228 BELL SYSTEM TECHNICA L JOURNAL 



criminator. The voltage from phase discriminator 15 controls oscillator 10 

 according to the difference of phase between the input from stage 9, and an 

 input of reference phase from amplifier 5. This difference of phase is pro- 

 portional to the time integral of the frequency error. The gross effect, 

 therefore, is to control the oscillator 10 according to the controller law, 

 proportional to frequency error -\- lime integral of frequency error, or, in the 

 terminology of feedback regulators, proportional + integral controls When 

 in equilibrium, the system operates with a static phase difference between 

 the phase discriminator inputs, a condition which can exist only when these 

 inputs are of equal frequency. The system is thus endowed with the 

 property zero frequency error, and the frequency at the output of modulator 8 

 is maintained in exact equality with crystal oscillator 2 frequency. Con- 

 sequently the intermediate frequency difference between input, /, and 

 controlled oscillator 10 is held precisely at the value 31,000 cycles. 



Automatic frequency control circuits of the phase sensitive type have 

 been previously described ' ' . 



The system of combined phase and frequency sensitive control in the 

 slave oscillator is superior to those which use only phase or frequency 

 sensitive control. In a control circuit which uses only a phase discriminator 

 and associated reactance tube, the controlled oscillator may lock-in at either 

 of two sideband frequencies. These are/ -|- 31 kc, and/ - 31 kc. Opera- 

 tion is at upper sideband when control stabilizes on the positive slope of the 

 phase discriminator output voltage curve in Fig. 5A, and at lower sideband 

 if control is along the negative slope. Thus an ambiguity of sideband 

 exists, though the attribute of zero frequency error is retained. When only 

 a frequency discriminator and reactance tube are used, lock-in is possible 

 at only one of the two sideband frequencies, determined by the poling of 

 the frequency discriminator output voltage. A frequency error, however, 

 is present. 



The combination in Fig. 5 of the two systems operating jointly utilizes 

 the phase sensitive discriminator to insure close control of oscillator fre- 

 quency, and the polarizing property of the coarser frequency discriminator 

 to eliminate the possibility of synchronization at the undesired sideband. 



The joint system of phase and frequency sensitive automatic control 

 has the further virtue of possessing a far greater degree of stability than is 

 obtainable with the phase discriminator loop acting alone. 



In the heterodyne circuit of Fig. 5B,/ + 31 kc from the automatic fre- 

 quency control circuit is modulated with/ - F, the variable local oscillator 

 frequency of the master oscillator. The frequency at the output of the 

 heterodyne circuit is /«^ + 31 kc, and this is modulated with frequency F 

 in the "5" and "X" modulators to produce the constant intermediate fre- 

 quency, 31 kc, in the measurement portion of the set. 



