Attenuation and termination circuit using impedance synthesis

The present invention discloses circuits for isolating and attenuating signals generated by a telephone network. In disclosed embodiments, a metering pulse signal is isolated from the terminals of the connecting device, and then attenuated with an impedance that is synthesized with a programmable digital signal processor. Embodiments also utilize the digital signal processor to synthesize a termination impedance for the connecting device. The termination impedance matches closely the characteristic impedance of the network, so as to minimize wave reflections and the like.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to circuitry for terminating and attenuating electrical signals. More particularly, the present invention relates to circuitry for isolating and attenuating incompatible signals present on a telephone network, such as a metering pulse signal.

2. Present State of the Art

The primary function of a telephone network (commonly referred to as the public switched telephone network, or PSTN) is to transmit audio signals. Accordingly, telephone networks have been optimized to carry signals having frequencies and amplitudes that fall within the audio spectrum. However this has limited the ability to utilize the telephone network as a transmission medium for other types of signals falling within different frequency spectrums. Similarly, problems can arise when non-traditional telephone equipment, such as modems, are connected to the telephone.

For a device to connect to the telephone network, it must be able to interpret the various signals that are present on the telephone network. In addition, the device must comply with various standards that are imposed by the telephone network. For instance, in the U.S., to connect to the PSTN, the central switching office places a direct current voltage of approximately −48 volts on the telephone line, and requires that a connecting device, such as a telephone or modem, draw little or no current when the device is not in use. This requirement is satisfied by designing the connecting device to have a large, specified impedance when the device is not in use, referred to as the on hook state. This large impedance effectively creates an open circuit which draws little current.

Another example of a PSTN mandated requirement is that when the device is being used (off-hook), the impedance of the device must closely match the characteristic impedance of the telephone network, typically 600 ohms. This ensures that the signal being transmitted over the telephone network is not reflected due to an impedance mismatch. Thus, a connecting device must be capable of presenting different impedances to the telephone network, depending on its connection state.

The design of a telephone network-connecting device becomes more complex when signals having frequencies and amplitudes that are different from conventional audio signals are present on the telephone network. One such signal is referred to as a “metering pulse signal.” While not present in the U.S. PSTN, in some countries, the telephone network generates metering pulse signals that have a carrier frequency at or above 12 kHz. The metering pulse signal permits entities such as small business to obtain instant billing information so that the cost of a telephone call can be identified. By instantly knowing the cost of a telephone call, the business can charge its customers appropriately.

The metering pulse signal, in addition to having a frequency that is typically higher than voice communications, has peak amplitudes on the order or 20 to 40 volts. As such, the signal can be detrimental to the operation of modems and other connecting devices. For example, the signal can saturate the receive amplifier, corrupt the data, and can physically damage electrical components of the connecting device. In addition, the metering pulse signal, like other signals present on the telephone network, is subject to certain specifications. In particular, the metering pulse signal cannot be attenuated by more than a prescribed amount of about 3 dβ at the input terminals of the connecting device.

One solution to the problem presented by signals such as the metering pulse signal is to use an external podule that contains an attenuation filter. However, this approach is not entirely satisfactory because of the additional cost incurred in order to manufacture, stock and distribute the podules. In addition, the customer is burdened with not only additional equipment that must be connected to the modem, but also with the additional cost associated with purchasing the podules.

Thus, it would be an advancement in the art to provide the ability to limit the detrimental effects of incompatible telephone network signals, such as metering pulse signals, on certain network connecting devices, such as modems. Moreover, it would be an advance in the art to provide a protection scheme that doesn't require extra external equipment, such as an external filtering podule.

OBJECTS AND SUMMARY

In view of the foregoing and other problems in the prior art, it is an overall object of one embodiment of the present invention to provide a system and method that allows a connecting device to operate on a telephone network in the presence of an incompatible network signal, by electrically isolating and attenuating the incompatible signal.

Another objective is to provide a system and method that accomplishes such signal isolation and attenuation without using equipment, such as filter podules.

