1. Field of the Invention
The present invention relates generally to the field of modem circuits, and more particularly, to a telephone line interface circuit.
2. Description of Related Art
Existing telephone line interface circuits use a passive network in the electronic inductor (EI) which provides feedback from the Tip and Ring voltage to the transistor""s bias and adjusts the line current so that the line voltage is maintained as low as possible. The circuit power dissipation is then calculated by using the worst-case voltage and current values. Components satisfying the maximum power requirements are then used. This power rating can be substantially higher than the rating that could be achieved by using an xe2x80x9cintelligentxe2x80x9d control of the current and voltage.
Additionally, problems arise when designing telephone line interface circuits for worldwide applications due to the specific V-I characteristics (i.e. requirements) of the telephone lines in different countries. These various V-I characteristics are commonly defined as xe2x80x9cDC masks.xe2x80x9d Specifically, various countries have different requirements as to the minimum and/or maximum levels of DC off-hook line voltage permissible for a given Tip and Ring circuit. For example, in the United States, the DC line current cannot exceed 6.6 volts at 20 mA. In France, the DC line current cannot exceed 60 mA while the voltage can be as high as 40 volts. Examples of various DC masks are shown in FIGS. 10(A)-10(D). The solid line on each graph represents a particular V-I boundary constraint within which the interface circuit must operate. These DC mask requirements, however, cannot be met with a single passive network.
Thus, there is a need for an intelligent line current control which can reduce the power rating over conventional telephone line circuits, and which can comply with various worldwide DC masks using a single circuit.
The present invention provides intelligent line current and voltage control that can adjust to the various requirements of worldwide telephone systems, and/or can reduce the power consumed by the interface circuit. In a basic configuration (FIG. 1), a voltage divider is connected across the rectified Tip and Ring voltage (VTR) and provides feedback of the line voltage Vtr to an electronic inductor and to an analog-to-digital converter (ADC) 10. The output of the ADC 10 is then provided to a controller 30. The controller 30 may be implemented as a microcontroller, using either hardware or software control. Based on the theoretical model of a telephone line network and the expected voltage/current characteristic, as dictated by the appropriate DC mask, the controller 30 sets the line current to an optimum value (via an output voltage) VDAC and causes the line voltage to adjust so as to minimize power dissipation in the circuit. The controller 30 can set the line current precisely via a digital-to-analog converter (DAC) 20, by changing the voltage at the base terminal of the electronic inductor transistor Q1 and measuring the emitter voltage Ve. The line current will be equal to the emitter voltage Ve divided by the emitter resistor Re.
Furthermore, the controller 30 can calculate the power dissipation in the emitter resistor Re from the value of the emitter voltage Ve, and determine the power dissipation in the transistor Q1. While determining the optimum voltage/current line setting, the controller 30 also takes into account the specific requirements of a particular DC mask, depending upon the country of operation. A switch S1 is enabled to increase the dynamic range of the ADC 10 with respect to VTR, by adding a resistor R3 in parallel with R2 so that a relatively large VTR can be measured within the limited voltage range of the ADC 10 (typically 0-4V).
In an alternative embodiment (FIG. 2), the ADC 10 reads the current between tip and ring (Itr) directly from the emitter of the transmitter Q1 by measuring Ve and dividing by Re. The controller 30 can select either switch S2 or S3 to read either voltage Vtrdc or Ve. In other embodiments, various feedback lines, controlled by the controller 30, are used to provide for greater current control. The electronic inductor transistor may also be configured as a Darlington pair. Various switches may be added to provide greater control over the operating range of the interface circuit, and are controlled by the controller 30.
The control logic of the controller may be implemented in software, and the circuit adjusted using either a static, dynamic or static-dynamic combination control method.