MEASrRIXG SYSTEM lOR VIDEO 225 



ferential detector constant. As the control action simultaneously affects 

 both "^"' and "A'" modulators uniformly, the system zero is undisturbed. 



Careful attention has been given to the problem of obtaining an electrical 

 match between '\S"' and "A'" modulators and coaxial cable lengths in the 

 high-frequency channels. (RG 6/U cable contributes a i)hase shift of 

 0.27inch at 3600 kc.) Consequently, with the apparatus under test re- 

 placed by a coaxial strap, a balance indication on the phase and transmission 

 difference indicators may be obtained which shifts less than 0.1 degree in 

 phase and 0.02 db in transmission when the master oscillator frequency is 

 varied over its entire band. 



Because of the frequency independence of the system zero and the auto- 

 matic frequency control of the slave oscillator, the master oscillator may be 

 swept through the entire frequency band for rapid appraisal of the network 

 performance by observation of the phase and transmission difference 

 indicators. 



The component chassis of the set are mounted in a specially designed con- 

 sole, shown in Fig. 4, which places all controls within easy reach of the 

 operator. This console houses as much apparatus as three 6-foot relay 

 racks within a floor space equal to that occupied by a 5-foot laboratory 

 l)ench. Though not visible, a full bay of apparatus is mounted behind 

 the central meter panel. Easily movable partitions and covers permit 

 accessibility to all units, thus expediting maintenance. 



Some of the signiticant design considerations are discussed separately 

 under the following headings: 



(1) Master Oscillator 



(2) Slave Oscillator 



(3) Modulators 



(4) Phase and Transmission Detector 



(5) Phase-shifter 



Master Oscill.\tor 



As indicated in Fig. 2 and Fig. 5C, the master oscillator is of the hetero- 

 dyne type. It employs 15,000 and 11,400-14,950 kc local oscillators. A 

 high degree of frequency stability has been achieved through special oscil- 

 lator circuit design. A motion picture film type scale, 300 inches in length, 

 calibrated every 10 kc, and further subdivided every 2 kc, covers the entire 

 frequency range 50-3600 kc without band-switching. A 0-10 kc inter- 

 polation dial with 100-cycle divisions, which operates on the fixed local 

 oscillator frequency, is used to interpolate between adjacent 2 kc graduations 

 on the main film scale. By oscilloscopic comparison with a 10 kc standard 

 of frequency, the oscillator can be set within 50 cycles of any desired fre- 



