938 THE B?:LL system TECHXICAL journal, JULY 1956 



The standard one milliwatt source of test power is used at the sending 

 end. The receiving end includes an amplifier, a set of adjustable resistance 

 pads which are relay controlled and an amplifier-rectifier with a measur- 

 ing relay (m) in its output circuit. Relay (m) is a polarized relay of a 

 type widely used in the telephone plant. 



The amplifier has a fixed gain of 19.9 db and it includes considerable 

 negative feedback so that its gain is constant. The pad components are 

 precision resistors to insure accuracy. 



The amplifier-rectifier consists of a two-stage amplifier followed by a 

 rectifier tube and a detector tube for controlling relay (m). This circuit is 

 designed so that the margin between the input power which will hold 

 relay (m) operated and the input power which will insure that relay (m) 

 will release is less than 0.1 db. The gain is adjusted, by means of a poten- 

 tiometer, so that relay (m) will operate when the test power level at the 

 output from the receiving pads in Fig. 4 is one milliwatt or higher and 

 so that it will release when the power level at this point is 0.1 db or more 

 below one milliwatt. This close margin between operate and release 

 permits relay (m) to be used as an accurate measuring device with a pre- 

 cision comparable with that of manual transmission measuring equip- 

 ment using direct reading meters. Negati^^e feedback, built into the 

 amplifier portion of the amplifier-rectifier, insures gain stability and the 

 amplifier-rectifier will maintain its gain adjustment over a long period. 



When making a transmission loss measurement, the power from the 

 sending end operates relay (m) in the amplifier-rectifier. The loss in the 

 receiving pads is then increased, by means of control circuitry, until the 

 power level at their output is reduced to one, milliwatt. In making this 

 adjustment, relay (m) is used as the power level indicating device. When 

 this adjustment is finished the trunk loss will be 



Intertoll Trunk Loss = 19.9 db — Receiving Pad Loss. 



Adjustment of Receiving Pads 



The receiving pads, shown in Fig. 4 consist of 9 individual pads 

 having losses of 10, 5, 4, 2, 1, 0.5, 0.4, 0.2 and 0.1 db. Each pad is in- 

 serted into the input circuit to the amplifier-rectifier by the operation 

 of a corresponding pad control relay. Adjustment of the pad loss takes 

 place in steps. 



When relay (m) operates on arrival of the test power, the control 

 circuit operates relay 10 to insert the 10 db pad. If this reduces the test 

 power level at the output from receiving pads to a \'alue below one milli- 



