RADIO BROADCAST COVERAGE OF CITY AREAS 



137 



showed a cutofif of only 20 TU, corresponding to an audio-frequency 

 current reduction to 0. 1 that of the value at resonance. The curves will 



Kilocycles FrorA Resonance at 900 kc. CXSSSM) 



Fig. 12 — Receiving set selectivity characteristics as measured from samples of 

 receivers having different types of selective circuits 



be seen to group themselves more or less into three classes in the order 

 of their selectivity merit as follows: 



1. The single-tuned circuit (non-regenerative and regenerative), and 

 the combination of two tuned circuits coupled together. 



2. Circuits employing radio-frequency amplification with tuned 

 circuits between stages. 



3. The double-detection or superheterodyne type of circuit. 



The curve for the double-detection type of circuit shows a "come- 

 back" which represents the familiar double-tuning effect. (Inci- 

 dentally, the admittance of this particular set, which was not a 

 commercial set, needs to be reduced by the use of more selectivity at 

 the radio frequency.) 



For comparison purposes there has been added to the figure the 

 curve marked "ideal selectivity characteristic," in accordance with 

 which the receiving set would pass without attenuation all frequencies 

 up to 5000 or 10,000 cycles and would cut off abruptly all frequencies 

 without this band. Attention is first called to the fact that the 

 various circuits attenuate within the desired transmission band of 



