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BELL SYSTEM TECHNICAL JOURNAL 



protection. All outside connections to the panel are made through 

 the terminal block in the lower right-hand corner. Wires supplying 

 power, together with those which are at a low a.-c. potential with 

 respect to ground, are run in a cable, while wires at a high a.-c. potential 

 are run directly from point to point in as short a path as possible in 

 order to reduce losses resulting from the capacity of these wires to 

 ground. 



Demodulator and Receiving Oscillator. The circuit of the demodu- 

 lator shown in Figure 35 is in many respects similar to that of the 



Oubpul 



P Incut 



H30' HSO' -24" i -22-5' -2V 



Figure 35 — Schematic of demodulator circuit and receiving oscillator 



modulator. The function performed by the demodulator is also 

 similar, being a translation from a high-frequency band to a lower 

 instead of the reverse. 



The oscillator which supplies the carrier to the demodulator is of the 

 same type as the modulator oscillator, and has been discussed in 

 connection with that circuit. No adjustable feature for balancing 

 the carrier is required in the demodulator circuit. The carrier sup- 

 pression needed in addition to the suppression inherent in the balanced 

 circuit is provided by the low-pass filter at the output. If the carrier 

 is not sufficiently suppressed, it will pass into the voice circuit or 

 across the hybrid coil into the associated modulator, causing in some 

 channels an objectionable beat tone. 



The transmission stability of the demodulator is obtained by the 

 same methods used in the modulator since the performance of the two 

 circuits is similar, and the transmission quality requirement is 

 essentially the same for both units. A typical demodulator charac- 

 teristic is shown in Figure 36. This characteristic at the higher 

 frequencies is controlled by the low-pass filter. 



