FILTERS FOR CA RRIER SYSTEMS 197 



band to prevent crosstalk of the adjacent message channels into the pro- 

 gram channel. 



The necessity of using quart/, crystal elements limits the maximum band 

 width of filter which can be realized. This limitation is the result of the 

 comparatively poor electromechanical coupling of quartz.- The resulting 

 filter band width is 8.5 kc with the upper cut-off located near the 88-kc 

 carrier. This is slightly greater than the S-kc nominal band width of the 

 system. 



The crystal band pass filter designed for the single sideband j^rogram 

 channel weighs approximately 30 lbs. and occupies 7 inches of mounting 

 space on a standard 19 inch relay rack. A total of 44 lilter components 

 are required for its construction, half of which are balanced quartz crystal 

 plates. The remaining components consist of eight adjustable air capaci- 

 tors, three fixed mica capacitors, seven balanced retardation coils, three of 

 which are adjustable, and four resistors. A schematic which shows the 

 relative placement of these parts in the filter is given in Fig. 1. 



The measured insertion loss characteristic of the filter between resistive 

 terminations is shown in Fig. 2. The pass band and the vicinity of the 

 upper cut-off are given in greater detail in the enlarged characteristics of 

 Figs. 3 and 4. The extreme sharpness of the upper cut-off is evident in the 

 latter figure. At 40 cps above the 88-kc carrier the discrimination has 

 reached 20 db while the slope of the insertion loss versus frequency curve 

 through this point is about 1 db per cps. Since at least two filters are con- 

 nected in tandem in any program circuit a minimum of 40 db discrimination 

 is provided to all frequencies in the unwanted sideband. The loss realized 

 at frequencies outside the band also is shown in Fig. 2. 



The delay distortion in the pass band of the filter, computed from the 

 slope of its measured insertion phase characteristic, is given in Fig. 5. For 

 short program systems, where no more than six filters are used in tandem, 

 the delay distortion would not exceed the limits set for a high quality system. 

 For longer systems it is necessar>' to equalize this delay distortion. The 

 design and performance of the delay equalizers for this purpose are given 

 in a separate paper.^ These equalizers also include some attenuation 

 equalization to correct for the systematic distortion of the filter. 



Figure 6 shows an exterior view of the filter. On both sides of the mount- 

 ing panel are metal containers which are provided with covers that can be 

 soldered on to make a hermetic-sealed enclosure. In a corner of one can 

 is a terminal box which contains the input and output terminals. These 

 terminals are of the metal glass seal type which are vacuum tight. Mounted 

 on brackets in each of the containers is a brass panel supporting the tilter 

 elements. One side of one of these panels is visible in Fig. 6, the other side 

 is shown in Fig. 7. 



