. The Bell System technical journal . illustrative of the generalmanner in which the properties of the vacuum tube may be appliedto the problem of 82 For other types of modulator circuits see a paper by R. A. Heising, Procd. Inst,Radio Engrs., Aug., 1921. THERMIONIC VACUUM TUBES 75 VIII. Thermionic Detectors Like the modulator the detector is a device for the production andseparation 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 2i
. The Bell System technical journal . illustrative of the generalmanner in which the properties of the vacuum tube may be appliedto the problem of 82 For other types of modulator circuits see a paper by R. A. Heising, Procd. Inst,Radio Engrs., Aug., 1921. THERMIONIC VACUUM TUBES 75 VIII. Thermionic Detectors Like the modulator the detector is a device for the production andseparation 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 ■bthe 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 mostreadily described by the consideration of a single pair of 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 circuitshown 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 76 BELL SYSTEM TECHNICAL JOURNAL difference frequency (See Equations 10 and 11) of the carrier of ampli- Atude AB and each side band of amplitude — and is therefore propor-tional to 2 .—. AB. The second term of the detected current rep- r
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