THERMIONIC VACUUM TUBES 



79 



40. Heterodyne and Homodyne Detection. In continuous-wave radio 

 telegraphy, the dots and dashes of the code are transmitted by a con- 

 tinuous carrier wave of a single frequency. Heterodyne reception 

 consists in supplying a slightly different frequency at the receiving 

 station, the transmitted and locally generated frequencies when ap- 

 plied to the detector acting exactly as the carrier and side band fre- 

 quency described above. The useful output of the detector is the 

 difference frequency which, of course, is chosen in the audible range. 



It follows from Sec. 38 that the heterodyne detecting current is 

 proportional to the product of the amplitudes of the transmitted and 

 locally generated waves. Because of this fact a feedback circuit may 

 be used to advantage as a means of increasing the strength of both 

 high frequency terms. In the usual type of feed-back detector, the 

 detector tube is also used as the source of local high frequency. Such 



Fig. 48 



a circuit is shown in Fig. 48. The oscillatory circuit is tuned to 

 differ in frequency from the incoming signal by an amount which will 

 give a satisfactory difference frequency in the telephone receiver T. 

 By varying the coupling at M, the intensity of the beat note can be 

 readily changed. It must, however, be sufficient to cause the circuit 

 as a whole to oscillate at the natural frequency of 0. A feed- 

 back arrangement is particularly applicable to those cases in which 

 the carrier frequency is much higher than the signal frequency and 

 is therefore generally used with a blocking condenser. 



In telephone systems, whether radio or carrier current, it is fre- 

 quently desirable to suppress the carrier frequency and transmit only 

 one 34 or both of the side bands. Detection with only the side bands 

 present would result in a double frequency detecting current which 

 obviously would not be permissible in a telephone circuit. Whenever 



34 For a discussion of the advantages of single side band transmission, see reference 

 given in footnote 13. 



