THE L3 SYSTEM TELEVISION TERMINALS 



931 



3.5 



4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 

 FREQUENCY IN MEGACYCLES PER SECOND 



8.5 



Fig. 10 — Envelope delay distortion of the transmitting terminal filters. 



3.5 mc. This filter has a low-pass shunt network at the input to maintain 

 its stopband impedance at 75 ohms. This is required because to produce 

 a uniform frequency characteristic the impedance facing the modulator 

 has to provide a reflection coefficient not exceeding 3 per cent. The re- 

 maining discrimination at frequencies less than 3.7 mc and the major 

 part of the vestigial sideband shaping are provided by a group of six 

 constant resistance equalizer sections, as shown on Fig. 9. The low-pass 

 filter to suppress carrier harmonics consists of 2 m-derived filter sections. 

 The delay distortion of the complete set of filters and loss equalizer 

 sections is shown on Fig. 10. This includes the equivalent delay distortion 

 of the video low-pass filter, translated in frequency for equalization after 

 the modulator. The distortion must be equaUzed to a constant delay 

 over the television band. A delay equalizer to do this is incorporated in 

 the filter. It was designed by a potential analogue method and consists 

 of 24 all-pass sections, each having the schematic as shown in Fig. 11. 

 A redundant capacitor is used to avoid excessively small capacity values. 



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11 — Schematic of a delay equalizer section, (dotted capacitors are para- 



