MEASUREMENT OF DELAY DISTORTION 255 



The signal oscillator frequency is then changed to/2 = /i + A/ and the slider 

 readiusted to the position X2 that again makes 4>c = <i>B' Then as above 



,' = ?^^[^ + 2.,-.] 



(7) 



where /3' is the phase shift in the unknown network at frequency f^. The 

 Helav in the network is, from (3) 



Substituting (6) and (7) in (8) yields ^ 



The first term in T is a constant of the set-up and can be dropped when the 

 delay distortion only is desired. The second term is small and can often 

 be neglected. The third term gives the major part of the difference between 

 the delay of the reference path and the delay of the network. 



It has been found that the slider position can be easily reset to dz 0.05 

 centimeters for each frequency. This would mean a maximum error of 

 =b 0.1 centimeter for the difference of two readings which corresponds to 

 an error in T of about ±0.4 X 10~^ seconds. However, this reset accuracy 

 will not be realized in overall accuracy unless a number of precautions are 

 observed in setting up the circuit of Fig. 5. In order to avoid stray coupling 

 the two branches of the system must be carefully shielded from each other. 

 At least 60 db net attenuation must be provided between the detectors via 

 the path through the balancing line, phase shifter and unknown network, 

 and 40 db attenuation in the path via the BO supply lines. All attenuators, 

 plugs, etc., must have voltage standing wave ratios of less than about 

 1.015 and the detectors and amphfiers in each branch must have identical 

 phase shifts over the range of variation of the IF due to imperfect tracking 

 of the oscillators. 



Delay Measuring Circuit 



The circuit of Fig. 5 is basically a phase measuring circuit. It can be 

 rearranged as shown in Fig. 8 to yield a circuit that will read delay directly 

 from a single setting of the slider. In Fig. 8 the signals from the two 

 oscillators are both sent through the circuit to be measured, and both are 

 sent through the reference branch. Any delay in either path will alter the 

 relative phases of the two signals in that path and this change in relative 

 phase shift will be passed on to the beat note formed in the detectors. If 



