Communications device with improved ring signal detection

Abstract

A communications device is presented which is able to detect a ring signal without the need for dedicated ring signal detection circuitry. The communications device includes a hookswitch coupled between a telephone line and a communications circuit. In an open mode of the hookswitch, an electrical resistance of at least 25 megohms exists between two switch terminals of the hookswitch, and the communications circuit is essentially disconnected from the telephone line at d.c. A capacitor positioned between the switch terminals of the hookswitch couples a.c. signals from the telephone line to the communications circuit when the hookswitch is in the open mode. A DSP of the communications circuit performs ring signal detection when the hookswitch is in the open mode using electrical power provided by a power supply. The input impedance of the communications circuit is substantially equivalent to a series combination of an inductance and a resistance. The series combination of the capacitor and the equivalent inductance and resistance of the input impedance of the communications circuit effectively forms an R-L-C highpass filter. The value of the capacitor is operably chosen such that: (i) the amplitudes of 20 Hz ring signals are reduced to below a 1.0 Vp-p maximum allowable input voltage range of common analog-to-digital converters, and (ii) the amplitudes of a.c. signals at Caller ID frequency shift keying frequencies of 1,200 Hz and 2,200 Hz are not increased or attenuated to a degree which would impair Caller ID circuit operation.