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BELL SYSTEM TECHNICAL JOURNAL 



until the beating oscillator gives the proper frequency, when the 

 action stops. 



Since this automatic tuning unit holds the carrier at intermediate 

 frequency in a fixed relation with respect to a crystal filter, which may 

 drift slightly in resonant frequency from time to time, and not in 

 synchronism with a local carrier oscillator, it is necessary that a sepa- 



INCOMING CARRIER 

 400 Kc. 



LOCAL CARRIER 

 400 Kc. + A 



90° PHASE 

 SHIFTER 



AMPLIFIER 



AMPLIFIER 



AMPLIFIER 



AMPLIFIER 



BALANCED 

 MODULATOR 



AOO 



BALANCED 

 MODULATOR 



TWO PHASE 



SYNCHRONOUS 



MOTOR 



)IC 



ON I.F. 

 OSCILLATOR 



+ B 



Fig. 8 — Schematic of automatic synchronizing equipment. 



rate mechanism be provided for synchronizing the local carrier if a 

 local carrier is to be used. A schematic diagram of the circuit for 

 doing this is shown in Fig. 8. The reconditioned carrier at inter- 

 mediate frequency is introduced through amplifiers to two balanced 

 modulators. The output of the local carrier oscillator is introduced 

 to the same modulators, to one of them directly and to the other 

 through a device which shifts the phase 90 degrees. The phases of 



