A SINGLE-SIDEBAND SHORT-WAVE SYSTEM 491 



considerable selective fading a satisfactory carrier was continuously 

 available. Tests made with these receivers showed that the elimina- 

 tion of one sideband at the receiver did not affect the intelligibility or 

 quality of reception to any extent if allowance were made for the 

 reduction in the received power. 



Description of Apparatus 



For the purpose of obtaining more complete quantitative informa- 

 tion on the improvement to be realized from single-sideband operation 

 and a better understanding of the requirements of commercial single- 

 sideband equipment, apparatus was constructed for a trial of a short- 

 wave single-sideband system across the Atlantic. Tiansmitter input 

 equipment was constructed which was capable of delivering a single- 

 sideband signal to the input of the water-cooled amplifiers used in the 

 short-wave double-sideband transmitters. This input equipment was 

 sent to Rugby, England, and with the cooperation of the British 

 Post Office installed in conjunction with one of the transatlantic 

 transmitters. For comparison purposes the normal double-sideband 

 output of this same transmitter was used. A single-sideband receiver 

 having a number of novel features was also constructed and installed 

 at the transatlantic receiving station at Netcong, New Jersey. During 

 the latter part of 1933 and the early part of 1934 comparative tests 

 of double and single-sideband transmission were conducted between 

 the British Post Office Headquarters in London and the Bell Telephone 

 Laboratories in New York City. 



Transmitting Input Equipment 



Figure 1 shows a rear view of the transmitting input equipment. The 

 equipment is mounted on three bays of panels in two welded steel 

 cabinets, each panel being the width of the usual telephone relay rack 

 panel. A schematic of the input equipment is shown in Fig. 2. The 

 incoming speech is applied to the balanced modulator No. 1, to which 

 is also applied voltage having a frequency of 125 kc, obtained through 

 a multivibrator from a 625 kc. crystal oscillator. The low-frequency 

 filter following the first modulator is of the lattice type of construction 

 and uses quartz crystals as elements ^ in order to obtain the necessary 

 attenuation to the carrier frequency and one sideband while passing 

 the other sideband. This filter passes frequencies from 125.1 kc. to 

 130 kc. The unwanted sideband is suppressed from 40 to 60 db and 



^ For information on the construction of such filters, see article by W. P. Mason, 

 "Electrical Wave Filters Emplo\ing Ouartz Crystals as Elements," Bell Sys. Tech. 

 Jour., Vol. XIII, No. 3, July, 1934. 



