 60 



BEAM CURRENT IN MILLIAMPERES 



70 



Fig. 28 — Low-level gain as a function of beam current. The helix voltage was 

 adjusted for maximum gain at each current. 



