Hybrid circuit and data communication apparatus

A MODEM comprising:

• a modulation/demodulation device having transmitting and receiving terminals for modulating signals onto the transmitting terminals and demodulating signals received over the receiving terminals;
• a DC-transmitting balancing bridge circuit provided between said transmitting and receiving terminals of said modulation/demodulation device and terminals of a two-wire circuit, the two-wire circuit being for carrying composite DC and AC currents that are transmitted through the bridge circuit;
• a transmitting isolation interface device, having a primary side and a secondary side, the primary side of the transmitting isolation interface device being operatively interconnected to said transmitting terminal of said modulation/demodulation device, and the secondary side of the transmitting isolation interface device being operatively interconnected between balancing points on a first side of said bridge circuit, the transmitting isolation interface device being for electrically isolating the primary side of the transmitting isolation interface device from the secondary side of the transmitting isolation interface device;
• and
• a receiving isolation interface device having a primary side and a secondary side, the secondary side of the receiving isolation interface device being operatively interconnected to said receiving terminal of said modulation/demodulation device and the primary side of the receiving isolation interface device being operatively interconnected between balancing points on a second side of said bridge circuit, the receiving isolation interface device being for electrically isolating the primary side of the receiving isolation interface device from the secondary side of the receiving isolation interface device;
• each of said transmitting isolation interface device and said receiving isolation interface device being an optically coupled device;
• each of said transmitting isolation interface device and said receiving isolation interface device including:
• a variable gain amplifier for amplifying an output of said isolation interface device;
• a current transmission rate measuring circuit for measuring a current transmission rate of said isolation interface device;
• and
• a gain control circuit for controlling gain of said variable gain amplifier based on a measured current transmission rate so that a maximum output can be obtained from said variable gain control amplifier when said measured current transmission rate corresponds to a predetermined value.

A MODEM comprising:

• a modulation/demodulation device having transmitting and receiving terminals for modulating signals onto the transmitting terminals and demodulating signals received over the receiving terminals;
• a DC-transmitting balancing bridge circuit provided between said transmitting and receiving terminals of said modulation/demodulation device and terminals of a two-wire circuit, the two-wire circuit being for carrying composite DC and AC currents that are transmitted through the bridge circuit;
• a transmitting isolation interface device, having a primary side and a secondary side, the primary side of the transmitting isolation interface device being operatively interconnected to said transmitting terminal of said modulation/demodulation device, and the secondary side of the transmitting isolation interface device being operatively interconnected between balancing points on a first side of said bridge circuit, the transmitting isolation interface device being for electrically isolating the primary side of the transmitting isolation interface device from the secondary side of the transmitting isolation interface device;
• and
• a receiving isolation interface device having a primary side and a secondary side, the secondary side of the receiving isolation interface device being operatively interconnected to said receiving terminal of said modulation/demodulation device and the primary side of the receiving isolation interface device being operatively interconnected between balancing points on a second side of said bridge circuit, the receiving isolation interface device being for electrically isolating the primary side of the receiving isolation interface device from the secondary side of the receiving isolation interface device;
• each of said transmitting isolation interface device and said receiving isolation interface device being an optically coupled device;
• each of said optically coupled devices including:
• a light emitting device;
• and
• a light receiving device for receiving light from said light emitting device and for generating an output voltage according to an intensity of said received light;
• and
• said MODEM having, at the secondary side of each of said transmitting isolation interface device and said receiving isolation interface device, a photo-coupler compensation circuit for compensating for variations and deviations of outputs of said light receiving device relative to a drive current of said light emitting device, said compensation circuit including:
• AC extracting means for extracting an AC signal component from an output voltage of said light receiving device;
• DC extracting means for extracting a DC signal component from an output voltage of said light receiving device;
• and
• dividing means for dividing said AC component by said component DC signal.

A MODEM comprising:

• a modulation/demodulation device having transmitting and receiving terminals for modulating signals onto the transmitting terminals and demodulating signals received over the receiving terminals;
• a DC-transmitting balancing bridge circuit provided between said transmitting and receiving terminals of said modulation/demodulation device and terminals of a two-wire circuit, the two-wire circuit being for carrying composite DC and AC currents that are transmitted through the bridge circuit;
• a transmitting isolation interface device, having a primary side and a secondary side, the primary side of the transmitting isolation interface device being operatively interconnected to said transmitting terminal of said modulation/demodulation device, and the secondary side of the transmitting isolation interface device being operatively interconnected between balancing points on a first side of said bridge circuit, the transmitting isolation interface device being for electrically isolating the primary side of the transmitting isolation interface device from the secondary side of the transmitting isolation interface device;
• and
• a receiving isolation interface device having a primary side and a secondary side, the secondary side of the receiving isolation interface device being operatively interconnected to said receiving terminal of said modulation/demodulation device and the primary side of the receiving isolation interface device being operatively interconnected between balancing points on a second side of said bridge circuit, the receiving isolation interface device being for electrically isolating the primary side of the receiving isolation interface device from the secondary side of the receiving isolation interface device;
• each of said transmitting isolation interface device and said receiving isolation interface device being an optically coupled device;
• each of said optically coupled devices including:
• a light emitting device;
• and
• a light receiving device for receiving light from said light emitting device and for generating an output voltage according to an intensity of said received light;
• and
• said MODEM having, at the secondary side of each of said transmitting isolation interface device and said receiving isolation interface device, a photo-coupler compensation circuit, the photo-coupler compensation circuit including:
• a field effect transistor, having a gate, a source, and a drain, for operating in a variable resistance area, with an output signal of said light receiving device supplied to the source;
• and
• an integrator, supplied with a reference voltage, for integrating a difference between said reference voltage and a DC voltage obtained between the source and the drain of said field effect transistor;
• said photo-coupler compensation circuit being for supplying an output of said integrator as a control voltage to the gate of said field effect transistor, controlling a DC voltage obtained between said source and said drain of said field effect transistor to become equal to said reference voltage, and obtaining from said source of said field effect transistor an output of a division which is an AC signal component of said light receiving device divided by a DC signal component of said light receiving device.

A MODEM comprising:

• a modulation/demodulation device having transmitting and receiving terminals for modulating signals onto the transmitting terminals and demodulating signals received over the receiving terminals;
• a DC-transmitting balancing bridge circuit provided between said transmitting and receiving terminals of said modulation/demodulation device and terminals of a two-wire circuit, the two-wire circuit being for carrying composite DC and AC currents that are transmitted through the bridge circuit;
• a transmitting isolation interface device, having a primary side and a secondary side, the primary side of the transmitting isolation interface device being operatively interconnected to said transmitting terminal of said modulation/demodulation device, and the secondary side of the transmitting isolation interface device being operatively interconnected between balancing points on a first side of said bridge circuit, the transmitting isolation interface device being for electrically isolating the primary side of the transmitting isolation interface device from the secondary side of the transmitting isolation interface device;
• and
• a receiving isolation interface device having a primary side and a secondary side, the secondary side of the receiving isolation interface device being operatively interconnected to said receiving terminal of said modulation/demodulation device and the primary side of the receiving isolation interface device being operatively interconnected between balancing points on a second side of said bridge circuit, the receiving isolation interface device being for electrically isolating the primary side of the receiving isolation interface device from the secondary side of the receiving isolation interface device;
• each of said transmitting isolation interface device and said receiving isolation interface device being an optically coupled device
• at least one of said receiving isolation interface device and said transmitting isolation interface device being configured in a push-pull configuration for canceling distortion in said receiving isolation interface device.