STEADY STATE DELAY A\D APERIODIC SIGXALS 



111 



at which it is a maximum. 7",., the time of maximum envelope, is then a 

 unique instant which is suitable for defining the time at which the dis- 

 turbance occurs. It is determined solely by the slope of the phase frequency 

 curve for the spectrum. 



The instant, T^, may be interpreted, in accordance with the principle 

 of stationary phase, as the one at which the sinusoidal components of (2) 

 are most nearly in the same phase, and so have the least destructive inter- 



Fig. 2— Graphical representation of the phase of an elementary disturbance 



ference. This condition will hold when the instantaneous phase angle is 

 changing least rapidly with frequency, that is, when 



5co 



(a>/ - 0) = 0, 



from which 



/ = 



The angle, Q\ — (s>\Q\, in (6), gives the phase of the wave at the instant, 

 T e , when its envelope is a maximum. The interpretation of this angle will 

 be aided by the geometrical construction of Fig. 2 which is similar to that 