It is a another object to provide a system and method that attenuates an incompatible signal present on a telephone network or other communication network by automatically generating and providing a synthesized impedance, thereby preventing the signal from interfering with the operation of a connecting device, such as a modem.

Another objective is to isolate and attenuate the incompatible signal, such as a metering pulse signal, in a manner that does not interfere with the operation of the telephone network, or in a manner that otherwise violates the operating characteristics of the network.

It is also an objective of embodiments of the present invention to automatically synthesize a termination impedance that substantially matches the impedance of the telephone network, thereby preventing wave reflections and signal losses.

In summary, these and other objectives are obtained with embodiments of the present invention, that include a circuit that isolates a network device from frequency-undesirable or frequency-incompatible signals that may be present in a telephone network, and that utilizes a synthesized impedance to attenuate the signal level of any undesirable network signal. Because the signal is attenuated, it does not interfere with the operation of the network connected device. Embodiments also provide the ability to synthesize desired termination impedance, so that the network connected device matches the characteristic impedance of the network.

One example of the type incompatible signal addressed is the metering pulse signal described above. As was noted, a metering pulse signal has a carrier frequency on the order of 12 kHz or more, and has peak amplitude of 20 to 40 volts or more, which can be incompatible with and even damaging to certain network connected devices, such as a modem.

The detrimental effects of this type of incompatible network signal are prevented when embodiments of the present invention are implemented in network connected devices. Presently preferred embodiments provide an electrical path for the metering pulse signal (or other incompatible signal) to electrically isolate it from the terminals of the connecting device. For example, in one embodiment a resistive circuit that is located such that the termination impedance of the connecting device is substantially unaffected performs the isolation. Moreover, the approach ensures that the metering pulse signal is not attenuated more than 3 dB across the tip and ring terminals, less than an amount that would violate the requirements of the PSTN.

Once the metering pulse signal has been electrically isolated, an impedance synthesis circuit automatically generates impedance. The impedance value is such that the metering pulse signal is attenuated by a predetermined amount at the end of the isolating resister away from the wire pair terminals, such as the previously mentioned tip and ring terminals. The impedance synthesis circuitry effectively performs the function of a notch filter, but does not require the use of physical circuit components typically used in filters, such as capacitors, inductors, and the like. Also, electrical components already existing within the modem or other network connected device can be used to perform the impedance synthesis function. This eliminates the need for any external, or additional parts, thereby reducing cost, manufacturing complexity, and operational complexity.

In addition, in preferred embodiments, the impedance synthesis circuitry is implemented so as to be much more precise than conventional filter podules using discrete components, because the impedance synthesis is implemented in a manner so as to preferably emulate the behavior of ideal circuit components by way of a digital signal processor. The impedance synthesis, in combination with the isolation resistor, effectively eliminates the harmful effects of the metering pulse signal (or similar signal) on connecting devices such as modems. In operation, the attenuation circuitry can cause the metering pulse signal to effectively disappear at the input of the receive amplifier of a modem, or similar network connected device. As such, the metering pulse signal has no detrimental effect on the operation of the device. Moreover, the metering pulse signal is not attenuated at the terminals of the connecting device in violation of the specifications of the telephone network.

Embodiments of the present invention also generate a desired termination impedance for the network connected device. Preferably, this synthesized impedance matches the characteristic impedance of the network, so as to avoid wave reflections, and thereby maintain compatibility with the network.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Devices that connect or interface with a telephone network are usually designed to comply with standards specified by that telephone network. For example, the specifications may require that signals have certain frequencies and amplitudes and/or may require that the connecting devices present a specified termination impedance to the network. A device that does not comply with specified requirements may not operate with the telephone network, and may have an adverse effect on the operation of telephone network.

The design of a telephone or other connecting device is complicated by the fact that many differences exist between the specifications of telephone networks in different countries. In some instances, a telephone network may produce signals that are not present on other telephone networks, resulting in connecting device incompatibility. The ability of a connecting device to function in more than one telephone network is dependent on its ability to deal with such incompatible signals.

