APPROXIMATE NETWORKS OF ACOUSTIC FILTERS 



337 



That the network shown on Fig. 7 is the equivalent in characteristic 

 impedance and propagation constant of that shown on Fig. 6, can 

 readily be verified by substituting the impedances of the lattice net- 

 work into the formulae for a lattice network 



Z = ^ZaZb; cosh P 



Zn + Za 

 Zb ~ Za 



(^') 



where Za is the impedance of one of the series arms, and Zb that of one 

 of (he lattice arms. A lattice network has a pass band when (he reac- 



Fig. 8 



tance of the series arm is of opposite sign to that of the lattice arm. 

 When the reactances of the two arms have the same sign, an attenua- 

 tion band results, while when the reactances of the two arms are equal, 

 an infinite attenuation constant results, since here the lattice will be a 

 balanced Wheatstone bridge. 



For example, suppose that a side branch impedance, equivalent to 

 an inductance and capacity in series, is used. The impedance of the 

 lattice arm has two zero impedance points — one of which is at an infi- 

 nite frequency — and two infinite impedance points- — one of which is at 

 zero frequency — as shown on Fig. 8. The impedance of the series arm 



