MEASUREMENT OF PHASE DISTORTION 



539 



sideband frequencies and, although it is not accurately the envelope 

 delay for the carrier frequency, it may be taken as such for all practical 

 cases when the modulating frequency is taken small enough so that the 

 slope of the phase shift-frequency curve for the carrier and the two 

 sidebands may be considered constant. 



AUXILIARY 

 PHASE SHIFTER 



PHASE SHITTER 



Fig. 7 — Arrangement for direct measurement of envelope delay. 



shifter shown R = SOttL = I/SOttC. 



In the phase 



Fig. 7 shows schematically a circuit for measuring the envelope delay 

 by measuring the phase shift of the envelope. ^^ The carrier frequency 

 is modulated with another frequency, 25 cycles for example, and then 

 transmitted through the system to be measured. At the receiving end 

 an ordinary amplifier-detector is used to demodulate the received wave 

 and obtain the modulating frequency. This source can then be 

 compared in phase with a reference frequency which is obtained from 

 the original source. In order to avoid including the effects of the 

 measuring apparatus itself, the change in phase shift so measured 

 through the system under consideration should be compared with a 

 similar measurement made with an artificial resistance line substituted 

 for the system under test. The difference of these two will, of course, 

 be the phase shift suffered in the system by the envelope of the modu- 

 lated current; and the envelope delay of the system in seconds at the 

 carrier frequency, /, is then given approximately by 



^ 360 p ' 

 '^U. S. Patent 1,645,618. 



