MEASl'KfA'G SYSTEM FOR VIDEO 



235 



The pluise-shiftcr's deviation from linearity is sulliciently small that no 

 correction is needed on the coarse scale. 



Both dials can be moved with respect to their shafts by releasinj^ friction 

 clutches. Thus the measuring system phase zero can be established by an 

 initial balance of the phase-shifter and restoration of the coarse and fine 

 scales to zero. This is onl}- possible because the scales are linear. Thus no 

 zero readings have to be subtracted from the measurement readings and the 

 need for a separate zero setting phase-shifter is avoided. 



The phase-shifter is calibrated by a method of substitution. As dis- 

 cussed previously the phase-indicator indicates balance uniquely in mul- 



11 — Phase shifltT scales and projected index. 



tiples of 360° phase-shift. E.xact sub-multiples of 360° can be generated 

 and used to calibrate the phase-shifter. 



For example (Fig. 12), to establish an exact 180° phase-shift the standard 

 phase-shifter is set to an arbitrar}- starting point. With the switches in 

 the position shown a null is obtained on the indicator by adjusting the 

 auxiliary phase-shifter. The network of nominally 180° phase-shift is 

 inserted and a null obtained on the indicator by adjusting the standard 

 phase-shifter. Now the network is removed and the null reestablished by 

 adjustment of the auxiliary phase-shifter. The 180° network again is 

 inserted and a null obtained by adjustment of the standard phase-shifter, 

 which now has been moved through twice the actual phase-shift of the 



