804 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



izers supplement the fixed delay equalizers described above and are 

 needed to trim the delay characteristic of the line in finer detail than 

 would be possible with fixed equalizers. 



2.223 Automatic EquaHzers 



The first step of automatic-gain equalization is provided at each 

 auxiliary repeater. The nominal gain characteristic of the repeater is 

 designed to match the loss characteristic of the coaxial at 55° F. The 

 cable loss varies with changes in temperature; the variations, however, 

 have a predictable characteristic, being very closely proportional in db 

 to the square root of frequency. To compensate for these changes, the 

 gain characteristic of the amplifier at auxiliary repeaters is adjustable 

 and follows the loss of the cable under the control of a thermistor. Two 

 types of circuits are used to control the current fed to the thermistor 

 as described in a later section. 



In the second step of automatic gain equalization, networks are pro- 

 vided to match system gain variations caused by electron tube aging 

 and by changes in repeater hut temperatures. The loss characteristics of 

 these equalizers are controlled by thermistors which in turn are con- 

 trolled by the 308-kc and the 2064-kc pilots. These equalizers are used 

 every 40 to 120 miles. 



In the final step of automatic gain equalization, networks are pro- 

 vided to compensate for second order effects of the first three rapid 

 variations described above, namely, cable loss variations, and repeater 

 gain variations due to hut temperature changes and electron tube aging. 

 The loss characteristics of these networks are under control of thermis- 

 tors acted on by the 556-kc, 3096-kc and 8320-kc pilots. These equalizers 

 are located at approximately 150-mile intervals. 



The thermistors which control the loss-frequency characteristics of 

 automatic equalizers are driven by regulators through a simple form of 

 analog computer. The design and operation of this circuit is described 

 in a companion paper.^" 



There is no automatic control of the delay characteristic in the sys- 

 tem except that provided by the automatic gain equalizers. Every effort 

 is made to have these equalizers match the transmission changes out- 

 side the band so that resulting delay changes in the band are minimized. 



2.23 Equalization System Considerations 



Whether the system is being eciualized for telephone or television 

 it is immediately apparent that the channel requirements described 



