50 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



Table VII — Required RF Receiver Selectivity versus 

 Antenna Separation 



is said to be desensitized. When two or more strong unwanted signals 

 are present desensitization also occurs, but in addition, extraneous fre- 

 quencies are generated by intermodulation in the receiver itself. As the 

 levels of the unwanted signals become greater than about 75 db below 

 one watt (1 or 2 millivolts across a typical receiver) the intensity of the 

 modulation products rises rapidly above the set noise. The resulting 

 interference can be 60 db or more above set noise and the number of 

 the modulation products increases by at least the cube power of the 

 number of operating channels. 



Ideally, the intermodulation interference in the receiver caused by 

 100-watt transmitters (20 db above one watt) can be eliminated by 

 20 + 75 = 95 db RF selectivity even when the receiver and the unwanted 

 transmitters are connected to the same antenna. In practice, the effect of 

 geographical separation assuming the free space loss given in Table I 

 reduces the RF selectivity requirement to the values given in Table VII. 



The RF selectivity requirements given in Table VII cannot be ob- 

 tained on nearby channels. The approximate RF bandwidths associated 

 with various amounts of RF selectivity in mobile receivers is shown in 

 Table VIII. For example, in mobile receivers it seems feasible to provide 

 40 db of RF selectivity at frequencies removed from the desired chan- 

 nel by about 3 mc in the 150-mc band and by about 10 mc in the 450-mc 

 band. At fixed stations the RF bandwidth required for a given selec- 



Table VIII — Frequency Spacing from Midband versus 

 RF Selectivity 



