Abstract:
Disclosed are improved modem apparatus and systems for interfacing with a Public Switched Telephone Network (PSTN). The modem includes a broadband data channel. A high-frequency transformer isolates the broadband data channel. An external power source provides one or more power levels for the broadband data channel. A transformerless embodiment using high voltage capacitors for isolation is also disclosed. The invention also provides a hybrid modem having a broadband data channel and a Voice Band (VB) data channel. Isolation is provided alternatively by a high-frequency transformer or high voltage capacitors.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to modems and, in particular, to broadband data modems for use with a Public Switched Telephone Network (PSTN). More particularly, the invention relates to improved apparatus and systems for providing power isolation for modems having broadband data capabilities.  
         BACKGROUND OF THE INVENTION  
         [0002]    Modem technology for data communication is in a state of rapid development, striving to accommodate voice band data and broadband data communications. Hybrid modems have been developed in an effort to provide Voice Band (VB) and broadband data communications, for example, Digital Subscriber Line service (DSL), using a single modem rather than two modems. Speed and efficiency, both in terms of cost and space, are two primary challenges facing hybrid modem technology.  
           [0003]    One of the major difficulties in integrating both broadband data and voice band data services in a single modem is caused by the requirement that the telephone line be isolated. The voice band data modem, typically operating in the range of up to about 3300 hertz, generally draws power directly from the telephone line and is thus unpowered when the telephone line is “on hook.” The broadband data modem, however, requires power whether the telephone line is in either “on hook” or “off hook” status.  
           [0004]    Existing solutions to providing the required power and isolation of both the broadband data and voice band data portions of hybrid modem typically employ two transformers. The Voice Band (VB) frequencies are low and thus the VB transformer needs to have either a large area or a large number of windings, and therefore tends to be bulky. The broadband transformer is required to operate with low loss at high frequencies, from about 30 kHz to 1 MHZ, and thus tends to be costly to manufacture. Another problem is that the transformers tend to be very large relative to the remainder of the modem circuitry.  
           [0005]    Accordingly, a need exists for a way of eliminating one or more of the transformers used for isolation in a broadband or hybrid modem. A modem using alternative means of providing isolation in place of one or more transformers would provide numerous advantages, including reducing system costs and size.  
         SUMMARY OF THE INVENTION  
         [0006]    The invention provides an improved modem apparatus and system with an interface to a Public Switched Telephone Network (PSTN). A codec having a voice band data portion and a broadband data portion is coupled to the interface for transmitting and receiving protocol-specific signals over the PSTN. The broadband portion of the codec draws power from an independent power supply. The voice band portion of the codec may draw power from the PSTN line. A coupling mechanism connects the codec with a processor facilitating data interface between the codec and external hardware.  
           [0007]    According to one aspect of the invention, the modem includes a broadband channel and a Voice Band (VB) channel. A pulse transformer forming an isolated DC-DC converter provides power for both VB and broadband channel. No power is drawn from the telephone line. A high voltage capacitive link isolates and transmits data to the processing unit.  
           [0008]    According to another aspect of the invention, the modem includes a broadband data channel and a Voice Band (VB) channel having a high-frequency transformer for isolating the broadband channel from the PSTN line in the signal path. An external power source provides power for the broadband data channel. Capacitive coupling isolates the VB channel. Power may be provided to the VB channel by the PSTN line.  
           [0009]    Another aspect of the invention includes a modem having a broadband data channel and a Voice Band (VB) channel with high-voltage capacitors in the signal path for isolating, and an external power source for powering, the broadband data channel. Capacitive coupling isolates the VB channel. VB channel power may be provided by the PSTN line.  
           [0010]    The invention disclosed provides many technical advantages in that the improvements facilitate the provision of a plurality of different voltages to the modem components while maintaining isolation from the PSTN line.  
           [0011]    Related technical advantages in size and cost are realized by the isolation and power features of the invention.  
           [0012]    Further technical advantages of the invention are achieved in that the improved hybrid modem uses components coupled to a single ground, making the modem of the invention amenable to manufacture as a single integrated circuit.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    In describing the features and advantages of the invention, reference is made to the following detailed description of the invention, taken in conjunction with the accompanying drawings of which:  
         [0014]    [0014]FIG. 1 is a block diagram showing an example of a hybrid modem of the invention;  
         [0015]    [0015]FIG. 2 shows a block diagram illustrating an alternative example of a hybrid modem of the invention; and  
         [0016]    [0016]FIG. 3 is a block diagram depicting another alternative example of a hybrid modem of the invention.  
         [0017]    Corresponding numerals and symbols in the various figures refer to corresponding parts in the detailed description unless otherwise indicated. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0018]    While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the invention.  
         [0019]    An example of a hybrid modem, according to the invention, is shown in the block diagram of FIG. 1. The hybrid modem  10  has an interface  12  to a Public Switched Telephone Network (PSTN) link  14 . Opposite the PSTN link  14 , a codec  16  is coupled to the interface  12 . The codec has a voice band data portion  18  and a broadband data portion  20 . The codec voice band and broadband portions,  18 ,  20  are capable of transmitting and receiving Voice Band (VB) and broadband, such as DSL, protocol signals, respectively.  
