IMPROVED CIRCUIT FOR TELEPHONE SET 619 



was fixed and the other varied over a wide range. The sidetone balance 

 under such conditions represented the best compromise that could be 

 made among a large number of uncontrolled factors. 



Let us examine next how the varistors affect this balancing problem 

 and consider the influence of long and short loops. From what has already 

 been shown, the varistor is a variable impedance element — that is, 

 both its dc and ac resistance depend on the direct current. If the loop 

 is long and composed of open wire the impedance looking toward it is 

 high, perhaps 1200 ohms, and the direct current is low. The dc resistance 

 of the Vi varistor is then so high (approximately 10,000 ohms) that it 

 has no appreciable effect on the impedance Zi. Even the ac resistance 

 of about 4,000 ohms does not have an appreciable shunting effect on Z\. 

 On short loops and local connections where the current is large and the 

 impedance looking toward the loop may be of the order of 900 ohms, the 

 dc resistance of 1,800 ohms has little effect. However, the ac resistance 

 of 600 ohms does have an appreciable effect on Zi, bringing about a 

 better impedance match. 



The impedance looking toward the balancing network and receiver 

 without V2 connected in the circuit is approximately 75 ohms, and is not 

 affected by the loop current. If we go through the same process of 

 examining the shunting effect of V2 on the impedance Z2, we see that at 

 small loop currents both the dc and ac resistance of V2 are too high to 

 have any appreciable effect. At large loop currents the dc resistance of 

 V2 is under 150 ohms, but the ac resistance has dropped to well below 

 50 ohms and has a decided effect on the impedance. 



Since the varistors are essentially pure resistances they tend to make 

 the impedances Z\ and Z2 presented to the coil more resistive than 

 they would be if the varistors were not present. This has the effect of 

 reducing the difference between loop impedances which are capacitative, 

 inductive, or resistive from the standpoint of phase angle and thereby 

 improves the impedance match. 



The overall effect of the automatic changing in resistance of the Vi 

 and V2 varistors is to provide better matching and thereby reduced 

 sidetone. This is shown in Fig. 6, where the loudness level of sidetone 

 of the 500D set is plotted against loop current. It will be noted that 

 mth Vi and V2 open circuited, there is a continuing increase in sidetone 

 loudness level with increasing loop current. When both Vi and V2 are 

 connected in the circuit a decrease in sidetone level of 10 db or more is 

 obtained at medium and large loop currents. The decrease is less at 

 small loop currents. The close interrelation between the varistors and 

 the circuit is well illustrated by the fact that if either the Vi or V2 



