Characteristics of Vacuum Tubes for Radar Intermediate 

 Frequency Amplifiers 



By G. T. FORD 



1. Introduction 



THE desired characteristics for vacuum tubes for use in broad-band 

 intermediate frequency amplitiers are primarily high transconductance, 

 low capacitances, high input resistance, and good noise figure. These 

 characteristics determine the frequency bandwidth, amplification, and 

 signal-to-noise ratio attainable with such an amplifier. The maximum 

 operating frequency is generally limited by the input resistance of the tube 

 which decreases as the frequency is increased, and, in some cases, by the tube 

 noise which increases with increasing frequency. Three other character- 

 istics which are also important are small physical size, low power consump- 

 tion, and ruggedness. The present paper describes how these character- 

 istics are related to the performance requirements for intermediate fre- 

 quency (IF) amplifiers used in radar systems and shows how the require- 

 ments were met in the design of the Western Electric 6AK5 Vacuum Tube. 

 In a coaxial cable carrier telephone system of the type which was initially 

 installed between Stevens Point, Wisconsin and Minneapolis, Minnesota^ 

 the upper frequency of the useful band is of the order of three megacycles 

 per second (mc) and the bandwidth is of the same order. The Western 

 Electric 386A tube was developed a number of years ago for amplifiers such 

 as those used in this system. It is characterized by high transconductance 

 and low capacitances. In radar receiving systems similar but more exact- 

 ing requirements must be met for the IF amplifier. It is desirable to operate 

 in many cases at a mid-band frequency of 60 mc, to have a bandwidth of 

 the order of 2-10 mc, and to have as close to the ideal noise figure as possible. 

 Additional considerations of great practical importance are low power 

 consumption, small size, and ruggedness. 



The choice of the mid-band frequency for the IF amplifier is influenced 

 by considerations, a detailed discussion of which is outside the scope of 

 this paper. For example, the characteristics of the beating oscillator and 

 ite relation to the operation of the automatic frequency control (AFC) 

 system, when AFC is used, are involved. The usual practice has been to 



1 "Stevens Point and Minneapolis Linked by Coaxial System," K. C. Black, Bell Labo- 

 ratories Record, January 1942, pp. 127-132. , , ^ „r z, n c . 



"Television Transmission Over Wire Lines," U. E. Stneby and J. F. VV enU, Bell System 

 Technical Journal, January 1941, Vol. 20, p. 62. 



385 



