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BELL SYSTEM TECHNICAL JOURNAL 



tions of the 

 be subjected 

 quency-time 

 of distortion 

 siderable. 



In Fig. 24 

 records obta 



vectors representing the direct and indirect waves will 

 to changes at every point where the slope of the fre- 

 curve departs from a simple sine relation. The degree 

 due to the presence of such irregularities may be con- 

 are shown some samples of "wobbled" carrier frequency 

 ined at Stamford, Connecticut. For these records the 



-^ 



pjg 24 Sample fast records showing distortion produced by intentional frequency 



modulation, a day record, b and c night records 



carrier was wobbled at the rate of about 10 cycles per second. There 

 is some uncertainty as to the range of frequency variation for these 

 records although it was probably in the order of a few thousand 

 cycles. By means of a constant frequency local oscillator the radio- 

 frequency wave was stepped down in frequency to audio values which 

 could be amplified and recorded. 



The record (a) of Fig. 24 represents stable day-time reception. 

 The record shows amplitude modulation due to the receiver char- 

 acteristic alone. If the receiver were, as is desirable, capable of 

 amplifying all the frequencies present in the received wave in the 

 same ratio this record would be of constant width. In the sub- 

 sequent examination of night records we must keep in mind the fact 

 that the terminal apparatus is responsible for a certain part of the 

 amplitude modulation. Its influence is readily recognizable. 



The night-time records shown in (b) and (c) reveal a distinct dis- 

 tortion of the envelope aside from that present in the daytime record. 

 Peaks appear and disappear within time intervals sometimes as short 

 as a fraction of a second. 



