ELECTRIC WAVE FILTERS 



271 



The critical region of the type "C" system band filter requirements 

 was the slope of the attenuation characteristic from the cut-f)!!" to the 

 peak nearest the carrier side of the transmitting band. The system 

 was designed to transmit one sideband only, and the band filters were 

 required to suppress the carriers at least 15 db. The transmitted side- 

 band for each filter extended from 250 to 2750 cycles from the carrier, 

 so that in every case the filter had to offer at least 15 db suppression 

 to the carrier frequency, which was only 250 cycles away from the 

 edge of the transmitting band. Where the carrier frequency was 



L| C| L4 <^4 Ls C5 



o^^yW^ k^TRJ«7^-| (-j-^Wff^ 



C6 



EFFECT OF ^7o SHIFT DOWNWARD 

 OF RESONANT FREQUENCIES OF 

 L| Cj, L4C4 AND L5C5 MESHES 



REQUIREMENTS 



NORMAL CHARACTERISTIC 



27 28 29 30 31 



FREQUENCY IN KILOCYCLES 



Fig. 6 — Attenuation characteristics of 38-N filter, showing effect of element variation. 



25 kc, the separation was only 1 per cent. This requirement necessi- 

 tated very steep slopes of the attenuation characteristic, especially for 

 the higher frequency filters; at these slopes a frequency shift of 1/2 per 

 cent produced a change in attenuation of 9 or 10 db, as may be ob- 

 served in Fig. 6. 



As the precision of adjustment for condensers was three or four 

 times that for coils, it was obvious that some improvement in coil 

 construction or adjustment would have to be made. Furthermore, 

 since the important limiting factors in the adjustment of both coils 



