210 



BELL SYSTEM TECIIMCAL JOURXAL 



the agenc\- of the resupplied carrier of the beating oscillator used to 

 l)riiig them down to audio-frequenc\'. If now we were to take two of 

 these frequencies shown on the record and multiply their amplitudes 

 together at each point we would obtain the amplitude of the signal 

 which would result if one of them were a single side-band and the 

 other its accompanying carrier. It is ob\ious that the fading \aria- 

 tions would thereby be increased in amplitude and rapidity. 



In order to obtain a comprehensiAC picture of the relative advantages 

 of radio transmission using a carrier and one side-band as compared 



Y 



Beating 



. OsciMator 



Audio -T for Sfow 



Frequency SID Paper 



_ Amplifier J Records 



.ow Pass 

 rilter 

 1100" 



Audio -J 



Frequeno a^ 

 AmoVfier'-l 



Fig. -49 — Diagram of system used to obtain records of transmission with carrier and 

 one side band and carrier and both side-bands 



with the common practice of transmitting both side-bands, the follow- 

 ing tests were made. The schematic diagram of the circuit arrange- 

 ment is shown in Fig. 49. At the transmitter the carrier and both 

 side-bands are transmitted and at the receiver they were selected out 

 by means of filters in the manner previously explained. The signals 

 from the filters corresponding to the carrier and lower side-band were 

 applied to the input of a detector circuit and from its output the de- 

 tected difference signal was selected by a low-pass filter. This signal 

 was equivalent to that which would be received if only the carrier and 

 one side-band w^ere transmitted. From the output of the radio receiver 

 a branch circuit goes to a low-pass filter which transmits only the 

 signal detected from the carrier and both side bands, suppressing from 

 this circuit the higher frequency signals corresponding to carrier and 

 side-bands produced by the beating oscillator and received signals. 



By making simultaneously a record of these two signals a direct 

 comparison is obtained of the effect of selective fading on their am- 

 plitudes. Fig. 50 shows samples of several such records made at 

 Riverhead, L. I. The modulating frequency for strips 1, 2 and 3 is 

 2o0-cycles, and for strips 4 and 5, 500-cycles. The record on strip 3 

 is shown on account of the peculiar chaiacteristic of the signal fading, 

 for considerable periods of time remaining at relatively low amplitude. 



