716 



BELL SYSTEM TECHNICAL JOURNAL 



by it is considerable. It is only where the rate of variation of the instan- 

 taneous frequency is high, as it can be in the case of a large noise transient 

 caused by impulse excitation, that the error in the assumption in question 



FROM 

 LIMITER 



■■=^-3 



r 



TO 

 AUDIO 



T 



Vi 



f 



Y2 



Bo< :4^C 



4>f 

 r 



y^i^.l-W] 



l_iM 



T) = DETECTION 

 EFFICIENCY 



ENVELOPE OF 

 FUNCTION OF 

 TIME,V 



Fig. 1 — Circuit of a balanced frequency detector. 



Fig. 2 — Voltages across tuned circuits of frequency detector. 



Fig. 3 — Output voltage of frequency detector. 



can become serious. A particularly subtle error that can arise from the 

 assumption is to fall into the habit of regarding the characteristic curve of 

 Fig. 3 as a frequency transmission curve of the sort obtained by measuring 

 the ratio of output to input of a linear network over a range of frequencies. 



