Abstract: In an FSK demodulator, the output of the phase detector of a phase locked loop (PLL) is capacitively coupled to one input of an FSK voltage comparator, the capacitive coupling blocking d.c. and d.c. being restored at the comparator input by diodes connected between the two comparator inputs. The other input of the comparator is supplied with a reference voltage corresponding to a nominal center frequency of the FSK signals. A buffer amplifier permits rapid charging of the coupling capacitor, which is set to a determined state when there is no phase lock of the PLL. The arrangement facilitates demodulation of narrow-band FSK signals whose center frequency is subject to change.

Abstract: Voice-band signals are coupled to and from a telephone subscriber line via a low-pass filter which has a shunt capacitor coupled across the line. In order to simultaneously couple above voice-band signals to and from the line, this capacitor is coupled to the line via windings of a transformer. The above voice-band signals are coupled via the transformer to and from the line, for example via series inductors which are series resonant with the filter capacitor, as reflected through the transformer, at above voice-band signal frequencies. A parallel resonant circuit, which has a low impedance in the voice-band, is connected in parallel with a winding of the transformer to prevent degradation of the filter characteristics in the voice-band.

Abstract: Voice-band signals are coupled to and from a telephone subscriber line via a low-pass filter which has a shunt capacitor coupled across the line. In order to simultaneously couple above voice-band signals to and from the line, this capacitor is coupled to the line via windings of a transformer. The above voice-band signals are coupled via at least one series inductor and the transformer to the line, the inductor being series resonant with the filter capacitor, as reflected through the transformer, at the above voice-band signal frequency. Two such series inductors provide for efficient coupling of signals to and from the line at two different frequencies, facilitating simultaneous transmission and reception of above voice-band signals, for example FSK data signals, via the line. Parallel resonant circuits are also provided to prevent degradation of the filter characteristics in the voice-band and to prevent coupling of receive frequency components from the transmit signal to the receive signal line.