MEASUREMENT OF DELAY DISTORTION 



249 



and curves (c) for coupled double tuned circuits. It is felt that the delay 

 curves of Fig. 2 are easier to interpret and give a better physical picture 

 of the distortion resulting from phase variations than the phase curves of 

 Fig. 1. Therefore, most of the following discussion will be in terms of 

 delay rather than phase. 



6.0 



5.5 



5.0 



4.5 



(0 4.0 

 o 



z 

 o 



HI 3.5 

 «0 



z 



.►3.0 



xCJo" 



2.5 



1.5 



1.0 



0.5 



0.4 0.5 0.6 0.7 



as 0.9 



1.0 1.1 



{J 

 Wo 



1.2 



1.3 



1.4 



1.5 



1.7 



Fig. 2 — Typical delay curves for various types of circuits 



a. A transmission line terminated in its characteristic impedance. 



b. A single tuned circuit. 



c. Two tuned circuits with approximately critical coupling. 



Thus far we have considered the general case and stated that the delay 

 must be constant over the essential frequency band for distortionless trans- 

 mission. It should be noted, however, that when delay distortion is pres- 

 ent different types of signals may be affected differently. For instance, in 

 the case of an amplitude modulated carrier of angular frequency wo, Fig. 

 2, we note that if the delay curves have arithmetic symmetry about coo, then 

 the sidebands at coo ± Aw will suffer the same delay and therefore will add 



