Method and apparatus for sending DC-balanced signals over a transmission line

1. A system for transmitting a DC-balanced signal to a receiver via a transmission line having an input and an output, the output of the transmission line electrically coupled to the receiver, the system comprising:

a transmitter having an output stage comprising differentially-connected transistors and respective load resistors, wherein the transmission line is connected to a node between one of the transistors and its respective load resistor, the transmitter operable to impose the DC-balanced signal on the input of the transmission line, the DC-balanced signal having a high state and a low state, each having a respective DC level; and

means, operable prior to the transmitter imposing the DC-balanced signal on the input of the transmission line, for holding the input of the transmission line at an intermediate DC level, the intermediate DC level intermediate between the DC levels of the high state and the low state of the DC-balanced signal, and the intermediate DC level not equal to zero volts, the means for holding having a switch operable to connect the load resistors in parallel.

2. The system of claim 1 , in which: A system for transmitting a DC-balanced signal to a receiver via a transmission line having an input and an output, the output of the transmission line electrically coupled to the receiver, the system comprising:

a transmitter comprises having an output stage comprising first differentially-connected transistors and respective load resistors and a first current sink connected to the first differentially-connected transistors; and

means, operable prior to the transmitter imposing the DC-balanced signal on the input of the transmission line, for holding the input of the transmission line at an intermediate DC level, the intermediate DC level intermediate between the DC levels of the high state and the low state of the DC-balanced signal, and the intermediate DC level not equal to zero volts;

wherein the transmission line is connected to a node between one of the first transistors and its respective load resistor; and

wherein the means for holding comprises:

second differentially-connected transistors connected to the load resistors,

a second current sink connected to the second differentially-connected transistors, and switches connected to deactivate the first current sink prior to the transmitter imposing the DC-balanced signal on the input of the transmission line, and to deactivate the second current sink in antiphase while the transmitter imposes the DC-balanced signal on the input of the transmission line.

3. A data transmission system, comprising:

the system comprising the transmitter and the means for holding, as claimed in claim 1 ;

the transmission line on which the transmitter imposes the DC-balanced signal; and

the receiver comprising an input coupled to the output of the transmission line.

4. The data transmission system of claim 3 , additionally comprising:

a coupling capacitor connected between the output of the transmission line and the input of the receiver; and

a biasing circuit connected to the input of the receiver.

5. A method for transmitting a DC-balanced signal via a transmission line having an input and an output, the method comprising:

imposing the DC-balanced signal on the input of the transmission line, the imposing changing an average DC level at the output of the transmission line; and

at the start of the imposing, additionally imposing a compensating pulse on the input of the transmission line, the compensating pulse compensating for the change in the average DC level at the output of the transmission line caused by the imposing.

6. The method of claim 5 , in which:

the output of the transmission line is DC coupled; and

the compensating pulse has the same polarity as a voltage difference between a static level of the input of the transmission line prior to the imposing of the DC-balanced signal thereon and the average DC level of the DC-balanced signal.

7. The method of claim 5 , in which:

the output of the transmission line is ac coupled; and

the compensating pulse is opposite in polarity to a voltage difference between a static level of the input of the transmission line prior to the imposing of the DC-balanced signal thereon and the average DC level of the DC-balanced signal.

8. A system for transmitting a DC-balanced signal to a receiver via a transmission line having an input and an output, the output of the transmission line electrically coupled to the receiver, the system comprising:

a transmitter operable to impose the DC-balanced signal on the input of the transmission line; and

a compensating pulse generator operable at the beginning of the transmitter imposing the DC-balanced signal on the input of the transmission line to additionally impose a compensation pulse on the input of the transmission line, the compensating pulse compensating for a change in DC level at the output of the transmission line caused by the transmitter imposing the DC-balanced signal on the input of the transmission line.

9. The system of claim 8 , in which:

the output of the transmission line is DC coupled; and

the compensating pulse generator is operable to generate the compensating pulse with the same polarity as a voltage difference between a static level of the input of the transmission line prior to the imposing of the DC-balanced signal thereon and an average DC level of the DC-balanced signal.

10. The system of claim 8 , in which:

the output of the transmission line is ac coupled; and

the compensating pulse generator is operable to generate the compensating pulse opposite in polarity to a voltage difference between a static level of the input of the transmission line prior to the imposing of the DC-balanced signal thereon and an average DC level of the DC-balanced signal.

11. The system of claim 8 , in which:

the transmitter comprises an output stage comprising first differentially-connected transistors and respective load resistors and a first current sink connected thereto;

the transmission line is connected to a node between one of the first transistors and its respective load resistor; and

the compensating pulse generator comprises switches operable to connect the load resistors to first supply voltages prior to the transmitter imposing the DC-balanced signal on the input of the transmission line and additionally operable at the start of the transmitter imposing the DC-balanced signal on the input of the transmission line to connect the load resistors to second supply voltages, at least one of the second supply voltages differing from at least one of the first supply voltages.

12. The system of claim 8 , in which:

the transmitter comprises an output stage coupled to the transmission line; and

the compensating pulse generator is operable at the start of the transmitter imposing the DC-balanced signal on the input of the transmission line to cause the output stage of the transmitter additionally to impose the compensating pulse on the input of the transmission line.

13. A data transmission system, comprising:

the system comprising the transmitter and the compensating pulse generator, as claimed in claim 8 ;

the transmission line on which the transmitter imposes the DC-balanced signal; and

the receiver comprising an input coupled to the output of the transmission line.

14. The data transmission system of claim 13 , additionally comprising:

a capacitor connected between the output of the transmission line and the input of the receiver; and

a biasing circuit connected to the input of the receiver.