98 BELL SYSTEM TECHNICAL JOURNAL 



(3). As already pointed out, good signal reproduction requires that 

 at the detector this shall not be too small relative to the side-bands. 

 However, it is merely a continuous alternating current, and does not 

 itself partake of the signal variations. It is therefore immaterial 

 whether it is transmitted from the modulator or is supplied to the 

 detector by a local source such as an oscillator. The elimination of 

 this component from the modulated wave at the sending station is 

 spoken of as "carrier suppression," and its re-introduction at the 

 receiving end as "homodyne" or "zero beat" reception. The term 

 homodyne implies supplying the same wave as distinguished from 

 heterodyne, meaning another. Zero beat refers to the bringing of 

 the local carrier into synchronism with the sending carrier by reducing 

 the beat note between them to zero frequency. While homodyne 

 reception is essential to carrier suppression, the reverse is not true. 

 The reception of an ordinary modulated wave may sometimes be 

 improved by the addition of carrier at the receiving end. 



The primary advantage of carrier suppression lies in the saving of 

 sending power which it makes possible, or, what is equivalent, the 

 increase in range made possible when all the power of a given station is 

 utilized in the side-band. Of the various ways in which this sup- 

 pression may be accomplished, the simplest is by the use of a so-called 

 balanced modulator as shown schematically in Fig. 3. Carrier fre- 



Fig. 3 — Balanced modulator 



quency from the source C is applied to the grids of two vacuum tubes 

 in the same phase, while signal currents, indicated at S, are applied 

 to the two in opposite phase. The two plate circuits are differentially 

 connected with a common output circuit. In the absence of signaling 

 current the amplified carrier frequency currents from the two tubes 

 neutralize each other and nothing is transmitted. With the applica- 

 tion of signaling current one grid is raised in potential and the other 

 lowered, with the result that more radio frequency is developed by the 

 first tube than by the second and the excess appears at O. The 

 magnitude of this radio frequency current is proportional to the in- 

 stantaneous value of the signaling current. Upon reversal of the 



