940 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1953 



The amplified signal is split into two branches by a hybrid trans- 

 former. Part of the signal is carried to the demodulator for detection to 

 vddeo frequencies while the remainder is employed in the carrier re- 

 . generation apparatus. The carrier supply oscillator output likewise is 

 split between the same two circuits; the portion supplied to the demodu- 

 lator providing the carrier energy for demodulation and the part sent to 

 the carrier regeneration circuits providing means for comparison with 

 the input signal. The carrier regeneration equipment comprises mainly 

 the phase comparison circuit together with means for changing elec- 

 tronically the frequency of the carrier supply oscillator. 



The phase synchronized carrier supply together with the associated 

 third harmonic supply provide demodulating carriers which translate 

 the carrier spectrum down in frequency to the original video spectrum. 

 The demodulator output also contains an unwanted upper sideband in 

 the 8 to 12-mc region. This signal and residual carrier leak from the 

 demodulator are removed from the detected signal by a low-pass filter 

 and the quartz crystal narrow band rejection filter tuned at carrier 

 frequency. Finally the signal is amplified at video frequencies to provide 

 moderately high video frequency transmission levels and transmitted 

 through the reversing relays of the polarizer to the receiver output. 



Fig. 20 is a photograph of two equipment bays each eleven feet high 

 which comprise at the left the T3 transmitting terminal and at the right 

 the R3 receiving terminal. 



Acknowledgement is made for the many contributions made by our 

 associates toward the successful development of these terminals. These 

 include both ideas related to transmission processes and important con- 

 tributions in the development of many new circuit components which 

 were necessary to the achievement of the quality and reliability ob- 

 jectives. 



RESTORER 



PILOT 

 FILTER 



MOP-UP HF 



EQUALIZER AMPLIFIER 



&4 



VIDEO 

 DEMODULATOR AMPLIFIER 



o 



CARRIER AND 



LOW -PASS 



FILTER 



POLARIZER 



C! 



CARRIER 

 FREQUENCY I— 

 CONTROL 



CARRIER 

 OSCILLATOR 



HARMONIC __ 

 OSCILLATOR 



Fig. 19 — L3 receiving terminal block diagram. 



