618 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



the loop is relatively high. To obtain acceptable transmitting levels, 

 the low impedance of the transmitter must be approximately matched 

 to the high impedance loop by providing a proper ratio of turns between 

 mndings A and B. Therefore, the impedance Zi looking toward the 

 balancing network must be made low and since an important element 

 of this network is the V2 varistor, this had to have an unusually low 

 resistance. This is the reason why it was necessary to undertake a 

 special development to make available a silicon carbide varistor having 

 suitable characteristics. 



Sidetone would be eliminated if the vectorial sum of the voltages in 

 the mesh which includes the receiver, and which arise as a result of 

 speech and noise picked up by the transmitter, were zero. The complete 

 elimination of sidetone is not desirable, but the objective is to keep it 

 at a low level by a balance of opposing voltages. To achieve this result 

 any voltage developed in the local transmitter is divided in the windings 

 A and B so that the voltages induced in winding C are opposing. Further- 

 more, the voltage across the network resistance arising from current 

 flowing in winding B is arranged to oppose this resultant of voltages 

 induced in winding C. The overall effect of this balance is that the 

 current in the receiver as a result of voltages developed in the transmitter 

 is small. Also, this result gives maximum transmitting levels because 

 there is little power loss in the receiver. However, to maintain the 

 balance which gives low receiver currents the impedances Zi and Z2, 

 as affected by the transformation ratio of the coil, must be comparable. 

 This is the key to good sidetone balance — that the impedances Zi and 

 Zi be effectively matched both in magnitude and phase. 



Now in the telephone plant, the impedance looking from the station 

 toward the loop varies widely from one installation to another, and 

 even from one call to another. The loop may be long or short, of small 

 gauge or large gauge, and composed of cable or open wire or combina- 

 tions of the two. Furthermore, the loop may be loaded or non-loaded 

 and it may be connected through central office circuits of different char- 

 acteristics to a trunk or distant loop and telephone set which also may 

 cover a range in impedance. It is quite obvious that under these con- 

 ditions the impedance looking toward the loop will not only vary over a 

 wide range in magnitude, but may be inductive, or capacitive or es- 

 sentially resistive. 



If we assume that the V2 varistor were not present, the impedance 

 7j2 looking toward the balancing network is not influenced by loop 

 current. This is the situation that has obtained in balancing sidetone in 

 preceding designs of telephone set. One of the impedances to be matched 



