1440 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1957 



tortion versus frequency. These displays are not available simultane- 

 ously with present equipment. To adjust the equalizer controls using 

 this equipment, it must be possible to adjust either gain or delay without 

 affecting the other. Thus, all the gain terms can be adjusted in suc- 

 cession using the gain display. Then the procedure is repeated with the 

 delaj^ display, adjusting the delay terms. Since a combination of leading 

 and lagging echos in equal amounts is reciuired for this procedure, an 

 arrangement of the controls to facilitate this is required. One way to 

 achieve this is to use stepped rheostats with the steps proportioned to 

 introduce ecjual amplitude changes in the echo voltage. With this ar- 

 rangement, gain changes can be introduced by rotating the two switches 

 corresponding to a pair of echos in the same direction an equal number 

 of steps. Delay changes can be obtained by similar rotation in opposite 

 directions. A further refinement consisting of mechanically ganging the 

 controls is possible but this was not done on these experimental models. 



The second method of adjustment would be to develop a special test 

 set similar to the one developed for the L3 system.^ This could produce 

 a meter reading proportional to the amount of gain and delay distortion 

 present in the circuit. Successive controls could then be adjusted for min- 

 imum meter readings. Experience with the L3 system cosine ecjualizers 

 has shown the desirability of continuously adjustable controls for such 

 a method. 



To test both methods under field-trial conditions, two versions of the 

 eciualizer were built — one with stepped rheostats and one with con- 

 tinuously adjustable rheostats. 



VIII. COAXIAL RHEOSTAT 



Since there were no available continuously adjustable rheostats sat- 

 isfactory for operation at 70 mc, a special rheostat was developed for 



FRAME 



r// '/////////////////////////////// : '/////////////////A 



TX 



SLEEVE 



I' ' ■////'/////////////////. '/7/////////////////////.'. '///A 



Fig. 7 — Schematic of coaxial rheostat. Moveable sleeve changes position of 

 inner contacts touching ceramic rod, changing resistance. Fixed outer contacts 

 maintain constant path length to frame. 





