328 BELL SYSTEM TECHNICAL JOURNAL 



to the output at any one time is lost. This is estimated to be equiva- 

 lent to not more than an increase of one decibel of transmitted power. 

 To offset this there must be considered the possibility of the delay 

 being out of adjustment for brief periods when changes in angle require 

 sudden changes in delay equalization. The necessity for close phase 

 uniformity between the various carrier and sideband amplifiers over 

 a wide range of automatic volume control voltage is also eliminated 

 when the automatic branch selector is used. Further, it is no longer 

 necessary to use the received carriers for demodulation of the various 

 branch outputs and a locally generated carrier of uniform amplitude 

 can be used with a resultant increased stability of output volume. 

 The practicability of trying to phase branch outputs by delay equaliza- 

 tion over a range of more than 3,000 cycles from the carrier has not 

 been demonstrated and consequently the use of automatic branch 

 selection with the channel whose sideband is spaced by one sideband 

 width from the carrier is to be recommended. The use of the branch 

 selector also permits simpler operation of the automatic angle ad- 

 justing equipment as will be explained later. For all of these reasons 

 it is to be expected that the automatic branch selector may eventually 

 be used to the exclusion of the delay compensating equipment. 



Automatic Angle Adjusting Equipment 



In the experimental musa receiver the rectified carrier output of 

 the monitoring branch was connected to one set of deflection plates of 

 a cathode ray oscilloscope and a sweep circuit mounted on the monitor- 

 ing phase shifter shaft connected to the other set of deflection plates. 

 The oscilloscope screen displayed a graph of the amplitude of signal 

 received for each phase shifter setting. The pattern frequently 

 changed rapidly from moment to moment so that only by constantly 

 observing the screen was it possible to determine at what phase shifter 

 settings the best signals were being received and to set the diversity 

 phase shifters accordingly. 



The attention necessary to operate satisfactorily the equipment in 

 this manner was believed to be too great for commercial operation, 

 particularly since it might vary widely from hour to hour. With a 

 mind to the fact that improper adjustment might give poorer reception 

 than would be obtained with ordinary receivers it was decided to make 

 the settings of the phase shifters of the diversity branches automatic. 



In Fig. 8 the motors A, B, C and D drive the phase shifters of the 

 corresponding branches through the vertical shafts. Motor D 

 operates continuously so as to vary the phase shifting system through 

 its complete range once a second, while motors A, B and C operate 



