426 



BELL SYSTEM TECHNICAL JOURNAL 



Experimental Results 



A number of filters have been constructed, during the past four 

 years, which employ quartz crystals as elements. Figure 18 shows the 

 measured insertion loss characteristic of a narrow band filter ^^ employ- 



149.0 149.2 149.4 149.6 149.8 150.0 150.2 150.4 150.6 150.8 151.0 

 FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 18 — Measured insertion loss characteristic of a narrow band-pass filter. 



ing only crystals and condensers. This filter employs two sections of 

 filter No. 3 of Fig. 6. It will be noted that in spite of the very narrow 

 band width, the insertion loss in the transmitted band is quite small. 

 A number of the broader band filters employing coils as well as 

 condensers and crystals have also been constructed. The frequency 

 range so far developed extends from 36 kilocycles to 1200 kilocycles. 

 Figure 19 shows the insertion loss characteristic of a band-pass filter 

 whose mid-frequency is 64 kilocycles and whose band width is 2500 

 cycles. The insertion loss rises to 75 db, 1500 cycles on either side of 

 the pass region. This filter was constructed from two sections of the 

 band-pass type described in Fig. 12. A similar insertion loss character- 

 istic, but shifted to a higher frequency, is shown by Fig. 20. The 

 insertion loss in the center of the band for this higher frequency filter 

 is considerably larger due to the smaller percentage band width. It is 

 interesting to note that practically all of this loss is due to the dissipa- 

 tion introduced by the coils. The useful percentage band width is 

 about one-half per cent and the filter reaches its maximum attenuation 



" The filters whose characteristics are shown on Figs. 18 and 21 were designed and 

 constructed by Messrs. C. E. Lane and W. G. Laskey. The author wishes to call 

 attention to the fact that they and others associated with them in the Laboratories 

 have made considerable progress in connection with the practical difficulties en- 

 countered in the design and construction of these filters such as working out the 

 high precision element adjustment methods required, in methods of mounting, and 

 in shielding methods. 



