THE L3 SYSTEM — TELEVISION TERMINALS 933 



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*Q" ADJUSTING CAA COIL Wm INDUCTANCE 



SCREW ADJUSTING 



SCREW 



Fig. 12 — Inductor with adjusting screws. 



The close limits on delay distortion can be met only by close control 

 of the adjustments on the individual all-pass sections. In order to obtain 

 reproducible results to the order of ±0.1° for the phase shift and ±0.01 

 db for the insertion loss of the individual sections, a special fixture is 

 employed to make the connection between the section and the measuring 

 circuit. This is shown in Fig. 13. The fixture can be clamped on the net- 

 work terminals quickly without soldering and provides coaxial patch 

 cords with plugs for connection to the measuring circuit. Each section is 

 mounted in an individual container with shielding between the inductors 

 to reduce couphng. Each inductor is resonated with its associated capaci- 

 tors and the dissipation is adjusted by adjusting the two cores. The 

 construction of a typical section is illustrated by Fig. 14. 



An important consideration in obtaining smooth loss and delay char- 

 acteristics for the delay equalizer is the reflection coefficient of each 

 section. Poor reflection coefficients cause reflections and interactions 

 between all-pass sections. Due to the large phase slope of the equalizer 

 these tend to produce frequency characteristics with large numbers of 

 loss and delay ripples across the frequency band for which transmission 

 requirements are most severe. Reflection coefficients of 2 per cent or 

 less at all frequencies in the TV band have been obtained for all delay 

 sections by taking the following precautions: 



1. Mutual coupling is limited between the two inductors in each sec- 

 tion by use of a shield in the section container. As little as 0.1 per cent 



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