Sn'P/l-S IX RADIO lUiOADCAST TRAXSMISSIOX 



195 



wobbling or rapid shifting l)ark and forth in frecinency of the aniph- 

 tude modulated carrier. In other words the carrier and side-hands, 

 without change in their relative frequencies, would be subjected to 

 "frequency modulation." 



This sort of thing should be clearly distingushed from the slow 

 wandering of frequency which, for instance, causes beat notes between 

 carriers of different stations to drift gradually in pitch. What we 

 have called "dynamic instabilit\'" is so rapid (being governed by the 

 cyclic variations of the modulator) that it is difficult to observe b\' 

 any aural method. Since the transmitter being used for our tests 



33 ~ 

 Oscillator 



Kaaio 

 Transmitter 



Master 

 Oscillator 



Beating 

 Oscillator 



Local Tone Signal g ^ 



Fig. 39 — Diagram of system used to measure frequency modulation 



was a member of this almost universal class which employs modulating 

 elements directly associated with the oscillator elements we deter- 

 mined to study this aspect of the transmission. 



The following test was made to find out the extent of the fre- 

 ciuency variation during the period of the modulating cycle. A 

 schematic of the testing circuit arrangement is shown in Fig. 39. 

 The plan was to modulate the carrier with 33 cycles, a tone so low 

 in frequency that it would not be efficiently transmitted through the 

 audio frequency amplifier connected to the output of the radio re- 

 ceiver. Then upon beating the received modulated carrier signal 

 down to a frequency of about 1,000 cycles, an oscillogram of this 

 signal would show a 1,000-cycle signal with a 33-cycle modulation in 

 amplitude. Frequency modulation, if present, should then be easily 

 discernible from the record. This experiment was made for day- 

 time transmission and oscillograms (A) and (B) shown in Fig. 40 

 were obtained, one with the frequency of the beating oscillator greater 

 than the carrier frequency, and the other with the beating oscillator 

 frequency less than the carrier frequency. Both of these oscillo- 

 grams show by the change in the frequency of the beat note signal 



