THERMIONIC VACUUM TUBES 



75 



VIII. Thermionic Detectors 



Like the modulator the detector is a device for the production and 

 separation of difference frequencies. The object of modulation is, in 



general, to transform a high frequency ■—- and a low frequency ;^- 



2tt 2ir 



into two high frequency side bands, v . Detection accomplishes 



2ir 



■b 

 the inverse operation of forming from a carrier frequency ~- and either 



2t 



or both side bands the original low frequency ~, detection often being 



referred to as demodulation. Detection, like modulation, can be most 

 readily described by the consideration of a single pair of frequencies. 

 When carried out by means of a vacuum tube it results from rectifi- 

 cation in either the grid circuit or the plate circuit. This rectification 



Fig. 43 



may arise either from unilateral conductivity or a curved current- 

 voltage characteristic as pointed out in the following paragraphs. 



38. Detection by Curved Plate Characteristic. Considering the circuit 

 shown in Fig. 43 and assuming an input voltage e = A (B-\-cos qt) cos pt, 

 it follows from Equation 4 that the output current, considering only 



those terms whose frequencies are of the order —, is 



2ir 



Jd= h^+r^ AKB cos qt+ \ cos 2 qt) - 



(13) 



The current Jd, known as the "detected current," therefore, consists 



of a term whose frequency is ^- and another term whose frequency is 



2tt 



twice this. The presence of these two frequencies is readily under- 

 stood. The detected current of frequency ^- corresponds to the 



2tt 