One example of a signal that is incompatible with certain connecting devices, such as modems, is known as a metering pulse signal. The metering pulse signal typically has a carrier frequency greater than 12 kHz and has peak voltages on the order of 20 to 40 or more volts. As described previously, the signal can disrupt the communication capabilities of the connecting device. In the case of a modem, the metering pulse signal may cause the communication to end prematurely or may introduce erroneous data into the communication. Additionally, the high voltages of the metering pulse signal can damage the modem's electrical components, which are usually designed to operate with voltages lower than 5 volts.

Embodiments of the present invention overcome the challenges presented by the metering pulse, or similar types of incompatible signals. Also, presently preferred embodiments can be integrated or incorporated with a connecting device, thereby eliminating the necessity of having external filters. Moreover, the number of additional circuit components is minimized by taking advantage of components and systems that may already be present on a connecting device. The elimination of podules and the utilization of existing subsystems lowers the overall cost and complexity of the connecting device.

Preferred embodiments of the present invention utilize systems and methods that introduce superior accuracy compared to solutions that use physical circuit elements because the metering pulse is attenuated by systems and methods that are comparable to ideal circuit elements. Also, preferred embodiments of the present invention can be easily adjusted to accommodate the various frequencies which may be present on different telephone networks. It will be appreciated that while the present invention is described in terms of a metering pulse signal, the systems and methods of the present invention are not limited to this particular signal, and can be applied to other such incompatible signals present in a telephone network, or other network such as a computer network.

Reference is first made toFIG. 1, which is an exemplary system or environment in which the present invention may be utilized or implemented.FIG. 1is intended to be illustrative of potential systems which may utilize the present invention and is not to be construed as limiting. The system ofFIG. 1illustrates a portable computer10having a PCMCIA compliant slot12which is configured to receive a PCMCIA compliant card14, which may be a modem, a network interface card, or any other card. The interface22of the card14is configured to detachably connect with a connector (not shown) inside slot12. Inserting the card14in slot12permits the card14to be in physical and electrical communication with computer10.

The card14also includes a connector24, which is illustrated inFIG. 1as an RJ type connector, but may be of any type, including but not limited to, a 15-pin connector or a coaxial cable connector. The connector24is configured to removably receive a plug26, which is connected to one end of the cable28. The other end of the cable28is connected to a plug26′, which is capable of detachably connecting with the jack30. The jack30is typically connected to a telephone network, a private branch exchange (PBX) system, or any type of computer network. A user needing access to the telephone network or other system gains access to that system through the jack30. Through the jack30, or through any other system access point, the computer10, and more specifically card14, is capable of communication with the network to which jack30provides access. With regard to the metering pulse signal, the jack30provides access to the telephone network and the card14is capable of receiving the signals generated by the telephone network through the jack30.

Reference is next made toFIG. 2, which is a block diagram of an exemplary system implementing a preferred embodiment of the present invention. The telephone network38comprises a central office40, which is responsible for generating the signals that are pertinent to the operation of telephone network38. The central office40is the source of the dial tone, the ringing signal and other signals; one example is a metering pulse signal, which is produced by the metering signal source42. The signals generated by central office40are transmitted over wire pair45, which comprises tip44and ring46in this embodiment. Wire pair45is an example of a telephone loop through which central office40applies various dc and ac voltages. Any device connecting to telephone network38usually sends and receives signals over the wire pair45and through the central office40.

A modem50is an exemplary connecting device that is capable of interfacing with telephone network38. As described previously, the modem50is required to comply with the specifications detailed by the telephone network38, including any requirements of the applicable regulatory agency. For example, the telephone network38may specify a characteristic impedance Z0, which is usually on the order of 600 ohms. In order to function properly, modem50must have a termination impedance of 600 ohms such that signal reflection is minimized and power transfer is maximized. The termination impedance specified by telephone network38is provided in this embodiment by termination circuitry60, described in further detail below.

