IIIJ ERS lOK CAKRIEK SYSTEMS 209 



The band elimination filter described herein was developed for the type A' 

 carrier system (Carrier-on-Cable) for which the hrst option mentioned above 

 was chosen. This filter operating at the line frequencies of the type A' 

 system is required to transmit frequencies from 12 to 31.6 kc and 44.2 to 

 60 kc while blocking those from 32 to 43.2 kc. Actually the filter will 

 transmit frequencies below 12 kc and above 60 kc but these do not appear 

 on the type A' line and therefore there are no requirements in these ranges. 



The filler which i)erforms these functions is shown schematically in 

 Fig. 10. Several factors made its design difficult. A high level of dis- 

 crimination of the order of 75 db is required over a wide frequency range of 

 about 12 kc. Also the allowable waste interval between wanted and un- 

 wanted frequencies is very small. The filter must transmit with a maxi- 

 mum distortion of 0.2 db to within 97.5% of the first unwanted frequencies 

 at which a discrimination level of 75 db is required. 



Because of the severe requirements the familiar image parameter design 

 method was not employed. In this, as is well known, the composite filter 

 first presented b\- Zobel" is made up of sections with matched image im- 

 pedances but different transfer constants depending upon the attenuation 

 requirements. Instead, it was felt that a design method proposed by Dar- 

 lington^ offered a better possibility of meeting the requirements with a 

 reasonably sized filter. This procedure known as the inserlion loss method 

 is based upon the determination of a four-terminal transducer of reactances 

 which, when inserted between definite resistance terminations, will produce 

 a specified loss characteristic. A filter so designed has an advantage over 

 image parameter filters in that the attenuation obtainable is greater for the 

 same effective cut-off and an equal number of elements. Elective cut-of 

 as used here means the last frequency of interest in the transmitted band. 

 It is possible, therefore, with an insertion loss filter to use fewer elements 

 for a given attenuation, or to obtain a wider transmission band with the 

 same number of elements. 



The advantage inherent in the newer design method is not derived from 

 a difference in structure. In configuration there is no way to distinguish 

 such a filter from one of conventional image design. The difference lies 

 solely in the element values. A simple way to visualize how the insertion 

 loss design varies from image design is to consider that the newer method 

 removes an arbitrary restriction placed upon the image theory to simplify 

 the mechanics of design. The restriction is that the nondissipative image 

 attenuation must be identically zero over continuous frequency ranges 

 including the transmitted bands and other than zero everywhere else. This 

 leads to the familiar ladder image filter composed of matched sections, or 

 the lattice filter with coincident critical frequencies. 



