324 BELL SYSTEM TECHNICAL JOURNAL 



branch circuits must follow the same law, i.e., if Hnear rectifiers are 

 used, the final demodulators should be linear, and if square-law recti- 

 fiers are used, the final demodulators should be square-law. When 

 linear demodulators are used the output noise is independent of the 

 strength of the incoming carrier and since the gains of all branch 

 amplifiers are the same the noise output of each branch will be the 

 same, assuming that received noise does not vary with the vertical 

 angle of reception. As a consequence the total noise will be equal to 

 the product of a single branch noise and the number of branches 

 regardless of the signal contributions of each branch. If square-law 

 detectors are used, however, the noise output of a branch will go down 

 when the carrier in the final demodulator of that branch goes down 

 and consequently the total noise will be proportional to the total 

 signal. In a three-branch diversity system a theoretical improvement 

 in signal-to-noise ratio varying up to 4.77 db can be had by using 

 square-law demodulators rather than linear. For this and other 

 reasons square-law final demodulators have been used in this 

 equipment. 



When delay equalization is used between the various diversity 

 branches it is essential that the received carrier be used in the final 

 demodulation process. Small changes in the lengths of the paths in 

 space traversed by the sidebands being received by the various 

 diversity branches make the phases at random and if all branches 

 were demodulated by a common carrier the audio outputs would not 

 add in phase. By using the carrier arriving over each path for the 

 demodulation of the accompanying sideband the random relation 

 disappears and the audio outputs can be added in phase. 



When using an automatic branch selector which discretely chooses 

 one branch at a time for connection to the line it is no longer necessary 

 to consider phases in the diversity branches and a local carrier is used 

 because it reduces output amplitude variations. With only one 

 diversity branch connected to the output, only the corresponding 

 volume control rectifier should be contributing to the automatic 

 volume control voltage if the output volume is to be held as constant 

 as possible. This result is obtained by putting a rectifier in the d.c. 

 output lead of each branch volume control rectifier so that only the 

 volume control rectifier having the highest amplitude will supply 

 current to the load resistance. 



Delay Circuits and Switches 



On account of the fact that waves arriving at different vertical 

 angles have taken different times in transit it is necessary to insert 



