I.LFXIRIC.IL TESTS ASP TIITIR ATI'LICATIONS 



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tennining which frot|uencii's making up the noise currents predomi- 

 nate. For both noise and crosstalk measurements, definite rules 

 must l)e followed in terminatinjj the distant ends of the circuits under 

 test in order to reduce terminal impedance irrejiularilies. 



'2\-Circuit Balance Tests. In describinjj the use of the bridge for 

 locating; im[H'dance irregularities, mention was made of the effect of 

 such irregularities on telejihone repeater operation. Since the making 

 of imiK'<l.ince runs on circuits invokes a considerable amount of time 

 and expense, a simple and (juick balance test, known as the 21-circuit 

 test, was devisetl in which the telephone repeater is made to function 

 as the testing set. The gain which can be obtained from a 21 or 22 

 type telephone repeater with good quality depends to a large extent 

 on the degree of balance, within the frequency range in\'olved, be- 

 tween the impedances of the telephone circuits and the impedances 

 of the corresponding balancing networks. The use of this balance 

 relation is illustratetl in the simplified circuit of Fig. 22 which shows 

 a 22 type repi-ater connected to make a 21-circuit balance test between 

 the "Fast" line and its balancing network. 



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S^^ort Of 

 ^4«l»wo^l( 

 West 



Output 



Trar^former 



West 



Ne 



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The line under test and its balancing network are connected as for 

 normal repeater operation, while the "West" line is opened. The 

 "West" line network terminals are either shorted or the network 

 left connected, the principle of the test being the same in either case. 

 The 3-winding transformer, when connected for normal repeater oper- 

 ation, as shown for the "Fast" transformer in Fig. 22, simply gives a 

 Wheatsione bridge relation, the input of the "West" amplifier being 

 connected to the balanced points of the bridge. The proportion 



