1064 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



generation in the middle stages of the amplifier. A peak-to-peak voltmeter 

 connected across one side of the balanced output is used to monitor the 

 transmission level of television signals. 



V. System Maintenance and Test Equipment 



A. General 



Most TD-2 stations are operated on an unattended basis. Test equipment 

 is provided at each terminal, auxiliary and main station to perform the 

 necessary maintenance functions. This consists of a radio test bay as shown 

 in Fig. 18 for each auxiliary, main and terminal station, and an FM test 

 console as shown in Fig. 19 at terminals and main stations where FM termi- 

 nal equipment is provided. The philosophy is to provide sufficient test 

 equipment at each station to isolate the trouble. When the unit in trouble 

 requires extensive tests or repair, a station spare is substituted and the faulty 

 unit is returned to a maintenance center. Maintenance centers are usually 

 located in existing telephone offices along the route. 



In maintaining the radio equipment each repeater bay is adjusted to 

 provide a transmission band 20 megacycles wide, flat to within two-tenths 

 of a db and centered about the assigned channel frequency. Trimming ad- 

 justments are provided on the receiver and transmitter to obtain this 

 characteristic. This test involves the use of a swept signal source which is 

 divided into a reference path and a path through the equipment under test, 

 each of which is terminated in an identical detector. The outputs of these 

 detectors are alternately appHed to the vertical deflection amplifier of an 

 oscilloscope at a 30-cycle rate, while a voltage proportional to the frequency 

 excursion is appUed to the horizontal amplifier. Generally, the vertical gain 

 of the oscilloscope is adjusted so that a separation of one inch between the 

 test and reference traces corresponds to a level difference of 1 db and the 

 horizontal gain is adjusted so that one inch corresponds to a 10-megacycle 

 frequency excursion. The reference trace is then matched to the test trace 

 by adjustments of the equipment under test. The waveguide attenuators 

 and directional couplers shown in Fig. 18 provide for testing over a wide 

 range of levels. 



B. Radio Test Bay 



The radio test bay contains a microwave swept frequency oscillator, a 

 combined microwave and IF power meter, a cathode ray oscilloscope, RF 

 and IF wave meters, detectors and attenuators and associated power supplies. 



The microwave sweep oscillator is adjustable in sweep range up to 70 

 megacycles over the 3700 to 4200 megacycle band. The frequency is swept 



