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BELL SYSTEM TECHNICAL JOURNAL 



As already mentioned, care has to be exercised in the mounting of 

 filters belonging to different systems in the same ofifice, so that no 

 crosstalk will be introduced from one system into another. A con- 

 siderable level difference may exist between two filters of different 

 systems, and it may be desired to mount these filters on adjacent bays. 

 In order that the crosstalk between these two systems may be kept 

 within desirable limits, the separation between the filters must, in 

 some instances, correspond in attenuation loss to the order of 120 

 TU, or one part in a million. To meet this exacting requirement, 

 the filters are totally incased in sealed copper boxes, the leads being 

 brought out through small holes to terminal blocks. 



Amplifiers. As previously mentioned, the amplifiers employed with 

 the type "C" system at the terminals are identical with those used 

 with the repeaters at intermediate stations. The following is, there- 

 fore, applicable to both cases: 



The number and size of tubes needed to deliver the necessary output 

 level or power are largely controlled by interchannel crosstalk require- 

 ments. With the grouping frequency arrangement, the three bands 

 which transmit in the same direction are amplified in a common 

 circuit. The different sideband frequencies in passing through the 

 common amplifier must not react upon each other to produce other 

 frequencies of sufficient magnitude to cause interference. For ex- 

 ample, second harmonics of the lowest band frequencies lie within 

 the range of the highest channel in the lower group. If these har- 

 monics are permitted to become too great, troublesome noise will be 

 present in the highest channel when speech currents flow in the lowest. 

 In order that this interference or crosstalk may not become excessive 

 the tubes used in this amplifier must be made of ample power capacity. 



Figure 42 — Amplifier circuit 



This example of interference caused by the second harmonic shows 

 the desirability of using a push-pull amplifier in carrier repeaters 