FIG. 2also illustrates how presently preferred embodiments include attenuation circuitry70, which is preferably designed to meet at least two objectives. First, attenuation circuitry70must comply with the requirements of telephone network38. Secondly, the attenuation circuitry70prevents the metering pulse signal (or similar incompatible signal) from interfering with the operation of the modem50or other connecting device.

With respect to the first objective, as noted previously the telephone network38typically mandates that the metering pulse signal must not be attenuated by more than a certain amount at the terminals of the connecting device. Presently preferred embodiments of the attenuation circuitry70addresses this objective by providing a circuit path for the metering pulse signal that is sufficiently isolated from tip44and ring46. The isolation of the attenuation circuitry70ensures that the metering pulse signal is not attenuated by more than the amount specified by telephone network38at the terminals of modem50. However, with respect to the receive amplifier82of modem50, the metering pulse signal is attenuated such that it does not interfere with the operation of modem50.

Basically, the function of attenuation circuitry70is similar to that of a physical notch filter or low pass filter circuit. A low pass or notch filter is used in some instances to prevent signals having particular frequencies from passing. The attenuation circuitry70functions like a low pass or notch filter in the sense that only desired signals are attenuated in a preferred embodiment, while the data and other audio signals are not affected by the attenuation circuitry70. Thus the loop action attenuates the signal such that the specifications of telephone network38are satisfied with regard to the attenuation of the metering pulse signal at the terminals of the connecting device and the operation of the connecting device is not impaired. Additionally, the attenuation loop, that includes attenuation circuitry70, prevents the metering pulse signal from saturating receive amplifier82and prevents the metering pulse signal from interfering with the data being transmitted to the modem50.

FIG. 3is a more detailed block diagram of a presently preferred embodiment of the termination circuitry60and the attenuation circuitry70. Because of the similarities between impedance synthesis circuitry92and impedance synthesis circuitry90, the following discussion, which often refers to impedance synthesis circuitry90, also applies to impedance synthesis circuitry92.

In this embodiment, the specified impedance that is to be provided by the termination circuitry60is equal to the characteristic impedance of the telephone network. Thus, impedance synthesis circuitry92in termination circuitry60generates an impedance equivalent to the characteristic impedance Zoof the telephone network to which modem50is connected. In a similar fashion, the impedance synthesis circuitry90in attenuation circuitry70generates an impedance that minimizes the adverse effects of the metering pulse signal on modem50and the data being transmitted and received by modem50. In both instances, the specifications of telephone network38are satisfied.

In the illustrated embodiments, the attenuation circuitry70and the termination circuitry60are not subject to the tolerances of physical components, because they are able to digitally emulate the behavior of ideal circuit components. More specifically, impedance synthesis circuitry90and92preferably utilize a programmable digital signal processor (DSP) or the like, and software components, to synthesize a desired impedance to attenuate the metering pulse signal or match the characteristic impedance. This approach is not subject to the variable tolerances of physical circuit components. Also, the impedance synthesized is more accurate, especially when the analog to digital converters (ADC) and the digital to analog converters (DAC) have high resolution.

With regard to the illustrated embodiment, the impedance (Zs) that is provided by the impedance synthesis circuitry92is substantially equal to the characteristic impedance Zoof the telephone network. The impedance provided by the impedance synthesis circuitry90is related to the metering pulse signal characteristics. By way of example, the impedance synthesis circuitry90uses a detection circuit to detect the line voltage (Vt) across tip44and ring46. This voltage Vtis then utilized to produce the specified impedance (Zs), by providing a corresponding current value (It). Thus, the current (It) present across the terminals of the wire pair or across the full wave bridge terminals is the sensed voltage divided by the specified impedance (It=Vt/Zs). By application of Ohms law, the impedance seen across tip44and ring46is: Vt/I=Vt/(Vt/Zs)=Zs. In this manner, impedance synthesis circuitry is capable of generating impedance across a pair of terminals or wire pair such as tip44and ring46.

