GRID CURRENT MODULATION 



131 



of its normal unbalanced value. To separate one sideband from the 

 other after the carrier has been suppressed, and to suppress unbalanced 

 components other than the carrier, filter attenuation is customarily 

 employed. 



In the usual type of balanced circuit there are two possible input 

 paths with corresponding output circuits, one connected to the two 

 grids in series; and known as the series path; the other to the two 

 grids in parallel, known as the shunt path or midbranch. When 

 carrier is impressed on the midbranch and signal on the series arm — the 

 present arrangement in commercial carrier systems using plate current 

 modulators — we designate the circuit, as a matter of convenience, as 

 the "Conjugate Input Type." The modulation product frequencies 

 are distributed as shown in Fig. 9a. When both signal and carrier are 

 impressed on the series branch the modulation product frequencies are 



CARRIER INPUT 



Fig. 9a. Conjugate Input Type 



SIDE BAND 

 OUTPUT 



a. (p±q), 



(2P+q),3Q 



SIGNAL AND 

 CARRIER INPUT 



RQ, 3R 3a 



(2P + a) (p + 2q) 



SIDE BAND OUTPUT 

 2 F? 2Q, (p+q) 



Fig. 9b. Common Input Type 



as shown in Fig. 9b and the circuit is called for convenience the 

 "Common Input Type." The phase of the modulation product of any 

 order may be determined from the consideration that the phase of the 

 frequency 



:r-\mp db nq\ 



depends upon the quantity (mdp ± ndg) where 9 represents the phase 



