A SINGLE-SIDEBAND SHORT-WAVE SYSTEM 501 



the outputs of these balanced demodulators will be in quadrature and 

 the frequency will be the beat frequency, A, between the incoming and 

 local carriers. These voltages operate a variable reluctance type 

 synchronous motor ^ which is mechanically connected to a condenser 

 which forms a part of the local carrier oscillator circuit. The motor 

 operates until the frequency of the local carrier oscillator is exactly the 

 same as the carrier at intermediate frequency, when the frequency 

 applied to the two-phase motor becomes zero. 



For distortion testing the receiver can be used as an harmonic ana- 

 lyzer, the frequency of the beating oscillator being shifted so that only 

 the desired distortion product passes through the narrow crystal 

 filter. Measurements made in this way when the transmitter and 

 receiver were close together checked very well with measurements 

 made using the monitoring unit previously described. 



A balanced second demodulator system was used, as the distortion 

 is much less than with other types. No attempt is made to separate 

 the incoming carrier from the sideband in the second demodulator, the 

 amplitude of the reconditioned carrier or the local carrier supplied to 

 the second demodulator being several times the amplitude of the 

 carrier transmitted with the sidebands. 



Experimental Results and Discussion 



To determine experimentally in a quantitative manner the relative 

 merits of two radio systems, such as the single and double-sideband 

 systems, is a matter of considerable difficulty. However, as a practical 

 matter, the percentage of increased commercial time and the increased 

 satisfaction which a customer may obtain are of great interest. Three 

 types of tests have been used in the past for rapidly obtaining in- 

 formation on the performance of radio circuits. They are: (a) deter- 

 mining the signal-to-noise ratio, (b) articulation tests, and (c) ob- 

 servations of circuit merit. 



A measurement of the signal-to-noise ratio is made by modulating 

 the transmitter a given amount and measuring the tone at the receiving 

 point. The tone is then removed and the noise measured with the 

 same equipment. When fading conditions are severe a considerable 

 degree of skill is needed to obtain consistent measurements. 



Articulation tests may be made in the manner which has been 

 described by Fletcher and Steinberg.-* They may consist of the reading 

 and recording of meaningless syllables, carefully chosen words inserted 



3 U.S. Patent No. 1,959,449. 



* "Articulation Testing Methods," H. Fletcher and J. C. Steinberg, Bell System 

 Technical Journal, October, 1929. 



