ACTIVE TERMINATION NETWORK

Abstract: An active termination for a transmission line comprising a reference impedance, a terminating impedance and a control circuit. The reference and terminating impedances are identical circuits made on the same integrated circuit in close proximity to one another. Both impedances are made of an active and a passive resistor in series. The active resistor is a CMOS transistor operated as a voltage controlled resistor. A control circuit senses the impedance of the reference impedance and generates a control signal to change the impedance of the reference and terminating impedances to be equal to the impedance of the transmission line. An alternate embodiment of the invention comprises an active resistor and a passive resistor in series to form a terminating impedance network. A control circuit senses the voltage on the transmission line and adjusts the active resistor to terminate the transmission line with the correct value of resistance.

Description: 1 ACTIVE TERMINATION NETWORK TECHNICAL FIELDTECHNICAL FIELD This invention relates generally to an active circuit for terminating a transmission line. 5 BACKGROUND OF THE INVENTION BACKGROUND OF THE INVENTION Computer and communications systems frequently use transmission lines to transmit data within and to other systems and subsystems. Transmission lines must be properly terminated to minimize distortion of the transmitted signals. This is particularly important as the operating * ^ frequencies of transmission lines are increased to match the higher operating frequencies of the latest integrated circuits. Many systems and integrated circuits currently have operating frequencies in the GHz range. Terminating networks are used to provide the correct terminating impedance. An off-chip precision resistor can provide transmission line termination. Off-chip resistors require significant 15 circuit board space and increase board complexity and cost. The distance between the off-chip resistors and the transmission line to be terminated can be large enough to cause signal reflection problems. The use of on-chip termination resistors is preferred, but high precision resistors are difficult to form using standard CMOS processes due to manufacturing process and thermal variations. A resistor made by current CMOS processes could have a tolerance range of as much 20 as plus or minus 30%, which is totally unacceptable for use as a transmission line terminator. Laser trimming of on-chip CMOS resistors is expensive and to be avoided, if at all possible. The use of active circuits to terminate a transmission line is known in the art and there have been many approaches to this problem. It is an object of the present invention to provide an active circuit for terminating transmission lines composed of imprecise, high tolerance 25 components configured to operate as a precision termination. SUMMARY OF THE INVENTION The present invention improves upon previous attempts to provide an active termination circuit for terminating transmission lines with a high degree of accuracy, especially when made 30 on an integrated circuit. This invention uses a reference impedance, which is a series combination of an active resistor and a passive resistor. The active resistor is a CMOS transistor operated as a voltage controlled resistor. A variable voltage is applied to the gate to control the drain to source resistance. The passive resistor is set to a fixed resistance. The current through the reference impedance is set to a fixed value. A control circuit with 35 a feedback loop senses the voltage across the reference impedance to generate a control voltage to control the resistance of the active resistor and thus control the resistance of the reference impedance. The terminating impedance and the reference impedance are made as identical circuits formed at the same time on the same integrated circuit and as a result have identical properties. The terminating and reference impedances are designed to have the same impedance and are controlled by the same control voltage generated by a feedback loop. An alternate embodiment of the invention does not use a reference impedance. An active resistor and a passive resistor in series form the terminating impedance circuit. A control circuit senses the voltage on the transmission line and adjusts the gate voltage to control the active resistor to terminate the transmission line with the correct value of resistance. The present invention has many advantages, which will become clear from the detailed 0 descriptions. DESCRIPTION OF THE DRAWINGS Figure 1 shows a first embodiment of a transmission line termination circuit. Figure 2 shows an alternate embodiment of the invention. 