TRAVELING WAVE TUBE FOR 6,000-MC RADIO RELAY 



1319 



severe interference with the signal being transmitted. We wU discuss a 

 particular example after consideration of the compression and AM-to- 

 PM conversion characteristics of the M1789. 



As in the case of compression, we must measure AM-to-PM conversion 

 dynamically. This is necessary because point-by-point measurements of 

 the shift in output phase as input level is changed include a component of 

 phase shift caused by changes in temperature of the ceramic support 

 rods and a consequent change in their dielectric constant. However, 

 this thermal effect does not follow AM rates of interest and therefore 

 does not produce AM-to-PM conversion. 



Fig. 24 shows a simplified block diagram of the test set used to measure 

 compression and AM-to-PM conversion. This equipment amplitude 

 modulates the input signal to the TWT under test by a known amount 

 and detects the AM in the output signal with a crystal monitor and the 

 PM with a phase bridge. A more complete discussion of this measurement 

 is given by Augustine and Slocum.^ 



Compression is given as a function of power input in Fig. 23(c) and 

 as a function of power output in Fig. 23(d). We see that compression 

 sets in more suddenly at higher helix voltages. Above about 2,500 volts 



REFERENCE 

 PHASE 



PHASE SHIFTER 



SIGNAL 

 SOURCE 



.1 



H 



AMPLITUDE 

 MODULATOR 



HYBRID 

 JUNCTION 



PHASE 

 BRIDGE 



OSCILLO- 

 SCOPE 



TRAVELING- 

 WAVE TUBE 



Fig. 24 — Simplified block diagram of test set used to measure compression and 

 conversion of amplitude to phase modulation. A ferrite modulator introduces one 

 db of 60 cps amplitude modulation into the test signal. The 60 cps rate is much 

 higher than that which can be followed by thermal changes in the TWT. Half of 

 the modulated signal serves as input to the TWT under test and half serves as a 

 reference phase for a phase detector. The signals at the phase detector input are 

 maintained equal and at constant level and nominally in phase quadrature. The 

 detector is essentially a bridge circuit, the output of which is a dc voltage propor- 

 tional to the phase difference of the two inputs. When operated with inputs in 

 quadrature it is not sensitive to amplitude changes of as much as two db in either 

 or both inputs. Phase modulation introduced by the amplitude modulator appears 

 at both inputs and thus does not produce an indication. The output of the de- 

 tector is therefore a direct measure of the phase modulation created in the TWT. 

 Compression is determined by comparing the percentage amplitude modulation 

 at the input and output crystal monitors. 



