Wide band modem for high speed data transmission

A modem transmitter comprising:

• means for modulating an AC signal with data signals having at least two levels to produce a modulated data signal,
• a transformer having first and second windings for coupling said modulated data signal to said communication line, said second winding being connected to said communication line,
• a first transistor having a linear region of operation and a saturated region of operation, a first terminal of said transistor being connected to a first terminal of said first winding, a second terminal of said transistor being connected to ground and said modulated data signal being connected to a third terminal of said transistor, said transistor applying said modulated data signal to said transformer when operating in said linear region and grounding said first terminal of said first winding when operating in said saturated region,
• means for biasing said transistor for linear operation when o=ta are to be transmitted and for saturating said transistor when data are not to be transmitted,
• a second transistor having a nonconducting region of operation and a saturated region of operation, a first terminal of said second transistor being connected to a power supply,
• means for biasing said second transistor so that it is saturated when data are to be transmitted and is nonconductive when data are not to be transmitted, and
• a low pass filter connected between a second terminal of said first winding and a second terminal of said second transistor, said filter applying to said winding power from said power supply when data are to be transmitted and, when data are not to be transmitted, said first transistor and said filter providing a short circuit for AC signals across said first winding of said transformer.

A communication system comprising at least two transmitters and two receivers interconnected by a coaxial line, each transmitter comprising:

• means for modulating an AC signal with uata signals having at least two levels to produce a modulated data signal,
• a transformer having first and second windings for coupling said modulated data signal to said communication line, said second winding being connected to said communication line,
• a first transistor having a linear region of operation and a saturated region of operation, a first terminal of said transistor being connected to a first terminal of said first winding, a second terminal of said transistor being connected to ground and said modulated data signal being applied to a third terminal of said transistor, said transistor applying said modulated data signal to said transformer when operating in said linear region and grounding said first terminal of said first winding when operating in said saturated region,
• means for biasing said transistor for linear operation when data are to be transmitted and for saturating said transistor when data are not to be transmitted,
• a second transistor having a nonconducting region of operation and a saturated region of operation, a first terminal of said second transistor being connected to a power supply,
• means for biasing said second transistor so that it is saturated when data are to be transmitted and is nonconductive when data are not to be transmitted, and
• a low pass filter connected between a second terminal of said first winding and a second terminal of said second transistor, said filter applying to said winding power from said power supply when data are to be transmitted and, when data are not to be transmitted, said first transistor and said filter providing a short circuit for AC signals across said first winding of said transformer.