5 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows a transmission line 1 terminated at the receiving end by terminating impedance network 8, which is made of an active resistor 9 in series with a passive resistor 10. Active resistor 4 and passive resistor 5 form a reference impedance 3, whose resistance is 0 controlled by a control signal coming from op amp 2. As part of a negative feedback loop, op amp 2 compares the voltage drop across resistors 4 and 5 against a reference voltage Vref to generate a control voltage which controls active resistors 4 and 9. Current source 7 limits the current flow through the reference resistance 3. The present invention relies on the well-known fact that a FET (a field effect transistor) 5 behaves like a resistor for low currents and voltages, but the channel resistance of a FET is nonlinear. The channel resistance of a FET is highly non-linear and varies as the drain-source voltage of the device is changed. Since a FET is used to terminate a transmission line, it is subject to this non-linearity caused by the voltage swing on the line. In order to reduce this voltage sensitivity, 30 this circuit incorporates a POLY resistor in series with the FET. The nominal resistance of the POLY resistor is chosen to approximately equal the nominal FET channel resistance, hence the maximum voltage variation seen across the FET is cut by half and it's overall non-linearity contribution is reduced by a factor of 2. _, , The ratio between the active and passive resistors need not be 1 to 1 , but can be adjusted during the design of the active terminator to suit a particular application. Current source 7 limits the current flow through the reference resistance 3 to 10 mA. Op amp 2 generates a control signal to keep the voltage drop at node 6 to 0.5 volts below the supply voltage Vdd. As a result, the resistance of 3 is equal to 0.5v/10ma, which is 50 ohms, which is a common characteristic impedance for transmission lines. For the feedback loop to function properly, the reference voltage Vref has to be 0.5 volts below the supply voltage Vdd. Selection of the specific circuit values will depend on the operating conditions of a particular transmission line. As an example of how this embodiment would function, if the signal on the transmission line is driven by a current source with a current limit of 10 mA, then the current source 7 has to have a value of 10 mA. This and other circuit design adjustments, such as accommodating transmission lines driven by a voltage source or having different termination impedances can be made by those skilled in the art. 10 Reference resistance 3 and terminating resistance 8 are designed as identical circuits on the same integrated circuit and will have identical properties. The closer that resistances 3 and 8 are in proximity to each other on the same integrated circuit, then the closer will be their operating temperatures and performance. As the control voltage is automatically varied by the feedback loop to keep the reference impedance 3 at a value of 50 ohms, the same voltage is used *■ to keep terminating impedance 8 at a 50 ohm value. Thus, the transmission line is terminated with an active impedance of the correct resistance. This invention is able to provide a termination impedance, which is plus or minus 5 % of the desired value. It is also possible for those skilled in the art, to use this invention for terminating 0 balanced transmission lines. Adopting this invention to be a series termination at the source of the transmission line is well known to those skilled in the art. Figure 2 is an alternate embodiment of the invention, in which a control circuit directly controls the terminating impedance. 25 Figure 2 shows a transmission line 21 terminated at the receiving end by a terminating impedance 24, which is made of an active resistor 25 in series with a passive resistor 26. As part of a negative feedback loop, peak detector 22 senses the voltage on the transmission line and generates a voltage which op amp 23 compares against a reference voltage Vref to generate a control voltage, which controls active resistor 25. 30 The reference voltage applied to op amp 23 is chosen so that the output of op amp 23 controls the resistance of FET 25 so that the resistance of the terminating impedance 24 matches the impedance of transmission line 21. Peak detector circuits are well known to those skilled in the art and their design and use need no further elaboration. 35 Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made without departing from the spirit and scope of the invention as defined solely by the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A transmission line termination circuit comprising: a connection to a transmission line, a reference impedance, comprising an active impedance and a passive impedance connected in series, a terminating impedance, comprising an active impedance and a passive impedance connected in series, wherein said terminating impedance is manufactured with properties proportional to the properties of the reference impedance, a control circuit for generating a control signal for controlling the impedance of the active impedance of the reference impedance, in order for the reference impedance to match the characteristic impedance of the transmission line, and said control signal is used to control the impedance of the active impedance of the terminating impedance in order for the terminating impedance to match the characteristic impedance of the transmission line. 2. The transmission line terminating circuit of claim 1, wherein the terminating impedance is connected in parallel at the receiving end of the transmission line. 3. The transmission line terminating circuit of claim 1, wherein the terminating impedance is connected in series with the sending end of the transmission line. 4. The transmission line terminating circuit of claim 1 , wherein the active impedance of the reference impedance comprises a transistor. 