378 BELL SYSTEM TECHNICAL JOURNAL 



of the currfiit delivered to the transformer from the "East" amphlier 

 which jiels t(j the input of the "West" amplifier depends, therefore, on 

 the degree of liridge balance furnished by the line under test and its 

 network. When the :i-winding transformer is ojiened on the line side 

 with either the network terminals shorted or witii the network con- 

 nected, as shown for the "West" transformir its action is the same as 

 a repeating coil. 



An internal path for currents which may produce repeater "singing" 

 or a sustained tone, is established if the gain of the two amplifiers is 

 just greater than the sum of the losses within the repeater circuit, that 

 is, the losses through the transformers and any other equipment in 

 the circuit. Theoretically, if the line and network were perfectly 

 balanced and there were no internal unbalances in the repeater, it 

 could not be made to sing since, due to the balance relations of the 

 "East" 3-winding transformer, there would be infinite loss from the 

 output of the "East" amplifier to the input of the "West" amplifier. 

 This ideal condition is, of course, not met with in practice, since it is 

 not practicable to design repeater circuits for perfect balance or to 

 construct artificial networks which will exactly balance the working 

 lines at all frequencies invoked. The amplification which can be 

 obtained in any instance without singing, then depends to a large 

 extent on the balance between the lines and networks. In the test 

 circuit shown in Fig. 22 the gains of the two amplifier elements are 

 increased until singing or a sustained tone is observed and the total 

 gain required for this gives an indication of the balance between the 

 "East" line and its balancing network. In liie same way the balance 

 between the "W'est" line and its network cm be determined by con- 

 necting this in the regular wa>' to tlic "West" 3-winding transformer 

 and disconnecting the "East" line. In making the tests in either 

 direction the "poling" of the repeater circuit is reversed in order to 

 give the lowest value of singing point which might occur imder service 

 conditions. 



In practice the tests described aliow iia\i' become of considerable 

 use and importance in the installation and maintenance of telephone 

 repeaters and the circuits associated with them. Methods are avail- 

 able for computing the estimated singing points which circuits and 

 equipment should gi\e with telephone repeaters under operating con- 

 ditions. These comi)ulations allow toll circuits and eciuipment to be 

 engineered intelligenlh' with respect to the gains which the repeaters 

 may be e-\])ccted to gi\e with good quality. .Micr iii>iallation, the 

 21-circuit tests furnish a means for checking com|)ulcd or estimated 

 singing points. When the estimated singing points cannot be obtained 



