14 BELL SYSTEM TECHNICAL JOURNAL 



measurements can be used. While a different amplifier-rectifier is 

 required for noise measurements the same meters and methods of 

 control can be employed as for transmission measurements. 



All of the methods so far described are manually operated in so far 

 as the recording of the results is concerned. There are occasions when 

 a fully automatic recording device is desirable.'* Such cases are the 

 making of transmission versus frequency runs on repeaters or circuits 

 where measurements are desired over a wide range of frequencies. 

 Another class of measurements are those in which the single-frequency 

 transmission loss of a circuit is to be determined over a long period of 

 time to obtain a measure of the stability of the circuit. For this 

 purpose the new method of measuring transmission is well adapted. 

 With a stable amplifier-rectifier having practically no frequency dis- 

 tortion, the indicating meter may be replaced by a recording meter 

 which will record continuously the received power expressed in db. 

 A fully automatic recording system is shown in Fig. 11. With this 

 arrangement an oscillator at one end of the line supplies testing power 

 to the line, the frequency of the power being changed continuously 

 by a synchronous motor. At the receiving end the recording meter, 

 also operated by a synchronous motor, plots the received power. A 

 complete transmission frequency run on a message telephone circuit 

 can be made in less than one minute. When records are to be made 

 of transmission vs. time, the oscillator frequency at the sending end 

 is fixed and the meter plots the transmission loss at that frequency. 



The above description has been limited to the types of measure- 

 ments commonly made on complete circuits or parts of circuits. In 

 addition to these there are a number of types of measuring apparatus 

 which are used in connection with the installation of new equipment, 

 changes in installations and the detailed running down of trouble. 



Transmission measurements have proved to be of great value in 

 maintaining satisfactory transmission performance of telephone cir- 

 cuits. Periodic routine tests avoid service impairment by detecting 

 troubles. Troubles which are thus detected or which cause service 

 complaint are located readily. Another large field of use is in the 

 adjustment of repeaters and complete circuits to prescribed trans- 

 mission characteristics. The importance of this work to the Bell 

 System is indicated by the fact that there are nearly 1000 of the 

 portable transmission measuring equipments and 1500 transmission 

 test boards now in use, with which several million measurements are 

 made annually. The new measuring systems enable this work to be 

 done more rapidly than in the past, and the reduced cost of the 

 equipment is resulting in its greater distribution. 



