I 



THE L3 SYSTEM — TELEVISION TERMINALS 



937 



Fig. 17 — Model of the carrier suppression filter. 



To avoid removing excessively broad bands of frequencies from the 

 television band, and thereby generating visible distortion in the trans- 

 mitted picture, narrow bandwidth crystal filters are used for both of 

 these applications. The pilot elimination filter has its 3 db loss points 

 at about 1,000 cycles on either side of the pilots. The carrier suppression 

 filter has its 3 db loss points at 150 cycles on either side of the carrier. 

 In the past, spurious or secondary responses in the crystal units could 

 not be Hmited to sufficiently low values to permit available crystals to 

 be used in broad band circuits without elaborate means for suppressing 

 the unwanted responses. In the LI system, for example, a balancing 

 circuit using hybrid coils was employed for this function. Techniques 

 for reducing unwanted responses in the crystal units by special contour- 

 ing of the blanks and by precise optimum area plating have been de- 

 veloped recently.^^ The use of these methods has resulted in crystal units 

 in which the unwanted responses are reduced to the extent that relatively 

 simple filters can be used for these applications. A simplified schematic 

 of the pilot elimination filter is shown on Fig. 16. Small ovens are pro- 

 vided to maintain close temperature control for the quartz cyrstal ele- 

 ments in order to stabilize the resonant frequencies. Mechanical arrange- 

 ments are illustrated on Fig. 17. 



Power Equipment 



Primary power for the television terminals is 60-cycle ac. It is derived 

 from the L3 motor-alternator power equipment at main repeater sta- 



