BELL SYSTEM TECHNICAL JOURNAL 



the side bands alone are transmitted, a locally generated high fre- 

 quency exactly equal in frequency and phase to the original carrier 

 frequency must be supplied. 35 This is known as homodyne reception. 



41. Measurement of Detection Coefficient. The constant a in the 

 relation Jd = ae 2 is called the "detection coefficient." Its measurement 

 by direct means is not difficult but as seen from Equation 13 it involves 

 so many factors that no satisfactory indirect methods of determina- 

 tion have been developed. The requirements of the direct method 

 are quite obvious, and for circuit details reference is made to the 

 Thermionic Vacuum Tube by van der Bijl. 



42. Detecting Efficiency. A knowledge of the detecting coefficient a 

 tells very little about the detecting efficiency of a tube, the efficiency 

 being defined as the ratio between the low frequency energy in the 

 output and the high frequency energy in the input. The efficiency 

 involves the input impedance of the tube which is a function of the 

 circuit constants as well as the tube. It is therefore impossible to 

 specify the detecting efficiency of a tube without certain data con- 

 cerning the circuit in which it is to be used; a is therefore without 

 much significance. 



Plate Voltage 

 Fig. 49 



43. Detecting Coefficient and Plate Voltage. The variation of the 

 detecting coefficient with plate voltage depends upon the type of 

 circuit. If detection is accomplished by a curved plate characteristic, 

 experiment shows (see Fig. 49) that the operation is best when the 

 effective voltage is about equal to the potential drop in the filament, 



"See J. R. Carson, Procd. Institute of Radio Engineers, Vol. II, p. 271, 1923. 



