120 BELL SYSTEM TECHNICAL JOURNAL 



ments can be made on operating systems to determine the performance 

 of the intermediate repeaters and regulators with respect to level and 

 equalization over the frequency range from 12 to 60 kilocycles. Loss 

 and gain measurements also can be made either between this point and 

 the voice-frequency four- wire test board, through the carrier terminal 

 equipment or through the next adjacent repeater or terminal office at 

 high frequencies. It is possible to test the high-frequency portion of 

 the terminal and to substitute a spare, by patching or rapid transfer, 

 for a defective or potentially defective group modulator, transmitting 

 amplifier, group demodulator or receiving amplifier. 



Some of the high-frequency testing equipment is shown mounted on 

 the middle bay of Fig. 9, B, of which one of the most important units 

 is the 1 to 150-kc. test oscillator located at the center of this bay. 

 It is a heterodyne type of oscillator which covers the frequency range 

 with a continuous film strip scale about 300 inches long. Its maximum 

 output is about one watt and this varies less than 1 db over the entire 

 range. It is provided with built-in calibrating features and can be set 

 to any frequency with an absolute accuracy of about 25 cycles. It is 

 used as the tuning control of the pilot level measuring circuit. An 

 auxiliary scale on the oscillator permits tuning the measuring circuit 

 directly in terms of frequency. 



The pilot level measuring circuit is of the double heterodyne type 

 and includes a copper-oxide modulator which is supplied with carrier 

 from the heterodyne oscillator, an intermediate frequency 130-kc. 

 crystal filter of 10 cycles band width, a high-frequency amplifier for 

 this frequency, a copper-oxide demodulator supplied with carrier from 

 a 129-kc. fixed frequency oscillator, a voice-frequency amplifier and 

 calibrating circuit. The input impedance of the measuring circuit is 

 high so that when it is bridged across a line pair at the high-frequency 

 testboard jack fields it does not produce appreciable loss to the line. 

 The circuit permits measuring each of the three pilot frequencies to 

 check levels and equalization of operating systems. The panels com- 

 prising the circuit are shown below the oscillator in Fig. 9, B. 



Mounted on a shelf just below the oscillator is the transmission 

 measuring set, which contains a highly accurate thermocouple and 

 meter combination with calibrating circuits, wide range repeating coils, 

 a test key circuit, and attenuators, one of which can be set in steps of 

 1 db up to a total of 90 db. 



Equipment Features 



Because of the large number of systems likely to be terminated in 

 an office, the jacks are concentrated in a group of bays located together 



