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BELL SYSTEM TECHNICAL JOURNAL 



Two circuits for measuring delay distortion will be described. The 

 first is a phase measuring circuit which, in principle, is an adaptation of 

 Fig. 4 to practical operation at intermediate frequencies. The second 

 circuit is a modification in which two frequencies differing by A/ are fed 

 through the circuit under test simultaneously in such a way that the differ- 

 ence in the phase shift at the two frequencies is measured. This arrange- 

 ment permits the calculation of the delay from a single measurement and 

 is, therefore, a delay measuring circuit. 



Fig. 5 — Schematic circuit for the precise measurement of phase shift in the interme- 

 diate frequency range. 



Phase Measuring Circuit 



Figure 5 shows a schematic diagram of a phase measuring circuit which 

 has been found to be suitable for precision measurements of wide-band 

 circuits in the intermediate frequency range of 50 to 80 megacycles. It 

 is a double detection system with an intermediate frequency of one mega- 

 cycle. The test signal oscillator and the beating oscillator are ganged to a 

 single control and adjusted to track so that they maintain a difference of 

 approximately one megacycle throughout their tuning range. The test 

 signal is fed to a sliding contact on a section of air dielectric coaxial trans- 

 mission line. The signal divides at this point to feed the test branch and 

 reference branch. 



The sliding tap on the coaxial line provides a high precision measuring 

 phase shifter if the coaxial line is well terminated at each end. The rela- 

 tive change A/3 in phase of the signals at the two ends of the line when the 



