. The Bell System technical journal. Telecommunication; Electric engineering; Communication; Electronics; Science; Technology. 624 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
. The Bell System technical journal. Telecommunication; Electric engineering; Communication; Electronics; Science; Technology. 624 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
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