Identifying receiver lifted condition of subscriber instrument - by measuring increase in direct current and filtering out tone frequencies in subscriber circuit

摘要

The d.c. source (UB) is connected to the subscriber lines (a,b) through two equally sized resistors (R1,R2). To the subscriber appts. (T) side of one resistor (R2) a voltage divider (R3,R4) is connected. From its tapping point connections are taken to a summing junction (A); one directly and one through a phase inverting filter (F). The summed output of these signals is applied to a level detector (E) whose output in turn is passed to the telephone exchange control circuits (A,St). The filter (F) passes operational tone frequencies, e.g. calling tones and frequencies associated with dialling, and phase inverts these to give a cancelling effect in the summing junction. By eliminating these alternating components superimposed upon the d.c. in the subscriber line the recognition of the receiver lifted condition is made more certain.