5. The transmission line terminating circuit of claim 4, wherein said transistor is a field effect transistor. 6. The transmission line terminating circuit of claim 1 , wherein the active impedance of the terminating impedance comprises a transistor. 7. The transmission line terminating circuit of claim 6, wherein said transistor is a field effect transistor. 8. The transmission line terminating circuit of claim 1, wherein the reference impedance and the terminating impedance are disposed on the same integrated circuit. 9. The transmission line terminating circuit of claim 1 , wherein the transmission line carries a balanced signal and is terminated by a balanced termination. 10. The transmission line terminating circuit of claim 1, wherein the reference impedance is manufactured to have properties essentially identical to the properties of the terminating impedance. 11. A transmission line termination circuit comprising a connection to a transmission line, a terminating impedance, comprising an active impedance and a passive impedance connected in series, and a control circuit for controlling the impedance of said active impedance in response to the signal on the transmission line in order for the terminating impedance to match the characteristic impedance of the transmission line. 12. The transmission line terminating circuit of claim 11, wherein the terminating impedance is connected in parallel at the receiving end of the transmission line. 13. The transmission line terminating circuit of claim 11, wherein the terminating impedance is connected in series with the sending end of the transmission line. 14. The transmission line terminating circuit of claim 11, wherein the active impedance of the terminating impedance comprises a transistor. 15. The transmission line terminating circuit of claim 14, wherein said transistor is a field effect transistor. 16. The transmission line terminating circuit of claim 11, wherein the transmission line carries a balanced signal and is terminated by a balanced termination. 17. A method of controlling the active impedance of a terminating impedance for terminating a transmission line, comprising the steps of: sensing the voltage across a reference impedance, comparing the sensed voltage to a reference voltage, generating a control voltage in response to variations in the sensed voltage, varying the resistance of an active resistor of said reference impedance in response to said control signal, said reference impedance comprising an active resistor and a passive resistor in series, and varying the resistance of an active resistor of a terminating impedance in response to said control signal, said terminating impedance being substantially identical to said reference impedance. 18. The method of claim 17 and further comprising applying said control voltage to the gate of a field effect transistor to vary the resistance of the transistor. 19. The method of claim 17 and further comprising connecting the terminating impedance in parallel at the receiving end of the transmission line. 10 20. The method of claim 17 and further comprising connecting the terminating impedance in series with the sending end of the transmission line. 21. A method of controlling the active impedance of a terminating impedance for 15 terminating a transmission line, comprising the steps of: sensing the voltage on said transmission line, comparing the sensed voltage to a reference voltage, generating a control voltage in response to variations in the sensed voltage, and ~~ varying the resistance of an active resistor of said terminating impedance in response to said control signal, said terminating impedance comprising an active resistor and a passive resistor in series. 22. The method of claim 21 and further comprising applying said control voltage to the gate of a field effect transistor to vary the resistance of the transistor. 25 23. The method of claim 21 and further comprising connecting the terminating impedance in parallel at the receiving end of the transmission line. 24. The method of claim 23 and further comprising connecting the terminating 30 impedance in series with the sending end of the transmission line. 35

Inventors: OLSON ERLAND, PARTOW ANTHONY S, OLSON, ERLAND, PARTOW, ANTHONY, S.
Applicants: BROADCOM CORP, OLSON ERLAND, PARTOW ANTHONY S, BROADCOM CORPORATION, OLSON, ERLAND, PARTOW, ANTHONY, S.
Application year: 2001
Main classification: H03H11
IPC codes: H03H11