Abstract:
In a conventional signal transmission device that transmits signals fed thereto to another device through a transformer, when the direction of the current flowing through the primary coil of the transformer is switched by switching devices connected in series between two different potentials, the timing with which the switching device that has been receiving current up to the moment is turned off is delayed from the timing with which the other switching devices are turned on or off to reduce overshoots and undershoots. This, however, may distort the square wave appearing across the secondary coil and make correct signal transmission impossible. To prevent this, a signal transmission device of the invention additionally has a waveform adjustment circuit that controls the delay time produced by a timing adjustment circuit according to changes in the states of control signals.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a signal transmission device that transmits signals fed thereto to another device through a transformer.  
           [0003]    2. Description of the Prior Art  
           [0004]    As an example of a conventional signal transmission device, a terminal adapter, which is needed to connect terminal equipment such as a personal computer to an ISDN (integrated services digital network) line, will be described. FIG. 4 is a diagram showing an outline of an example of a configuration that permits connection between an ISDN line and terminal equipment. A similar configuration is disclosed in Japanese Patent Application Laid-Open No. H11-330937.  
           [0005]    As shown in this figure, to connect terminal equipment TE to an ISDN line, it is necessary to use a terminal adapter  10  that converts the signals fed thereto from the terminal equipment TE into a signal format adapted for the ISDN line and a digital service unit  20  (hereinafter referred to as the “DSU  20 ”) that serves as a terminal interface device by converting the signals from a telephone company, which are adapted for network transfer, to signals adapted for distribution inside a household so as to be ready for bus-based wiring.  
           [0006]    The interface I/F between the terminal adapter  10  and the DSU  20  is composed of a transformer employing coils, and the ISDN standard sets strict requirements regarding overshoots and undershoots that occur at such a signal conversion point.  
           [0007]    [0007]FIG. 5 is a diagram showing an outline of the configuration of an example of a conventional terminal adapter  10 . A logic circuit  11  that converts the signals from the terminal equipment TE into a signal format adapted for the ISDN line is connected through an output circuit  12  to the primary coil L 1  of the interface I/F. The output circuit  12  feeds the coil Li with current based on the signals output from the logic circuit  11 , and the output stage of the output circuit  12  is composed of a P-channel MOS transistor QH 1  and an N-channel MOS transistor QL 1  that are connected in series between a supply voltage line and a GND (ground) line and a P-channel MOS transistor QH 2  and an N-channel MOS transistor QL 2  that are similarly connected in series between the supply voltage line and the GND line.  
           [0008]    The drains of the transistors QH 1  and QL 1  are connected together through a resistor R 1  to one end of the coil L 1 , and the drains of the transistors QH 2  and QL 2  are connected together through a resistor R 2  to the other end of the coil L 1 . The sources of the transistors QH 1  and QH 2  are connected through constant-current source circuits CC 1  and CC 2 , respectively, to the supply voltage line, and the sources of the transistors QL 1  and QL 2  are connected to the GND line. The resistors R 1  and R 2  both serve to limit the current that flows through the coil L 1 , and the constant-current source circuits CC 1  and CC 2  both serve to limit transient fluctuations in the current that flows through the coil L 1 .  
           [0009]    The gate of the transistor QH 1  is connected directly to a first output terminal O 1  of the logic circuit  11 , and the gate of the transistor QH 2  is connected directly to a second output terminal O 2  of the logic circuit  11 . The gate of the transistor QL 1  is connected through a timing adjustment circuit T 1  to the first output terminal O 1 , and the gate of the transistor QL 2  is connected through a timing adjustment circuit T 2  to the second output terminal O 2 .  
           [0010]    Here, when the logic circuit  11  outputs at its first and second output terminals O 1  and O 2  a H (high) level and a L (low) level, respectively, the transistors QH 1  and QL 2  are off, and the transistors QL 1  and QH 2  are on. Accordingly, the constant current produced by the constant-current source circuit CC 2  flows through the transistor QH 2 , resistor R 2 , coil L 1 , resistor R 1 , and transistor QL 1  to the GND line, and thus the voltage F appearing across the coil L 2  is positive (in a H state).  
           [0011]    By contrast, when the logic circuit  11  outputs at its first and second output terminals O 1  and O 2  a L level and a H level, respectively, the transistors QH 1  and QL 2  are on, and the transistors QL 1  and QH 2  are off. Accordingly, the constant current produced by the constant-current source circuit CC 1  flows through the transistor QH 1 , resistor R 1 , coil L 1 , resistor R 2 , and transistor QL 2  to the GND line, and thus the voltage F appearing across the coil L 2  is negative (in a L state).  
           [0012]    When the logic circuit  11  outputs a L level at both of its first and second output terminals O 1  and O 2 , no current flows through the coil L 1 , and thus no voltage appears across the coil L 2  (in a M (middle) state). It never occurs that the logic circuit  11  outputs a H level at both of its first and second output terminals O 1  and O 2 .  
