MEASUREMENT OF DELAY DISTORTION 



253 



slider is moved a distance Aa: is two times the phase shift corresponding to 

 the movement of the sUder at the working frequency or 



A^ = 



nof^x 



degrees 



where c is the velocity of light. 



For Ax = 0.1 centimeter and/ = 65 megacycles, AjS is 0.156 degrees. 



The measuring branch and reference branch feed through the network to 

 be measured and a balancing line respectively to identical detectors and 

 one megacycle amplifiers. The amplifiers are connected to the plates of a 

 cathode ray oscilloscope which is used as a phase comparator. A cathode 



x = A cos cji 



Y = A SIN (Cjt + 9) 

 S = 2A SIN e 



IF A = 3"AND e- o.r 



THEN S ^ 0.01" 



Fig. 6 — Calculation of the sensitivity of a cathode ray oscilloscope as a phase com- 

 parator. 



ray oscilloscope has the advantage that the phase comparison with this 

 instrument is independent of either the relative or absolute amplitudes of 

 the two signals. A straight line on the oscilloscope always indicates that 

 the two voltages are in phase (or phase opposition) regardless of the relative 

 amphtudes, which determine the slope of the fine. The sensitivity, of 

 course, is a function of the amplitudes. Figure 6 illustrates how the sensi- 

 tivity of an oscilloscope phase comparator can be calculated. If each 

 signal alone produces a 6-inch deflection, then 0.1 degree phase difference 

 produces an opening of the pattern of 0.01 inches, which is sufficient to be 

 detected on a "rocking" or in-out basis. 



It is obvious that a circuit of this type measures the difference in the phase 

 shifts of the two branches. The measured phase shift is, therefore, the 

 absolute phase shift in the measuring branch less the phase shift in the ref- 



