THE TYPE PI CARRIER SYSTEM 389 



the carrier assignments for the various frequency arrangements on a 

 one and two-crossarm route using the Rl transposition design. Use of a 

 transposition system giving better carrier frequency crosstalk perform- 

 ance than the Rl design is expected to permit the apphcation of a number 

 of additional channels over those shown in Fig. 2. 



It will be noted that the grouped arrangement provides four channels 

 less on a two-crossarm basis than the stackable arrangement. From a 

 transmission standpoint, equal numbers of channels would be possible 

 by assigning one or two channels to a pair, but these arrangements will 

 usually be uneconomical for grouped systems because of the high cost 

 per channel of repeaters. 



7.5 Line Networks 



The location of the line networks and filters, which are a permanent 

 part of the carrier layout, will be designated by the Plant Engineer, and 

 the location and type of the remaining networks, which will vary with 

 changes in subscriber service, will be selected by the plant forces. The 

 low-pass filters or carrier blocking networks used on the carrier lines are 

 simple resonant circuits designed to match given ranges of capacitance 

 that will be presented by the drop wire or open-wire side leads. Since 

 this capacitance varies considerably with various lengths of facility, a 

 method will be provided by which the total capacitance of the drop can 

 be determined and the proper network chosen. The other line networks 

 are applied to the line as necessary to achieve their particular functions. 



8. INSTALLATION AND MAINTENANCE 



A portable field test set has been developed which will simplify the 

 installation and maintenance of the PI carrier sj^stem. The new 

 set, known as the 7F test set, will provide the carrier and audio frequen- 

 cies and a means of measuring them required to align and troubleshoot 

 units of the system. The set, which is battery operated, contains a carrier 

 oscillator to supply test frequencies from 10 to 100 kc, an audio fre- 

 quency oscillator having six selected frequencies in the range of 250 to 

 2,500 cycles, a modulator to modulate the carrier frequency signal with 

 the audio signal, a demodulator for calibrating the modulated signals, 

 and a wide-band amplifier-detector for making level and transmission 

 measurements. The model of the set shown hi Fig 31 included a pre- 

 cision dial for signaling testing which was subsequently found mmeces- 

 sary and eliminated. An ac operated set providing the same desired fa- 

 cilities is now under development. 



