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BELL SYSTEM TECHNICAL JOURNAL 



make the resistance of the filter approximately constant over the 

 program band, and a 2-terminal network placed in shunt at the end 

 to cancel the reactance of the filter in this band. The inductance and 

 capacitance of the coils and condensers of the 4-terminal network are 

 related to the coefficients of a power series expansion of the right-hand 

 part of equation (3) in the manner explained in the article by H. W. 

 Bode.'' The 2-terminal shunt network at the apparatus end is de- 

 signed so that, while canceling the reactance of the filter in the program 

 band, it resonates just above the band to produce a peak or sharp 

 maximum of attenuation. It thus supplies the sharp selectivity 

 required to produce an abrupt change from free transmission of the 

 program frequencies to high attenuation of the carrier frequencies. 



°-^«^nti^fcii 



TO LINE 

 AND TO : 

 HIGH-PASS 

 FILTER 



IMPEDANCE •— IH 

 CORRECTING „'' 

 SECTION i^tAt\ 



DELAY AND LOSS EQUALIZING 

 LATTICE SECTION 



Fig. 6 — Schematic diagram of filter. 



At the line end, the impedance correcting section is designed for 

 parallel connection with the high-pass line filter. The high-pass filter 

 itself acts as the shunt reactance-canceling network. 



The peak section shown at the left of the delay and loss equalizing 

 section in Fig. 1 provides attenuation which rises rapidly with fre- 

 quency above the program band in such a way as to add to the selec- 

 tivity of the filter. It is a ladder section of a type often employed in 

 filters for its selectivity. 



The filter is designed to match the average impedance of the open- 

 wire lines. The impedance of the office apparatus, however, is slightly 

 higher than that of the lines and the filter. An autotransformer 

 therefore is used at the end of the filter connected to the office appa- 

 ratus, to effect the required change in impedance. A schematic 

 diagram of the complete filter is shown on Fig. 6, the parts being 

 marked for identification in accordance with the foregoing discussion. 



