EXPERIMENTAL TRANSVERSAL EQUALIZER FOR TD-2 



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Fig. 8 — Model of coaxial rheostat with cover removed. 



this purpose. It employ's a ceramic rod, | inch in diameter, coated with 

 a pyrolytic carbon film, asa centermember of a coaxial structure. A metal 

 sleeve which is moved longitudinally by a lead screw carries sliding con- 

 tacts along the rod. These parts are supported inside a rectangular hous- 

 ing which forms the outer conductor. A second set of fixed contacts at- 

 tached to the rectangular housing makes contact with the sleeve. This 

 arrangement maintains a substantially constant length of path from 

 the input end of the rod to the housing, which forms the ground, inde- 

 pendent of the position of the sleeve. The schematic of the rheostat is 

 shown in Fig. 7. A model of the rheostat is shown in Fig. 8. 



To obtain uniform adjustment of amplitude in decibels, a resistance 

 that varies exponentially with length or with rotation of the lead screw 

 is required. Such a resistance characteristic is realized by varying the 

 thickness of the carbon film along the rod. This produced a total re- 

 sistance which varied from 20 ohms at the low setting to about 350 

 ohms at the high resistance setting. .After an initial wearing-in period 

 of 1,000 cycles of moving the contacts over their full travel, the resist- 

 ance was changed less than 1 per cent by another 9,000 cycles. This 

 amount of wear is estimated to be greater than that encountered in 

 twenty years of normal operation. 



The housing and rod were dimensioned to form a 7o-ohm transmission 

 line. Measurements of the impedance at the input connector, made at 

 frequencies from (iO to 80 mc, showed that this impedance can be approx- 

 imated by a resistor terminating 6.7 cm of 7o-ohm coaxial cable. For 

 the 7o-ohm setting, the reflection coefficient of the rheostat is less than 

 2 per cent across this frequency band. 



