MEASUREMENT OF PHASE DISTORTION 535 



the approximate value of the envelope delay for the system under con- 

 sideration. 



In the three patents referred to, circuits for different purposes are 

 given. In all of these methods both ends of the system to be measured 

 must be available to the tester. The first describes a circuit in which 

 the method of measuring is quite similar to that just described except 

 that by means of a reversing switch the frequency interval is found 

 corresponding to a change in phase shift of one half cycle. This 

 method is suitable for measuring relatively large delays. The second 

 circuit referred to is much the same as the first except that a definite 

 phase shift can be introduced in the path which contains the system 

 under consideration by means of a reactance inserted between two 

 artificial resistance lines of considerable length. The method of opera- 

 tion is exactly the same as before except that here frequency intervals 

 can be measured for changes in phase shift which are not integral 

 multiples of one half cycle. This method is suitable for measuring 

 much smaller values of delay than the first circuit referred to, but is not 

 particularly suited to measuring very small delays. The third method 

 is adaptable to measuring very small values of delay, such as those 

 introduced by separate units of equipment. Here a phase shifter is 

 introduced in the path containing the system under consideration and 

 the change in phase shift through the system for a particular frequency 

 interval is measured. The phase shifter for this purpose should be 

 continuous in its operation ; and in the circuit referred to, the relative 

 phases of the received currents from the two paths are compared by 

 means of a vacuum tube device which indicates a zero condition on a 

 meter when the two received currents are in quadrature. 



c. Direct Measurement of Envelope Delay 



Here the phase shift of the envelope of a modulated wave is measured 

 under steady-state conditions and this gives a direct measurement of 

 the envelope delay when the measuring set is properly calibrated, inas- 

 much as the delay of the envelope of the modulated wave is closely 

 related to the differences in phase shift for the component frequencies 

 of the modulated wave transmitted. Before describing the details 

 of the measuring circuits, some of the principles underlying the trans- 

 mission of simple modulated waves will be considered; and for this 

 purpose envelopes produced by sine wave modulations will be used. 

 It is assumed in this discussion that the modulations in the transmitted 

 current are repeated periodically and that the attenuation of the system 

 used for transmission is completely equalized for all the frequency 

 components. 

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