         [0020]    A DC power supply circuit  26  provides constant power to a power supply block  24  and, in turn, to the broadband data codec portion  20 , and preferably to the VB data portion  18 . The power supply block  24  is configured to provide power, regardless of whether the PSTN line  14  is “on hook” or “off hook”. An isolating connection, preferably a chopper amplifier circuit  28 , couples the DC power supply  26  with the broadband data codec  20 . Of course, it will be apparent that various configurations of chopper or DC-to-DC converters are possible without departure from the concept of the invention. The isolating connection  28  provides isolation for the DC power source  27  of the power supply circuit  26 . Preferably, the power supply block  24  is connected to the secondary winding  30  of a switching transformer  32 . The DC power supply  26  feeds power to the transformer  32 . The coupling of the power supply block  24  with the secondary winding  30  may be combined with appropriate components, typically rectifier diodes and filter capacitors (not shown), in order to provide a plurality of different voltages to the broadband data codec  20 , VB data codec  18 , and to line drivers  42 . In general, the amplifiers and 2-to-4 wire conversion elements are known as the “front end”  52  of the circuit.  
         [0021]    The voice band portion  18  of the codec  16  preferably includes high voltage components  22 , as known in the arts, such as transistors and diodes for providing Data Access Arrangement (DAA) functionality.  
         [0022]    A processor, preferably a digital signal processor  36 , is connected to the codec  16  through a coupling mechanism  38 , preferably high voltage capacitors in the range of 100-300 picofarad. A low voltage AC coupling capacitor  40  is preferably used to block DC current and PSTN signals from entering the broadband path.  
         [0023]    Of course, many variations are possible within the concept of the invention, including but not limited to the configuration of the power supply block  24  and line drivers  42 . In operation, the switching transformer  32 , along with capacitive coupling  38 , provides isolation of the broadband data path  13  and VB data path  12  from the telephone line  14 . The digital signals generated from the broadband data codec  20  are transmitted through the capacitive coupling  38  to the DSP  36 . Optionally, the VB portion  12  of the circuit may be omitted, providing a broadband data modem.  
         [0024]    Now referring to FIG. 2, a block diagram illustrates an alternative example of a hybrid modem  10  of the invention. As can be seen in the figure, the broadband portion  13  of the circuit contains a digital codec  50 , preferably implemented in a CMOS chip and having a VB interface. A front end portion  52  of the circuit is coupled to the secondary winding  54  of a high frequency wideband transformer  56 . Isolation of the broadband data path  13  is achieved by the use of wideband transformer  56 . Each block in the broadband data path  13  is directly supplied with DC power. Low voltage isolation capacitors  40  block DC current and PSTN ring signals from entering the broadband data path  13 .  
         [0025]    The voice band data portion of the circuit  12  draws its power from the PSTN line  14 , preferably using high voltage components  22 , such as bipolar transistors and diodes to perform the necessary DAA functions, for example, DC termination, AC termination, two to four wire conversion, ring detect, and features such as caller ID. A VB codec  58  accommodates VB traffic. All voice channel communications are done through the capacitive coupling  38  isolating the VB portion  12  of the circuit  10 . Preferably, the capacitive coupling  38  is provided with high voltage capacitors in the range of 100-300 pF. Thereafter, the VB signals pass through the digital codec/interface chip  50  to the signal processing circuit  36 .  
         [0026]    With reference to FIG. 3, another example of an embodiment of the invention is shown. The broadband data portion  13  of the modem  10  contains a digital codec and VB interface chip  50 . A front end portion  52  of the circuit is coupled to the phone line  14  with high voltage capacitors  60 , preferably about 68 nF. Isolation of the broadband data path  13  is achieved by the use of the high voltage capacitors. Each block in the broadband data path  13  is directly supplied with DC power. No transformer is required for this embodiment of the invention. It is believed that low frequency attenuation may be encountered due to coupling capacitor. This may optionally be compensated for by adjusting the transmit spectrum accordingly. Longitudinal balance resulting from the mismatch of termination resistors and the coupling capacitors may be an issue in this configuration and this configuration may require very high precision termination resistors and coupling capacitor.  
         [0027]    The voice band portion of the circuit  12  draws its power from the PSTN line  14 , preferably using high voltage components  22 , such as bipolar transistors and diodes to perform the necessary DAA functions as above. A VB codec  58  codes and decodes the VB communications. Capacitive coupling  38  isolates the VB portion  12  of the circuit  10 , preferably using high voltage capacitors in the range of 100-300 pF. The VB data signals pass through the digital codec/interface chip  50  to the signal processing circuit  36 . It will be understood by those skilled in the arts that the VB portion  12  of the circuit may be omitted, providing a broadband data modem.  
         [0028]    The embodiments shown and described above are only exemplary. Many details are often found in the art such as variations in digital protocols and standards. Therefor many such details are neither shown nor described. It is not claims that all of the details, parts, elements, or steps described and shown were invented herein. Even though numerous characteristics and advantages of the present inventions have been set forth in the foregoing description, together with details of the structure and function of the invention, the disclosure is illustrative only, and changes may be made in the detail, especially in matters of arrangement of the functional steps within the principles of the invention to the full extent indicated by the meaning of the terms used in the attached claims.