70 



ANALOGIES 



volume currents of all frequencies from infinity down to a certain frequency 

 termed the cutoff frequency /c and which bars currents, velocities or vol- 

 ume currents of all lower frequencies. 



A band pass filter is a system which passes currents, velocities or volume 

 currents that lie between two cutoff frequencies fc\ and Jci and bars cur- 

 rents, velocities and volume currents of all frequencies outside this range. 



A band elimination filter is a system which bars currents, velocities or 

 volume currents that lie between the two cutoff frequencies /ci and/c2 and 

 passes currents, velocities or volume currents of all frequencies outside 

 this range. The transmission characteristics of low pass, high pass, band 

 pass and band elimination filters are shown in Fig. 4.8. 



v. 



FREQUENCY 

 LOW PASS 



FREQUENCY 

 HIGH PASS 



FREQUENCY 

 BAND PASS 



FREQUENCY 

 BAND ELIMINATION 



Fig. 4.8. Transmission-frequency characteristics of low pass, high pass, band pass and 



band elimination wave filters. 



C. Response Frequency Characteristics of Wave Filters ^' ^. — The ideal 

 or nondissipative filters must consist entirely of pure reactances. The 

 primary object is the determination of those combinations of reactances 

 which will give a single or double transmitted band of frequencies. The 

 most important type of structure is the ladder type, that is, a certain com- 

 bination of reactances in series with the line and another combination in 

 shunt with the line. The series reactance and shunt reactance are desig- 

 nated by Zi and Zi respectively. It has been shown in treatises on wave 

 filters that attenuation occurs when Zxjzi is positive and if Zxjzi is nega- 

 tive and is greater in absolute magnitude than four. Nonattenuation 

 occurs when Zi/z2 is negative and is less in absolute magnitude than four. 

 Therefore, a nondissipative recurrent structure of the ladder type having 

 series impedances Zi and shunt impedances Zi will pass readily only cur- 

 rents of such frequencies as will make the ratio Zi/z2 lie between and — 4. 



D, Low Pass Wave Filters. — Electrical, mechanical and acoustical low 

 pass filters are shown in Fig. 4.9. 



^Johnson, "Transmission Circuits for Telephonic Communication," p. 180, 

 D. Van Nostrand Co., New York. 



* Shea, "Transmission Networks and Wave Filters." D. Van Nostrand Co., 

 New York. 



