TRAVELING WAVE TUBE FOR 6,000-MC RADIO RELAY 



1335 



TWT in an FM radio relay was investigated briefly. In this measurement 

 an FM receiver tuned to the carrier frequency was used to detect the 

 noise modulation present in the TWT output. The noise figure was deter- 

 mined in the usual manner from the ratio of receiver outputs with the 

 noise source turned off and on. When the TWT was operated in the 

 linear region, this measurement gave the same result that our first 

 method did. With the TWT operated in the non-linear region it gave 

 a value within a few db of that obtained from the first method. 



The cause of the high noise output observed for low magnetic flux 

 densities at the cathode is at the present time not clearly understood. 

 Fried at MIT and Ashkin and Rigrod at Bell Laboratories have all 

 probed the beam formed by guns of the M1789 type and have found 

 certain anomalous efl"ects. Normally one would expect to find a standing 

 wave of noise current along the electron beam. For the M1789 gun they 

 find instead that after about two minima of the standing wave pattern, 

 the noise current on the beam begins to grow and continues to do so 

 until a saturation value is reached. The noise current at this saturation 



LOW NOISE 

 TRAVELING- 

 WAVE TUBE 



SIGNAL 



POWER 



MONITOR 



NOISE 

 LAMP 



SIGNAL 

 SOURCE 



RECEIVER 



y/C 



FILTER 



HYBRID 



H 



GK-r 



SIGNAL 

 6000 MC 



X 





TRAVELING- 

 WAVE TUBE 

 UNDER TEST 



RECEIVER LOCAL 



OSCILLATOR 



6170 MC 



FILTER 

 6080 



BAND 

 6100 



20 MC 



Fig. 40 — Block diagram of noise measuring equipment. Tiie noise source con- 

 sists of a fluorescent lamp the output of which is amplified by a low-noise TWT so 

 as to bring the noise level to about 35 db above kTB at the M1789 input. The out- 

 put from the M1789 is passed through a 20-mc bandpass filter which eliminates 

 both the single frequency test signal and the noise in the image band of the re- 

 ceiver. The noise figure is measured by noting the difference in noise level at the 

 receiver output with the noise source off and on, in a manner similar to that used 

 in a conventional noise figure measurement. 



