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BELL SYSTEM TECHNICAL JOURNAL 



may be built which will require tuning of the beating oscillator at very 

 infrequent intervals, perhaps only two or three times a day. For 

 receivers in which the carrier is to be separated from the sideband by 

 a narrow filter, a much higher degree of frequency stability is required 

 in both the transmitter and the receiving beating oscillator if fre- 

 quent or almost continuous tuning is to be avoided. Rather than 

 endeavor to obtain the high-frequency stability required, it was de- 

 cided to arrange that the incoming carrier automatically tune the 

 beating oscillator of the receiver in such a manner that the carrier at 

 intermediate frequency would always pass through the narrow crystal 

 filter in a satisfactory manner. The manner in which this is accom- 

 plished is shown in Fig. 6. The reconditioned carrier is introduced 



BALANCED 

 MODULATORS 



WATT HOUR 

 METER 



Fig. 6 — Schematic of automatic tuning device. 



in push-pull fashion on the grids of a balanced modulator system. 

 The same carrier is passed through a circuit having a 90-degree phase 

 shift, through a narrow band suppression filter, and applied to the same 

 two grids in parallel. A small 60-cycle voltage is also applied to the 

 grids in parallel. The 60-cycle output voltage of the balanced modu- 

 lators is applied through a transformer to the rewound current coils 

 of a watt-hour meter. When the carrier frequency is that of maximum 

 suppression for the narrow filter, equal voltages e\ and e^ will be applied 

 to the grids of the two tubes forming the balanced modulator. If the 

 carrier frequency shifts from this position, the voltage applied to each 

 grid will be the vector sum of ei or ei, and a voltage of variable magni- 

 tude and phase e^, which appears in parallel on the two grids. The 



