238 BELL SYSTEM TECBNICAL JOURNAL 



the first and last stages respectively are coupled to separate tuned cavities 

 iu order to provide double-tuned terminals for the amplifier. This results 

 in the overall structure shown schematically in Fig. V-16. 



The required bandwidth is obtained by a process of stagger-coupling the 

 circuits so that the individual responses are as indicated schematically at 

 the top of Fig. V-16. Because there are available continuously variable 

 adjustments of tuning, loading, and coupling for each circuit, it is possible 

 to obtain a very smooth and symmetrical overall response characteristic as 

 shown in Fig. V-17. The corresponding measured delay distortion charac- 

 teristic is shown in Fig. V-18. For this type of tuning the output power is 

 about .7 watt at ^ db of compression. More power is, however, obtainable 

 if more compression is tolerable; and when used in the repeater for the 

 transmission of FM signals, the amplifier is driven to an output of 1 watt. 



Since the circuits are of fairly high Q, the frequency characteritics of the 

 amplifier are markedly affected by changes in temperature of the cavities. 

 In order to minimize such detuning effects, the amplifier has been placed in 

 a temperature controlled compartment. Since about 45 watts are dissipated 

 at the collector of each tube by the high- voltage electron beam, the collectors 

 must be cooled by a forced air blast. In order to keep the cooling system 

 separate from the temperature control system, the collector ends of the tubes 

 project through a wall of the temperature controlled compartment into an 

 external air duct. Figure V-19 shows a complete r.f. amplifier with the 

 cover of the temperature control box removed. This is the amplifier de- 

 veloped for the New York-Boston circuit.* The electron-gun ends of the 

 tubes face the reader. Short lengths of flexible coaxial cable couple the 

 input and output of the amplifier to the associated equipment. 



Testing of Components and Repeater Amplifier — Many special measuring 

 and testing techniques had to be devised for the research work on each com- 

 ponent. In addition, standard production tests had to be worked out and 

 measuring equipment constructed. 



Impedances were measured at RF with standing wave detectors, and at 

 IF with three fixed taps on a coaxial line. It was found that input and 

 output SW ratios could be held to 1.7 db or less over the 10 megacycle band. 

 Both point by point and swept oscillator methods were used at RF and IF for 

 measuring amplitude characteristics. Particularly as a result of the de- 

 velopment of swept oscillator measuring techniques it was found practicable 

 to adjust each unit to ±0.1 db amplitude variation over the 10 megacycle 

 band. Noise figures^'' of IF equipment were measured by the noise diode 

 method.^® 



* Developed by F. E. Radcliflfe and R. C. Carlton. 

 " Loc. cit. 



"H. Johnson, "A Coaxial Line Diode Noise Source for U.H.F.", R. C. A. Review, 

 vol. VIII, No. 1, pp. 169-185, March 1947. 



