1150 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1956 



any instant. An optical system, Fig. 9(a), produces on the unexposed 

 film an image of the illuminated portion of the master. As the oscillator, 

 its film, and the master advance, the markings on the master can be re- 

 produced on the new film. 



The problem in control is to cause each mark on the master film to 

 pass the slit just as the oscillator goes thru the corresponding value of 

 frequency. To do this we drive the oscillator and its scale together at a 

 constant linear speed. The oscillator frequency increases steadily but 

 not at a constant rate. Its rate of increase varies according to the law of 

 its particular cavity. So our problem reduces to causing the master film 

 to move according to that same law. 



The method is to time the passage of known points in the oscillator 

 frequency spectrum, and then to pace the movement of the master film 

 to maintain precise correspondence. The pacing is done by detecting 

 small differences in times of arrival at corresponding points and correct- 

 ing the speed of the master film to keep successive differences small. 

 Fig. 10 is a block schematic of the automatic control system. The varying 

 oscillator output passes through multiples of 10 kc at a rate near five mul- 

 tiples per second. When it is compared in a balanced modulator with 



APERTURE 



sow. PROJECTION 

 LAMP 



UNEXPOSED - 

 FILM 



(A) 



DQDai! 



/aDaDDDDDDDDDDaDD 



1.200 



1,300 



i I I I I I I I I I I I I I I I I I I I I I I 

 aDDaDDDDaDDDDDDDDDD 



(B) 



Fig. 9 — Film scale calibrator, (a), optical system schematic; (b) section of 

 master film. 



