MEASUREMENT OF PHASE DISTORTION 527 



taining a resistance line (introducing only constant attenuation) and 

 the other the system under consideration, to a detector which measures 

 the magnitudes of the currents received from the two paths. Initially, 

 by adjustment of the attenuation in the distortionless path the 

 magnitudes of the two received currents are made the same. Then by 

 operation of the switches the vector sum and difference of the two 

 received currents can be measured. From the amount of attenuation 

 introduced in the common path to the input of the detector to make the 

 sum and difference equal, the difference in phase of the two received 

 currents can be computed. This is the insertion phase shift of the 

 system under consideration. This method of measurement has been 

 only briefly described here, as all of its details are described in the 

 patent referred to. This method does not involve an elaborate set-up 

 of apparatus and gives accurate results, and is particularly useful 

 where very small amounts of phase shift are involved. 



2. Measurement of Delay 



A number of measuring methods will now be described which vary 

 somewhat in the amount of apparatus required and in the convenience 

 with which the measurements can be made. The method of measuring 

 envelope delay from impedance measurements is given first because of 

 the very small amount of apparatus required, and for certain interest- 

 ing steady-state phenomena which will appear in discussing the method 

 of its operation. Following this, several modifications of a method 

 will be referred to for determining the envelope delay from phase shift 

 measurements. The application of this method and the determination 

 of the results are usually quite laborious, but are given here since the 

 apparatus required is fairly small in amount and usually readily avail- 

 able in a laboratory. When a number of delay measurements are to 

 be made so that the saving of time during measurement is of impor- 

 tance, direct measurements of envelope delay can be made using 

 somewhat complicated measuring circuits which require considerable 

 time for building and calibrating, but which will allow measurements 

 to be made simply with the loss of relatively little time. Several 

 such circuits will also be considered. 



a. Determination of Envelope Delay from Impedance Measurements 



A method is given here for determining the envelope delay from 

 steady-state impedance measurements. The method is limited to 

 the measurement of systems which are capable of transmitting in 

 both directions, and in certain cases it is further restricted in that both 

 terminals of the system under test must be readily available at the 



