258 BELL SYSTEM TECHNICAL JOURNAL 



movement. This is helpful in judging the effect of adjustments of the 

 unknown network. The condition (15) corresponds to the optimum 

 condition for the circuit of Fig. 5 which requires that the average delays of 

 the two branches be equal. In the circuit of Fig. 8, the condition (15) can 

 be reaHzed by varying either I or a. If a is made zero and (15) is fulfilled 

 by adjusting t, errors due to variations in A/ are minimized. However, if 

 A/ is held sufficiently constant, the balancing line I can be omitted entkely 

 and a 360° variable phase shifter introduced in the beat note circuit of one 

 branch to vary a. 



The measuring interval A/ is determined by the difference in the two 

 signal oscillators. Since A/ has an important influence on the measurement, 

 oscillator tracking cannot be relied upon to maintain A/ with sufficient 

 accuracy. A one-megacycle crystal frequency checker has therefore been 

 included in the equipment as shown in Fig. 8. A sample of the signal from 

 one of the amplifiers is compared with the crystal oscillator and a trimmer 

 on one of the oscillators adjusted for zero beat before measuring each 

 point. When T is so large that a balancing fine is not practicable, as in 

 the case of loop circuits including radio paths or long transmission fines, 

 A/ must be held constant to about 1 part in 10^. This has been accom- 

 pfished in a modification of this equipment built by Messrs. W. J. Alber- 

 sheim and J. P. Shafer of these laboratories, by deriving the two measuring 

 frequencies from a crystal oscillator. 



In order to obtain the absolute value of T, the constants I, v, and a must be 

 known. The value of ^may be found from the physical length of the line; a 

 can be measured by measuring the movement of the slider required to rebal- 

 ance the circuit when the connections to the two detectors are reversed. The 

 absolute delay is particularly sensitive with respect to the difference be- 

 tween 2x and a. If a is measured as accurately as possible, then the exact 

 setting of the sUder corresponding to condition (15) can be found by re- 

 versing the connections of the slider at points A and B in Fig. 9. In this 

 way a reference value of x may be found which will yield accurate results 

 in spite of a small error in the value of a. 



Successful operation of this circuit depends on low standing waves 

 throughout and upon sufficient padding for satisfactory isolation of the 

 oscillators. It will be noted that in the analysis it was assumed that /i 

 reached the slider via the path AC, Fig. 9. There must be an attenuation 

 for/i in the path AFBC sufficient to render the signal traversing this path 

 negligible. Similar unwanted paths exist for /2 from B to C and also for 

 /i and /2 to point F. Attenuation inserted as shown in Fig. 8 can be ar- 

 ranged to make these unwanted signals negligible. 