           [0013]    Now, the operation of the timing adjustment circuits T 1  and T 2  mentioned above will be described. The timing adjustment circuits T 1  and T 2  are provided for the purpose of reducing the overshoots and undershoots that occur in the square wave appearing across the coil L 1 . The timing adjustment circuits T 1  and T 2  are so configured that, when the input thereto rises to a H level, they turn their output to a H level irrespective of a clock CLK but, when the input drops to a L level, they turn their output to a L level after counting a predetermined number of pulses in the clock CLK.  
           [0014]    For example, in a case where the timing adjustment circuits T 1  and T 2  receive, as their clock CLK, the clock signal (for example, having a frequency of 6 MHz) used for the logic circuit  11 , and are so configured that they turn their output to a L level in synchronism with the second rising edge in the clock CLK, the timing adjustment circuits T 1  and T 2  produce a delay time t of about 170 nanoseconds.  
           [0015]    In this way, the timing adjustment circuits T 1  and T 2  serve to shift level switching points of the digital signals that are applied to the gates of the transistors QL 1  and QL 2 , and desired timing adjustment can easily be achieved by using, for example, flip-flops. Thus, it is possible to produce a short delay stably, without slowing down the data transfer rate.  
           [0016]    [0016]FIG. 6 is a timing chart showing the waveforms of signals observed at relevant points in the terminal adapter  10 . In this figure, reference symbols A, B, C, and D indicate the digital signals that are applied to the gates of the transistors QH 1 , QL 1 , QH 2 , and QL 2 , respectively, and reference symbol F indicates the voltage that appears across the coil L 2 .  
           [0017]    As described previously, in the terminal adapter  10  configured as described above, as shown in the figure, level switching points of the digital signals B and D applied to the gates of the corresponding transistors are shifted by the timing adjustment circuits T 1  and T 2 . More precisely, when the individual transistors are switched between on and off, any transistor that has been receiving current from the coil L 1  up to that moment is switched from on to off with a delay of t from the time point at which the other transistors are switched between on and off.  
           [0018]    In the terminal adapter  10  configured as described above, it is possible, indeed, to reduce transient fluctuations in the current flowing through the coil L 1  and thereby reduce the back electromotive force induced by the inductance of the coil. As a result, it is possible to reduce the overshoots OS and undershoots US (indicated by a dash-and-dot line in the figure) that occur in the square wave appearing across the coil L 2 .  
           [0019]    However, in the terminal adapter  10  configured as described above, on occasions when the levels of the two signals output from the logic circuit  11  are switched in opposite directions, i.e. when one signal turns from a H level to a L level and simultaneously the other signal turns from a L level to a H level, the square wave appearing across the coil L 2  may be distorted as indicated by reference symbol S in FIG. 6. This problem arises when the delay time t produced by the timing adjustment circuits T 1  and T 2  is too long, and the signal INFOL on an ISDN line or the like is particularly prone to such distortion.  
           [0020]    Distortion as described above of the square wave appearing across the coil L 2  leads to increased jitter, which not only degrades signal transmission quality but also, in some cases, brings signals out of synchronism, making correct signal transmission impossible.  
         SUMMARY OF THE INVENTION  
         [0021]    An object of the present invention is to provide a signal transmission device that can reduce overshoots and undershoots without distorting the waveform of a square wave appearing across a coil constituting a transformer.  
           [0022]    To achieve the above object, according to the present invention, a signal transmission device is provided with: a transformer having primary and secondary coils; switching devices that are connected in series between two different potentials and that are turned on/off individually according to a control signal; a coil driving circuit that switches the direction of the current flowing through the primary coil by controlling the switching devices; a timing adjustment circuit that delays the timing with which, of all the switching devices, the one which has been receiving current up to now is turned off relative to the timing with which the other switching devices are turned on or off; and a waveform adjustment circuit that controls the delay time produced by the timing adjustment circuit according to changes in the state of the control signal. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    This and other objects and features of the present invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the accompanying drawings in which:  
         [0024]    [0024]FIG. 1 is a diagram showing an outline of the configuration of a terminal adapter embodying the invention;  
         [0025]    [0025]FIG. 2 is a logic circuit diagram showing an example of the configuration of the timing adjustment circuits T 1  and T 2  and the waveform adjustment circuit  13 ;  
         [0026]    [0026]FIG. 3 is a timing chart showing the waveforms of signals at relevant points in the terminal adapter  1 ;  
         [0027]    [0027]FIG. 4 is a diagram showing an outline of an example of a conventional configuration that permits connection between an ISDN line and terminal equipment;  
         [0028]    [0028]FIG. 5 is a diagram showing an outline of the configuration of an example of a conventional terminal adapter  10 ; and  
         [0029]    [0029]FIG. 6 is a timing chart showing the waveforms of signals observed at relevant points in the conventional terminal adapter  10 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    As an example of a signal transfer device embodying the embodiment, a terminal adapter, which is needed to connect terminal equipment to an ISDN line, will be described. FIG. 1 is a diagram showing an outline of the configuration of a terminal adapter embodying the invention.  
