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BELL SYSTEM TECHNICAL JOURNAL 



frequency /m the impedance of the filter is 



K 



tan 



7r/2 



1 + 



2/1 



tan 



1 + 



/i 



k 



1 4- — 

 ^2/1 



4/1 



—j- Zoi for narrow bands. (18) 



-^.' 



Such a filter would be useful for an interstage coupler to couple to- 

 gether the plate of one screen grid or pentode tube to the grid of another 

 one. One such arrangement is shown in Fig. 3. This method of 

 coupling together two stages of vacuum tubes has an advantage over 

 using a coaxial conductor or a coil and condenser as a tuned circuit 

 on several counts. In the first place the width of the band passed can 

 be accurately controlled and a flatter gain characteristic is obtained. 

 As will be shown later, distributed capacity in the plate and grid of the 



Fig. 3 — Coaxial band-pass filter used to couple vacuum tubes. 



vacuum tube can be absorbed in the filter by making the line length h 

 shorter. Since only half of the total distributed capacity has to be 

 absorbed on each end of the filter, a higher impedance can be built up 

 in the filter for the same band width, and hence more gain per section 

 can be obtained than with a tuned circuit. 



The filter will have other pass bands at 2fm, Sfm, etc., but these do 

 not usually cause any trouble at the short-wave frequencies because the 

 gain in the vacuum tube is falling off' very rapidly and no appreciable 

 signal is passed. If desired, however, another band-pass filter can be 

 added which has the same fundamental bands but different overtone 

 bands, and this will eliminate the effect of the additional pass bands. 

 For very narrow bands, the line length h can be made longer and hence 

 more realizable by making the characteristic impedance Zo, lower. 

 If anything is to be gained by making the impedance on the plate side 

 different from that on the grid side this can be accomplished by making the 

 filter an impedance transforming device, as discussed in the next section. 



