1. Field of Invention
This invention relates to communication interfaces and design methods therefor. More particularly, the invention relates to a telephone line interface which simultaneously shapes received signals to reject unwanted signals and synthesized matching impedances for different frequency bands on a connecting telephone line and a design method therefor.
2. Description of Prior Art
Communication interfaces, more particularly, telephone line interface circuits, are required to provide a particular set of functions associated with connection to a telephone cable pair carrying an analog/digital telephone signal. This set of functions includes the following:
1. Termination of the telephone cable pair with a specified impedance. The impedance is generally different in different countries. In some cases, the impedance is complex with a magnitude that decreases with increasing frequency. In addition, some countries require that the terminating impedance be greater in magnitude than some specified value at one more frequencies above the voice band.
2. Maintenance of an appropriate receive frequency response. Generally, the objective is to maintain a frequency response as flat as possible, i.e., with as little variation in frequency as possible.
3. Removal of certain network-generated signals which are used in telephone systems of some countries. A major example is the so-called metering-tone, a signal above the voice frequency band (typically a 12 kHz. or 16 kHz.) that must be filtered out in the receive signal path.
Classically, telephone line interface circuits have been implemented with a transformer in the signal path. In such circuits, metering-tone signals are commonly removed using a passive LC filter between the transformer and the telephone line. The LC filter simultaneously provides a high impedance to the telephone line at the metering-tone frequency. In interface circuits of this type, the terminating impedance in the voice band may be realized in a number of ways. However, when the terminating impedance must be complex, there is an unavoidable roll-off introduced in the receive response. That is the magnitude of the receive response is not flat but decreases with increasing frequency. The roll-off must be compensated elsewhere in the receive path using extra circuitry.
Recently, there has been growing interest in telephone line interface circuits that are physically small and that employ a single hardware design that meets requirements of all countries, either statically or by reconfiguration of the hardware under control of software. Such circuits cannot employ transformers in the signal path in the usual way, because transformers are physically large (relatively speaking) and interfere with the ability to program certain of the telephone line interface circuit characteristics. Such circuits cannot employ LC metering-tone filters because the inductors in these filters are physically large; different sets of filter components are required for different metering-tone frequencies; and the ability to reconfigure these filters accordingly, is extremely difficult and costly to realize. Finally, such circuits suffer from the problem of roll-off of the receive response with complex-impedance termination and require additional circuitry to compensate for roll-off. Recently, active filters using transconductance amplifiers with feedback have been used to synthesize terminating impedances and may be programmed to serve a wide variety of terminating conditions. The application of such active filters to the problem of receive signal roll-off would advance the state of the art.
Prior art related to terminating impedances for telephone interface circuits using a transconductance amplifier with feedback includes the following:
U.S. Pat. No. 5,500,894 issued Mar. 19, 1996, discloses a telephone line interface circuit with AC and DC transconductance loops for providing selectable impedances to a cable pair. The '894 patent does not disclose a cascade of sub-filter networks which simultaneously shapes a receive path response to prevent roll-off from unwanted signal components and matching terminating impedances for different frequency bands on the cable pair.
U.S. Pat. No. 4,894,864 issued Jan. 16, 1990, discloses a transconductance loop including a filter network for synthesing a terminating impedance into real and imaginary components for a single frequency band and does not simultaneously shape a receive path response to prevent roll-off.
U.S. Pat. No. 4,351,060 issued Sep. 21, 1982, discloses a transconductance feedback loop including a filter for synthesizing a matching impedance for a single frequency on a connecting telephone circuit, but which does not simultaneously shape receive path response to prevent roll-off.
U.S. Pat. No. 4,381,561 issued Apr. 26, 1983, discloses impedance matching for an interface circuit using a digital filter and D/A converter in a negative feedback loop to match a full duplex analog telephone line to a digital switching system. The '561 patent does not simultaneously shape (a) the synthesized matching impedance and (b) a receive path response to prevent roll-off.
U.S. Pat. No. 4,359,609 issued Nov. 16, 1982, discloses an interface circuit having a feedback loop, either current or voltage controlled, by which the circuit impedance can be easily set to match a telephone circuit. The '609 patent does not disclose simultaneously shaping (a) synthesized matching impedances to different frequency bands and (b) a receive path response to prevent roll off on a connecting cable pair.
U.S. Pat. No. 4,961,219 issued Oct. 2, 1990, discloses synthesing an impedance across a telephone line by detecting a voltage difference across the telephone line and using an RC circuit to develop a pair of impedance output signals matching the impedance of the telephone line circuit. The '219 patent does not disclose a transconductance loop and filter for simultaneously shaping (a) synthesizing matching impedance to that of different telephone line circuits and (b) a receive path response to prevent roll off.
U.S. Pat. No. 5,598,467 issued Jan. 28, 1977, and filed Dec. 9, 1994, discloses shaping of the receive and transmit frequency responses in addition to the termination impedance. However, the '467 patent does not disclose simultaneously shaping (a) synthesized matching impedances for different frequency bands and (b) a receive path response to prevent roll-off.
None of the prior art, alone or in combination, disclose or suggest a communication interface in which a receive signal from a telephone line circuit is taken from a point within a feedback loop for transconductance amplifier to control the shape of the receive signal to prevent roll-off while the frequency response of the terminating impedance is shaped to match the transmission impedance of a connecting telephone line circuit carrying different frequency bands.