MEASUREMENT OF DELAY DISTORTION 



251 



repeaters for this service an accuracy of better than 10-^ microseconds in 

 measuring the relative delay over a band of frequencies will be desirable. 

 The phase shift, A/3 in Fig. 3, which corresponds to a delay of 10-^ seconds 

 is a function of the measuring interval Aco = IttAJ. If A/ is small AjS will 



FREQUENCY, U) *' 



Fig. 3 — Factors involved in calculating the delay of an electrical circuit 



Fig. 4 — Basic circuit for measuring the phase shift in a network 



be small and difficult to measure. If A/ is large the average slope measured 

 will not be the true slope in the center of the interval. For a 5-megacycle 

 video signal the intermediate and radio frequency bands of interest will 

 be in excess of 10 megacycles wide. These considerations led to a choice 

 of 1 megacycle as a reasonable value for A/. If A/ = 1 megacycle a phase 

 shift A/3 = 0.36° will result from a delay of 10'^ seconds. 