As is illustrated inFIG. 3, the attenuation circuitry70preferably includes a resistor75, which has a value that is large in comparison to the characteristic impedance of telephone network38. Resistor75serves to isolate the attenuation circuitry70from the tip44and the ring46leads of the telephone line. Thus, the impedance of attenuation circuitry70has a minimal effect on the synthesized impedance of termination circuitry60. In this way, the primary function of resistor75is to comply with the requirement of the telephone network that the metering pulse signal, as well as other signals, not be attenuated at the terminals of modem50or other connecting device as described previously.

The transmit amplifier80causes the Rx and Tx signals to be summed together as a form of feedback for the attenuation circuitry70. More specifically, the Tx signals may be separated from the RX signals by summing the appropriate level of negative Tx signal to the combined Rx and Tx signals, which allows for an Rx signal with a greatly reduced Tx content at the receive amplifier82.

Reference is next made toFIG. 4, which is a schematic diagram showing one embodiment with the relevant components for attenuation circuitry and termination circuitry. The telephone communication network associated with wire pair45, comprising tip44and ring46lines, delivers a metering pulse and data signal to the connector from metering and signal source42. The connector must not attenuate the metering pulse signal by more than 3 dB on wire pair45. Additionally, the characteristic impedance41must be properly terminated on the connector side for efficient data transfer. In this embodiment a DSP or modem data processor50performs the attenuation of the metering signal via notch filter70and the generation of characteristic impedance via characteristic impedance synthesis circuitry60. Some of the functions of the attenuation circuitry include, but are not limited to, preventing the metering pulse signal from saturating the receive amplifier82and preventing the metering pulse signal from interfering with the data being transmitted to the modem50. The attenuation circuitry70is isolated from wire pair45by resistor75, which has a value that is large in comparison to the characteristic impedance of telephone network. The termination circuitry60produces impedance substantially equal to the characteristic impedance (Zo)41of the telephone network. Resistor75prevents the impedance of attenuation circuitry70from affecting the value of the impedance synthesized by termination circuitry60. As such, in the absence of a metering pulse signal, which is transmitted over wire pair45at determined intervals, the attenuation circuitry70is effectively an open circuit compared to the impedance of the line. The attenuation feedback loop is summed with the transmit signal of modem50at the transmit amplifier80. The digitally synthesized output of transmit amplifier80is filtered by termination filter60. The output of the termination filter60is first converted to a controlling voltage by DAC64and then into a controlling current by V-I converter68, which controls current source66. The current source66generates a current Itso as to provide the desired characteristic impedance across wire pair45as described above.

Likewise, when a metering pulse signal is present on the wire pair45, the data processor receives a line voltage from ADC58and produces an output voltage that is calculated based upon a desired low pass or notch impedance (Znotch) needed to sufficiently attenuate the metering pulse signal. DAC74converts the output of notch filter70into a control voltage, which is converted into a controlling current by V-I converter78, which controls current source76. Thus the control voltage output by DAC74controls current source76. The current generated by current source76corresponds to the desired notch or low pass impedance value and attenuates the metering signal accordingly.

Thus the attenuation circuitry and termination circuitry configuration depicted inFIG. 4creates several advantages. First, many connection devices incorporate a DSP, ADC and DAC interfaces, and memory. Thus, the number of extra electrical components is reduced, thereby reducing manufacturing cost and complexity. Second, the attenuation performed by the processor based attenuation circuitry has much more accuracy as compared to attenuation circuitry composed of physical circuit components or notch filters. Finally, the necessity of an external filter podule and its associated costs are eliminated. While the impedance synthesis has been generated with a DSP or modem data processor, it is understood that other configurations, including digital circuits, filters and other circuit configurations may be used. Additionally, it is understood that the impedances synthesized by the attenuation circuitry and the termination circuitry may be synthesized using the same ADC, DSP, and DAC that are frequently already present on the connecting modem device.