IMPEDANCE CORRECTION OF WAVE FILTERS 



779 



made by O. J. Zobel with his invention of "m-type" sections.^ The 

 schematic of a typical low-pass ^ filter terminated with these sections 

 is shown in Fig. 6. The terminating networks are enclosed by the 



(wm _r: — 



Fig. 6- 



-A typical low- pass filter terminated in "w-derived" sections; m 

 The reflection coefficient of this filter is given on Fig. 7. 



.512. 



broken lines. At one end these sections match the normal filter image 

 impedance, as in Fig. 4-^. The approximation at the other end to the 

 ideal block type characteristic of Fig. A- A is shown by Fig. 4-C. 

 The actual reflection coefficient of the filter of Fig. 6 is given on Fig. 7. 



35 



I- 

 Z 

 Ol 



U 

 a. 



LU 

 0. 



UL 

 bL 

 lU 



o 



o 



z 



O 



U 



UJ 



_l 

 ti. 



UJ 



a. 



30 



25 



20 



15 



10 



500 



1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 

 FREaUENCY IN CYCLES PER SECOND 



Fig. 7 — Reflection coefficient of low-pass filter shown on Fig. 6. 



^ See Bell System Technical Journal, Jan. 1923. 



^ The m-type sections are applicable to all types of filters, low-pass, high-pass, 

 or band-pass, and giv-e very similar results in all cases. For example, the curve of 

 Fig. 4-D can be considered as being a combination of two curves like that of Fig. 4-C, 

 with a slight distortion of the frequency scale. If we allow for this distortion in 

 scales the approximation to the ideal characteristic over a given percentage of the 

 transmitted band is the same for low-pass and band-pass filters. 



