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BELL SYSTEM TECHNICAL JOURNAL 



(2) The phase shift from the transmission Hne to the phase shifter 

 stage must be aHke in all six circuits, independent of wave-length. 



In order to simplify the experimental job it was decided to dispense 

 with the selectivity afforded by high-frequency amplifiers and to use 

 the simple circuits shown in Fig. 10. The capacitive coupling to the 

 transmission line is a convenient means of matching the low-impedance 

 lines to the high-impedance circuits. Plug-in coils (L) are used to 

 cover the range from 4.5 to 22 megacycles. 



The first detectors are of the two-tube balanced type which sup- 

 presses interference from two signals differing by the intermediate 



ADJUSTABLE 



COUPLING 

 CONDENSER 



INTERMEDIATE 



FREQUENCY 



CIRCUITS 



PARALLEL-BALANCED 

 DETECTOR TUBES 



TO BRANCH A 

 PHASE SHIFTER 



TO BRANCH B 

 PHASE SHIFTER 



TO BRANCH C 

 PHASE SHIFTER 



Fig. 10 — Input circuit, first detector, and first intermediate-frequency tubes. 



frequency and isolates the beating oscillator supply from the input 

 circuits. The latter prevents crosstalk between the six inputs, and 

 assures independence in the tuning of the input circuits. The beating 

 oscillator voltage is introduced, at low impedance, between cathodes ^® 

 by means of the distributing system of equal length coaxial lines shown 

 in Fig. 1 1 . This distributing system gives equiphase beating oscillator 

 inputs to all detectors and makes requirement (2) attainable by having 

 nominal similarity in the remaining parts of the six circuits. 



Requirement (1) is met by feeding a test oscillator of 78 ohms im- 

 pedance into the first circuit jack and adjusting the tuning condenser 



^^ W. A. Harris, " Superheterodyne Frequency Conversion Systems," Proc. I. R. E., 

 vol. 22, pp. 279-294, April, 1935. 



