218 



BELL SYSTEM TECHNICAL JOURNAL 



The phase simulating network is shown in schematic form in Fig. 15. 

 The network is a balanced structure and consists of the following pieces 

 of apparatus connected in tandem: 



1. An input repeating coil to improve the longitudinal balance at the 

 sending end, 



2. A half-section high-frequency cut-off low-pass filter to mop up the 

 phase shift introduced by two repeating coils of the band elimination 

 filter, 



24 28 32 36 40 44 48 



FREQUENCY IN KILOCYCLES PER SEC0N3 



Fig. 16— Phase shift-frequency characteristic of the phase simulating network. 



3. A resistance pad to equalize the over-all loss level of the all pass 

 network to within dzO.l db of the pass band loss of the band elimina- 

 tion filter, and 



4. Three delay sections, self equalized for loss, for simulating the phase 

 shift of the band elimination filter. 



The network simulates the phase shift of the band elimination filter over 

 the frequency ranges covered by message channels 1 to 4 and 10 to 12 to 

 within 20 electrical degrees as shown in Fig. 16. As phase simulation is 



