1050 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



from other channels for transmission 

 through a common antenna. The 40-mega- 

 cycle shift from receiving to transmitting 

 frequencies is introduced in order to reduce 

 the effect of crosstalk between transmitting 

 and receiving antennas. 



Main and auxiliary station repeaters dif- 

 fer in the following respects: An auxiliary 

 repeater simply receives a particular channel 

 signal, ampUfies and transmits it to the next 

 station. Here a common beating oscillator 

 source for the transmitter and receiver, 

 together with a stable 40-megacycle shifter, 

 results in a systems frequency stability, for 

 auxiliary stations alone, which is dependent 

 only upon the stability of the 40-megacycle 

 oscillator. 2 This feature cannot be used in 

 the repeaters of a main station since each 

 radio section between main stations must 

 be independent of other sections for switch - 

 ing, branching, maintenance and termi- 

 nating purposes. Here it is necessary to 

 provide an independent oscillator source for 

 each modulation process. In such an ar- 

 rangement, errors in frequency add through- 

 out the system and, therefore, the individ- 

 ual stability requirements for the oscillators 

 are severe. In the TD-2 System this fre- 

 quency stability is obtained by the use of 

 a crystal controlled oscillator and harmonic 

 generators. Two such microwave generators 

 with temperature controlled crystals are 

 used in each repeater bay at main stations, 

 while one microwave generator and a 40- 

 megacycle oscillator and shifter unit are 

 used in each auxiliary station repeater. 



A repeater bay using a 9-foot cable duct 

 type framework is shown in Fig. 8. The 

 top half of the bay contains the compo- 

 nents which comprise the signal, path 

 through the repeater. These are the channel 

 separation filters, image suppression filter, 



Q-W 



Fig. 8a 

 Fig. 8— Repeater bay. 



