THE L3 SYSTEM — TELEVISION TERMINALS 



919 



frequencies where the upper side band is suppressed. A second modula- 

 tion then translates the vestigial sideband signal back down in frequency 

 to the final band. In contrast the L3 terminals employ only a single step 

 of modulation to convert the signal directly to the assigned band as 

 shown in Fig, 2. In general this can be accomplished if the carrier fre- 

 quency is at least half the sum of input and vestigial band\vidths. Then 

 the lower modulation sideband does not fold over the zero frequency 

 axis to produce frequencies which fall back into the vestigial or upper 

 sidebands. Some foldover is evident in the L3 case shown in Fig. 2 at 

 low frequencies of the modulator output. Single step modulation is 

 advantageous in that the very high frequencies encountered in the 

 multi-step process are avoided. The disadvantage of the one step process 

 is that many extraneous products of modulation, which in the two step 

 process can be suppressed with filters, must be reduced to tolerable levels 

 by balances in the modulator. 



The modulator, Fig. 3, is a combination of two double balanced modu- 

 lators of a form often employed for modulation of telephone signals.'' 

 The effect of a double balanced varistor modulator of the type repre- 

 sented by either of the two in Fig. 3 is to multiply the input signal by a 

 square wave function having the period of the particular carrier fre- 

 quency. Since the square wave contains all odd harmonic multiples of 



VIDEO 

 INPUT 



HARMONIC 

 INPUT 



CARRIER MODULATOR 



►1 :^-^Vv 



AAAr 



]__L 



HYBRID 

 NETWORK 



n 



HYBRID 

 TRANS- 

 FORMER 



AAV 



AAAr 



DOUBLE 



SIDEBAND 



OUTPUT 



THIRD HARMONIC 

 MODULATOR 



Fig. 3 — Modulator for L3 terminals with third harmonic carrier product bal- 

 ance feature. 



