Miscellaneous active electrical nonlinear devices – circuits – and – Signal converting – shaping – or generating – Current driver
Reexamination Certificate
1999-12-23
2001-04-17
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Signal converting, shaping, or generating
Current driver
C327S110000
Reexamination Certificate
active
06218872
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to transmission line drivers, and specifically to line drivers for data transceivers.
BACKGROUND OF THE INVENTION
Digital Subscriber Line (DSL) systems are commonly used as one of the methods of transmitting data when there is a wired connection, such as a twisted wired pair, between a transmitter and a receiver. Modems which operate in an Asymmetric DSL (ADSL) mode typically use a multitone signaling technique known as discrete multitone (DMT) signaling, and a DMT signal may be considered to be composed of a large number of sinusoidal signals. Each sinusoidal signal has a relatively small varying amplitude and phase, but the DMT signal which is the sum of the sinusoidal signals typically has a large dynamic range. In other words, even though the average signal voltage amplitude is low, the DMT signal contains high and infrequent voltage peaks. To avoid signal distortion, the peaks must be transmitted and received accurately.
Typical ADSL modems utilize a relatively high voltage power supply in order to transmit the DMT signal without distortion. Accordingly, line drivers of typical modems use power inefficiently, since most of the time the signal being sent has a low voltage amplitude. The resultant low efficiency of the line drivers means that the power consumed is large. Thus, when many modems must be deployed in a single location, as is the case, for example, in the central office of a regional telephone company, the heat generated because of the low modem efficiency limits the number of modems that can be installed in a given space.
For efficient operation, all modems need to have good impedance matching at the output of the modem. Typically, a resistor in series with the output of an operational amplifier, which acts as a line driver signal amplifier, is used to define the output impedance, which is set equal to the load impedance to achieve good impedance matching. In this situation the resistor dissipates half the power delivered by the amplifier, and the load only receives half of the amplifier's voltage output.
Methods are known in the art to decrease the power consumption of line drivers for signals with a high dynamic range. In an article entitled “Line Driver Economically Synthesizes Impedance,” by Koren, in the Jan. 6, 1994, issue of
Electronic Design News
, which is incorporated herein by reference, there is a description of a method of synthesizing the output impedance so as to reduce the value of the resistor in series with the output of an operational amplifier. The method utilizes two feedback loops, both using resistors. In order for the driver to work correctly, all resistors in the system must have very close tolerances and must be carefully matched.
SUMMARY OF THE INVENTION
It is an object of some aspects of the present invention to provide line driver apparatus having reduced power consumption.
It is a further object of some aspects of the present invention to provide line driver apparatus wherein the output series resistor is eliminated.
It is a further object of some aspects of the present invention to provide a modem using line driver circuitry such that received signals are delivered to receiver circuits with a high amplitude, while transmitted signals are largely prevented from reaching the receiver circuits.
It is a yet further object of some aspects of the present invention to provide line driver apparatus that has a well-defined, predetermined output impedance.
In preferred embodiments of the present invention, a transmit circuit of a modem, preferably an Asymmetric Digital Subscriber Line (ADSL) modem, is coupled by a line driver to a transmission line. The line driver comprises an operational amplifier, a transformer which is most preferably connected as an autotransformer, and a balancing network. An inverting input of the amplifier is shunted by the network, and also receives feedback from the amplifier output via a primary coil of the transformer, so that the feedback is negative. A secondary coil of the transformer is connected in series between the amplifier output and the line being driven. A transmit port is formed between the non-inverting input of the amplifier and a ground, and a receive port, for conveying signals received over the line to a receive circuit of the modem, is formed between the inverting input of the amplifier and the line.
This configuration of the line driver circuit enables the impedance of the balancing network to be synthesized conveniently, as a function of the turns ratio of the transformer, so as to couple the modem to the line with enhanced efficiency, relative to line drivers known in the art. The balancing network impedance is most preferably set to be equal to the turns ratio times an impedance of the transmission line. When this is the case, the negative feedback provided from the transformer to the amplifier input, and the shunting of the input by the balancing network, mean that line drivers constructed as described above have the following properties:
High signal rejection between the transmit and receive ports, i.e., there is high transmit port to receive port isolation.
Zero voltage loss between the output of the operational amplifier and the line driver output.
All of the voltage received from the line is transferred to the receive port.
The output impedance of the line driver is a function of the impedance of the receive port, which can thus be set to give optimal impedance matching to any particular line.
There is therefore provided, in accordance with a preferred embodiment of the present invention, a line driver for coupling a data transceiver to a line having a load impedance, including:
a transformer, including a primary coil and a secondary coil with a turns ratio of N:1 therebetween;
a shunt impedance set to a value equal to a predetermined function of N and of the load impedance, coupled between one end of the primary coil and ground; and
a differential amplifier, having a first input coupled to be driven by a transmit circuit of the transceiver, and having an output coupled to drive the line through the secondary of the transformer, and having a second input shunted to ground through the shunt impedance and coupled to receive feedback from the output through the primary coil of the transformer.
Preferably, the second input of the differential amplifier is coupled to a first connection of a receive circuit of the transceiver, and the line is coupled to a second connection of the receive circuit.
Preferably, an output impedance of the line driver is approximately equal to an impedance of the receive circuit divided by N plus one.
Preferably, the value of the shunt impedance is chosen as a function of N, so as to isolate the receive circuit from the transmit circuit.
Further preferably, the value of the shunt impedance is approximately equal to N times the load impedance.
Preferably, the transformer is connected as an autotransformer, having a common node coupled to the output of the amplifier.
Alternatively, the transformer includes a tertiary coil, insulated from the primary coil and the secondary coil and-coupled to a receive circuit of the transceiver.
Alternatively, the shunt impedance includes a voltage-divider having a voltage-tap coupled to the first input of the differential amplifier.
Preferably, the second input of the differential amplifier is an inverting input.
There is further provided, in accordance with a preferred embodiment of the present invention, a method for coupling a data transceiver to a line having a load impedance, using a transformer having primary and secondary coils in a turn ratio of N:1, the method including:
coupling a transmit circuit of the transceiver to a first input of a differential amplifier;
coupling an output of the differential amplifier to drive the line through the secondary coil of the transformer;
coupling the output of the differential amplifier through the primary coil of the transformer to provide feedback to a second input of the differential amplifier; and
shunting the se
Orckit Communications Ltd.
Tran Toan
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