ELECTRICAL WAVE FILTERS 



421 



in a range not practical with electrical filters, or with crystals alone. 

 Another important characteristic of the filter is its iterative im- 

 pedance. For a lattice filter this is given by ^ 



Zi = VZ1Z2, 



where Zi is the impedance of the series arm and Z2 that of the lattice 

 arm. For a dissipationless filter, this is shown by Fig". 12£, as can be 

 easily verified by a consideration of the reactance curves of Fig. 12 C 



ELECTRICAL 



STRUCTURE 



A 



PHYSICAL 



STRUCTURE 



B 



REACTANCE 



CURVES FOR 



EACH ARM 



C 



ATTENUATION 



CHARACTERISTIC 



D 



ITERATIVE 

 IMPEDANCE 



E 



BAND -PASS FILTER 



FREQUENCY 



/M 



FREQUENCY 



fi fal 



I / 



Fig. IS^Lattice network band-pass filter employing parallel coils. 



This type of filter results in a relatively low impedance, for example 

 about 600 ohms for a filter whose mid-band frequency is 64 kilocycles 

 and whose band width is that shown on Fig. 19. Since the band width 

 is decreased by adding more capacitance, it is evident that smaller 



