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BELL SYSTEM TECHNICAL JOURNAL 



phase characteristics for typical band pass and high pass filter sections. 

 The curves of Fig. 6 are for all-pass lattice type network sections. 

 The frequency fr is the resonant or anti-resonant frequency of the 

 series arms and cross-arms of the network. 



The above four figures show that the phase characteristics are 

 curved in every case over a considerable portion of the transmitted 

 frequency range.** A curved phase characteristic like that shown in 

 Fig. 3, curve B, for example, may be represented as the sum of a 

 distortionless phase characteristic, Bi, of the type shown in Fig. 1 

 and another curved one, B2, which is the difference between it and the 



Fig. 5 — Image transfer phase characteristics of typical high-pass filter sections. 



original,^ i.e., B = By + Bi. The slope of the straight line charac- 

 teristic is the minimum slope of the original, i.e. the slope at very low 

 frequencies. B\_ introduces at all frequencies a definite delay without 

 distortion given by its slope. Bo, to which no delay as a whole may be 

 ascribed may be called the phase distortion characteristic of the net- 

 work, and its derivative {dBjdo))/ — (dB/d(jo)min. the delay distortion 

 characteristic or simply the delay distortion. This procedure is equiva- 

 lent to regarding the low pass filter as consisting of two parts in tan- 

 dem the first part introducing a delay without distortion and the second 

 part a distortion. If, after subtracting such straight line portions 

 from low pass filters the remaining curves are the same, the phase 



* In discussing the phase characteristics of filters only the characteristics m the 

 transmitting range are considered, since in general the frequency components in this 

 range only contribute noticeably to the received signal. 



' See Appendix II. 



