4 BELL SYSTEM TECHNICAL JOURNAL 



"flip flop" principle. The use of these dials generally requires that 

 the measuring instrument be built as a unit and that measurements 

 be made at its location. The latter requirement often prevents its 

 installation at the most desirable point because space is not available. 

 For toll circuit testing in the larger ofiftces where a considerable 

 number of routine and trouble locating tests are made, the required 

 frequency range and loss range are beyond the scope of a simple 

 copper-oxide rectifier and meter. The ability to make level measure- 

 ments is also a desirable feature. To meet these requirements, an 

 amplifier is provided in the receiving circuit. By applying the nega- 

 tive feedback principle to the amplifier and rectifier of the latest toll 

 transmission measuring system a remarkably stable circuit has been 

 obtained so that the meter can be calibrated directly in db and oper- 



INPUT >600'*J 



Fig. 2 — Simplified circuit of reverse feedback amplifier rectifier used for toll circuit 



maintenance. 



ated for long periods without adjustment. This circuit, which is 

 shown in simplified form in Fig. 2, consists of a high-impedance input 

 transformer T bridged across a 600-ohm terminating resistance which 

 may be removed when level measurements are made, two pentode 

 tubes 1 and 2, a copper-oxide rectifier R and a meter M. This com- 

 bination has much more amplification than is required, so the excess 

 is used to improve stability by introducing part of the output voltage 

 into the input circuit of the first tube in such phase relation with 

 respect to. the applied input voltage that the net input voltage is 

 reduced. This reverse or negative feedback voltage is introduced into 

 the grid circuit of the first tube through resistances A, B and C by 

 connecting one of the rectifier terminals to the movable arm of the 

 potentiometer P. These resistances, and resistance D form the cath- 

 ode drop resistance of the grid circuit and any potential applied across 

 them affects the potential on the grid of the tube. In the position 

 shown, the reverse feedback is a maximum and the net amplification 



