5 YNCHRONIZA TION S YSTEM 



475 



reduced to an equilibrium value, after which the motor continues to 

 run at constant speed in accordance with the frequency of the os- 

 cillator. 



Operating tests on the circuit show that the motor will hold in step 

 over line voltage ranges from 100 to 125 volts and will be self-synchron- 

 izing over somewhat narrower voltage limits. Thus, under normal 

 conditions all that is necessary from an operating standpoint is to 

 close the switch and wait for the motor to pull into step. 



Control Oscillator 



The control oscillator is a standard type of vacuum tube oscillator 

 having a frequency precision of the order of 1 part in 1000, when 



PLATE 

 CURRENT 



/effective 



I VALUE 



90° 



PHASE ANGLE BETWEEN 

 PLATE AND GRID VOLTAGES 



Fig. 5 — Phase detector tube characteristic. 



180' 



delivering the negligible output of .005 watts to the grid circuit of the 

 detector tubes. This frequency is delivered directly to the motor 

 circuits at one end of the line and is transmitted over a separate cable 

 pair to the control circuits at the other end of the line. It was found 

 that the detector tubes would operate successfully over a considerable 

 variation in power level, provided the minimum oscillator output was 

 sufficient. 



An interesting alternative method was developed in which the 

 synchronizing channel between stations may be omitted entirely, 

 but this method was not used in the present system as the additional 

 cost was not justified. The method, however, is described as it may 

 prove of value if television transmission over long distances is con- 

 sidered. 



