540 BELL SYSTEM TECliSlCAL JOURNAL 



where p = the modulating frequency in cycles per second 



and M = the phase shift of the envelope of the modulated wave in 



degrees. 



In order to measure the value of M, some method of comparing the 

 phases of various currents must be used. Also it is convenient to have 

 in the measuring circuit a phase shifter or some means of controlling 

 the phase of the modulating frequency. 



The value used for the modulating frequency will vary somewhat with 

 the frequency used for measurement and with the conditions under 

 which the measurement is made. Of course, other things being equal, 

 the greater the value of this modulating frequency the greater will be 

 the frequency difference for which the phase shift is measured and the 

 accuracy of the measurement will be correspondingly increased. This 

 is true, however, only when the envelope delay is changing very slowly 

 within this frequency interval. In most cases where the envelope de- 

 lay is changing quite rapidly with frequency, it is necessary, therefore, 

 to use as small a value for the modulating frequency as will give the 

 required accuracy. In practice, both conditions of measurement will 

 be encountered so that some sort of compromise value should be chosen 

 for a particular measuring set which will do fairly well for its require- 

 ments. \'arious modifications of this circuit for loop and straightaway 

 measurements are given in the patent referred to. \ arious methods of 

 modulation and detection may be used. 



(1) The set-up '" shown in Fig. 7 has been used extensively for loop 

 measurements on systems, including various telephone circuits and 

 phase correcting networks. The details of the circuit of this set are 

 not given here, but certain phases of its makeup and operation will be 

 discussed. A frequency of 25 cycles from a tuning fork is used for 

 modulation. In measuring the phase shift of the transmitted envelope, 

 a dynamometer detector and phase shifter are used as described in the 

 patent referred to.^-' 



When the modulated wave as transmitted over the system is de- 

 tected, the modulating frequency is obtained. This will, in general, 

 differ in phase from the original modulating frequency. If the de- 

 tected frequency and the original frequency are now put into the dyna- 

 mometer detector, the phase of one of these frequencies can be shifted 

 by means of the phase shifter until these two frequencies are 90° out 

 of phase, which is indicated by zero reading of the dynamometer. 

 The amount of phase shift which has been introduced in order to bring 

 about this condition is a measure of the delay of the system being 



1* Compare " Phase Compensation III — Nyquist Method of Measuring Time 

 Delay da/dw," E. K. Sandeman and I. L. Turnbull, Electric Communication, Vol. \'II, 

 p. 327, 1929. 



