Electric power conversion systems – Current conversion – Including d.c.-a.c.-d.c. converter
Reexamination Certificate
2000-06-20
2002-01-29
Han, Jessica (Department: 2838)
Electric power conversion systems
Current conversion
Including d.c.-a.c.-d.c. converter
C323S364000, C323S316000, C323S208000
Reexamination Certificate
active
06343024
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates to a line driver having a self adjustable output impedance and, in particular, to a transformer line driver with a hybrid.
2. Description of Related Art
Line drivers having a controlled output impedance are well known in the art. See, B. Nauta, et al., “Analog Video Line Driver with Adaptive Impedance Matching,” ISSCC98, pp. 318-19, 1998. A simplified schematic of one such driver
10
is illustrated in FIG.
1
A. The driver
10
(also referred to as a “buffer”) comprises an operational amplifier
12
whose negative input terminal receives an input voltage Vin. The output terminal of the operational amplifier
12
is connected to the gates of two field effect transistors
14
and
16
, where the illustrated “N” value is equal to the ratio of their respective drain currents. The sources of the field effect transistors
14
and
16
are connected to a reference voltage Vdd. The drains of the field effect transistors
14
and
16
are connected to each other by a resistor (R
1
)
18
. The drain of the field effect transistor
14
is connected in a feedback fashion to the positive input terminal of the operational amplifier
12
, and is also connected to ground through a resistor (R
2
)
20
. An output voltage Vout is supplied from the drain of the field effect transistor
16
to drive a transmission line
22
having a characteristic resistance equal to the load resistance (RL)
24
. By properly selecting the values of the resistors R
1
and R
2
for the driver
10
in a well known manner (and as illustrated) with respect to the “N” value and the value of the load resistance RL, the value of the output impedance from the driver may be set (i.e., controlled) substantially equal to the load resistance RL. An advantage of this driver is its reduced power dissipation which makes it very attractive for implementation in an integrated circuit. However, with respect to an integrated circuit fabrication, the precise resistance values needed to achieve substantial matching of driver-line impedance are very difficult to consistently obtain.
It is recognized that it would be advantageous to be able to exercise some adjustment control over the output impedance of the driver following the setting of the resistance values. The driver of
FIG. 1A
may be modified, as shown in
FIG. 1B
, to provide for such an adjustment mechanism. Controllable source degeneration (through circuit
30
) is applied to the transistors
14
and
16
. The current ratio value “N” is electrically tunable (through circuit
30
) via application of the voltage Vtune. In this implementation, the driver adapts to match the load resistance RL using a control loop
28
that integrates the current from the output of the transconductance amplifier (
28
), which results from the voltage output of the drain of transistor (
16
) from the transistor
16
to generate Vtune for application to circuit
30
resulting in an adjustment to the source current of transistor
14
and a change in the value of N. At low frequencies, the control loop
28
forces Vout to equal Vin, in which case the gain of the driver is one. By then setting the resistances R
1
and R
2
as discussed above, approximate matching of the output impedance to the load resistance RL is obtained, with the control loop
28
further refining the matching.
Most telecommunications devices utilize transformer decoupling of the driver and the transmission line. Because transformer driver-line decoupling is typically utilized in the push-pull configuration, a direct current output signal related to the load resistance is not available to be integrated by the control loop
28
and produce the adjustment signal Vtune. Furthermore, if the transmission line is relatively long, its direct current resistance is substantially different from the characteristic impedance. In such situations, the precision of the impedance adjustment provided by the
FIG. 1B
circuit is not sufficient.
One solution to this problem is presented in R. Mahadevan, et al., “A Differential 160 MHz Self-Terminating Adaptive CMOS Line Driver,” ISSCC2000, pp. 436-37, 2000, where the gain of the transformer push-pull driver is adjusted to unity by using the low frequency content of the transmitted signal. In this implementation, the driver output signal is filtered and compared with the input signal. Responsive to that comparison, the driver transistor ratio is adjusted to set the gain to unity. This method of driver gain adjustment is effective if the load of the driver is a transmission line having a matched termination at the opposite end. However, in a full duplex architecture where transmission and reception occur through the same line, a similar driver should be located at the opposite end of the transmission line. Typically, at the beginning of the adjustment procedure neither one of these drivers is matched to the line. This causes significant reflections on the signals, which affect the amplitude of the signal at the driver output, and the simultaneous adjustment of both drivers becomes a complex multi-step routine.
In some applications, transmission and reception take place simultaneously through the same transmission line. A hybrid device or circuit is typically connected to split the transmitted and received signals. It is conventional to utilize voltage mode drivers in modern wireline communications devices. In such cases, additional resistors are often connected in series with the line driver to effectuate line impedance matching. As an example, these additional resistors may be used to build a balanced bridge hybrid circuit. Unfortunately, the differential output of such a circuit has a common mode voltage equal to the transmitted signal, and this results in a substantial increase in transmitted signal echo. As a further drawback, if such a hybrid circuit is used in a self-terminated driver there is a substantial reduction in power saving efficiency.
There is accordingly a need for a line driver possessing a self-tuned output impedance and operable in an efficient manner with a hybrid for application in communications devices where transmission and reception occur simultaneously over the same transmission line. Such a driver would preferably be inexpensive to fabricate and present a relatively simple method for tuning gain, adjusting output impedance and balancing the hybrid.
SUMMARY OF THE INVENTION
A line driver circuit with hybrid is provided for connection to a signal transmission line. The circuit includes a controlled or synthesized impedance buffer. The line driver circuit further includes an adjustment circuit that processes an output from the hybrid during training mode and generates an adjustment signal for application to an adjustable controlled current source within the buffer. By manipulating the adjustable controlled current source with the adjustment signal, the output impedance of the buffer can be made to substantially match the characteristic impedance of a transmission line connected to the driver.
REFERENCES:
patent: 4254458 (1981-03-01), Griffith
patent: 4798982 (1989-01-01), Voorman
patent: 5121080 (1992-06-01), Scott, III et al.
patent: 5249225 (1993-09-01), Williams
patent: 5459440 (1995-10-01), Claridge et al.
patent: 5510751 (1996-04-01), Nauta
patent: 5585763 (1996-12-01), Navabi et al.
patent: 5936393 (1999-08-01), Nauta
patent: 5973490 (1999-10-01), Nauta
B. Nauta et al., “Analog Video Line Driver with Adaptive Impedance Matching”, ISSCC98, Session 20, SA 20.1 Feb. 7, 1998.
R. Mahadevan et al., “A Differential 160MHz Self-Terminating Adpative CMOS Line Driver”, ISSCC2000, Session 26, WP 26.6, Feb. 9, 2000.
D. Johns et al., “Integrated Circuits for Data Transmission Over Twisted Pair Channels”, 1997 IEEE Journal of Solid-State Circuits, vol. 32, No. 3, Mar. 1997, pp. 398-406.
Han Jessica
Jorgenson Lisa K.
STMicroelectronics Inc.
Szuwalski Andre
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