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BELL SYSTEM TECHNICAL JOURNAL 



greater in amplitude than the other at the input to the Umiter the relative 

 difference in level is increased by an additional amount of about 6 db at the 

 limiter output. The limiter output may be considered as the stronger sig- 

 nal frequency-modulated by the weaker signal. For small modulation 

 indices the amplitude of the first order sideband is approximately one-half 

 the modulation index thus explaining the 6 db added difference in level at 

 the limiter output. As the input levels approach equality the added level 

 difference decreases to zero. A block diagram indicating the arrangement of 

 such a diversity system is shown in Fig. 35. Tests were made of both paral- 

 lel and series connections of the two discriminator outputs. With a parallel 

 connection the discriminator having the greater output blocks the rectifier 

 output of the other discriminator and thus gives a sharp diversity selection 

 characteristic. However, the level ratio of the channels at which a switch 



ANTENNA 



U 



LU 



RADIO FREQUENCY INPUT 

 RECEIVER CONVERTER FILTER 



DISCRIMINATOR 

 AND RECTIFIER 



fA 





COMMON 

 LIMITER 



^ B -■ 



Fig. 35. — Block diagram of dual-diversity FS receiving system using a common limiter. 



takes place is affected not only by the ratio at the limiter input but also by 

 frequency drift and discriminator slope. With the series connection only 

 the input level ratio at the limiter input affects the diversity switching; 

 this arrangement was therefore selected as the preferred method although its 

 selection characteristic is not sharp. The series and parallel combining 

 characteristics are shown in Figs. 36 and 37. 



Various tests were made on a terminal having a 1500-cycle bandwidth 

 and using the series combining method to determine the signal-to-noise 

 characteristics under different conditions of diversity fading. A frequency 

 shift of 850 cycles was used and the midband frequencies of the two diver- 

 sity channels at the common limiter input were v30 and 35 Kc. Figure 38 

 shows the distortion versus signal-to-noise ratio characteristics of each 

 channel separately and in diversity combination for various relative level 

 conditions of the two channels. During diversity operation equal noise 

 levels were maintained in the two channels and various combinations of 

 level differences of the two channels were preserved as the whole signal 

 level combination was varied. The level differentials are indicated in 