         [0031]    As shown in this figure, the terminal adapter  1  of this embodiment is, as compared with the conventional configuration (see FIG. 5), additionally provided with a waveform adjustment circuit  13 , which enables the terminal adapter  1  to reduce overshoots and undershoots without distorting the square wave appearing across the secondary coil L 2  constituting the interface I/F.  
         [0032]    Accordingly, here, such circuit blocks and elements as are found also in the conventional configuration shown in FIG. 5 are identified with the same reference numerals, and their explanations will not be repeated. In the following descriptions, a special emphasis is placed on the configuration and operation of the waveform adjustment circuit  13 , which is the circuit block peculiar to the present invention.  
         [0033]    The waveform adjustment circuit  13  controls the operation of the timing adjustment circuits T 1  and T 2  according to the logic levels of the digital signals fed from the timing adjustment circuits T 1  and T 2  to the waveform adjustment circuit  13 . FIG. 2 is a logic circuit diagram showing an example of the configuration of the timing adjustment circuits T 1  and T 2  and the waveform adjustment circuit  13 .  
         [0034]    As shown in this figure, the timing adjustment circuit T 1  is composed of a D flip-flop D 1  and an OR circuit OR 1 . The first output terminal O 1  of the logic circuit  11  is connected to the data input terminal of the D flip-flop D 1  and to one input terminal of the OR circuit OR 1 . The output terminal of the D flip-flop D 1  is connected to the other input terminal of the OR circuit OR 1 . The output terminal of the OR circuit OR 1  is connected to the gate of the transistor QL 1 .  
         [0035]    Similarly, the timing adjustment circuit T 2  is composed of a D flip-flop D 2  and an OR circuit OR 2 . The second output terminal O 2  of the logic circuit  11  is connected to the data input terminal of the D flip-flop D 2  and to one input terminal of the OR circuit OR 2 . The output terminal of the D flip-flop D 2  is connected to the other input terminal of the OR circuit OR 2 . The output terminal of the OR circuit OR 2  is connected to the gate of the transistor QL 2 . The D flip-flops D 1  and D 2  both receive a common clock at their respective clock terminals.  
         [0036]    On the other hand, the waveform adjustment circuit  13  is composed of two NAND circuits NAND 1  and NAND 2  and an AND circuit AND 3 . The first output terminal O 1  of the logic circuit  11  is connected to one input terminal of the NAND circuit NAND 1 , and the output terminal of the D flip-flop D 2  is connected to the other input terminal of the NAND circuit NAND 1 . The second output terminal O 2  of the logic circuit  11  is connected to one input terminal of the NAND circuit NAND 2 , and the output terminal of the D flip-flop D 1  is connected to the other input terminal of the NAND circuit NAND 2 . The output terminals of the NAND circuits NAND 1  and NAND 2  are respectively connected to the two input terminals of the AND circuit AND 3 , and the output terminal of the AND circuit AND 3  is connected to the reset terminals of the D flip-flops D 1  and D 2 .  
         [0037]    Thus, the waveform adjustment circuit  13  outputs, as its output signal E, a L (low) level when the digital signal A output from the logic circuit  11  at its first output terminal  01  and the digital signal Cd (a delayed version of the digital signal C) output from the D flip-flop D 2  are both at a H (high) level, or when the digital signal C output from the logic circuit  11  at its second output terminal  02  and the digital signal Ad (a delayed version of the digital signal A) output from the D flip-flop D 1  are both at a H level. Otherwise, the waveform adjustment circuit  13  outputs a H level. When the output signal E turns to a L level, the D flip-flops D 1  and D 2  are both reset.  
         [0038]    Next, the operation of the waveform adjustment circuit  13  described above will be described in more detail with reference to FIG. 3. FIG. 3 is a timing chart showing the waveforms of signals at relevant points in the terminal adapter  1 . In this figure, reference symbols A, B, C, and D indicate the digital signals that are applied to the gates of the transistors QH 1 , QL 1 , QH 2 , and QL 2 , respectively, and reference symbol E indicates the output signal of the waveform adjustment circuit  13 . Reference symbol F indicates the voltage that appears across the coil L 2 .  
         [0039]    As this figure shows, on occasions when the levels of the two signals (the digital signals A and C) output from the logic circuit  11  are switched in opposite directions, first the digital signal C or A turns from a L level to a H level, and then, with a delay, the digital signal Ad or Cd turns in the opposite direction. Thus, a period is produced in which both the digital signals A and Cd, or both the digital signals C and Ad, are at an H level simultaneously. As a result, as described previously, the output signal E of the waveform adjustment circuit  13  turns to a L level, resetting both the D flip-flops D 1  and D 2 .  
         [0040]    This circuit configuration permits the length of the delay time t produced by the timing adjustment circuits T 1  and T 2  to be controlled according to the logic levels of the two signals output from the logic circuit  11 . Specifically, it is possible, exclusively on occasions when the levels of the two signals output from the logic circuit  11  are switched in opposite directions, i.e. when the direction of the current flowing through the primary coil L 1  is inverted, to make the delay time t produced by the timing adjustment circuits T 1  and T 2  include only a delay time ti (several tens of nanoseconds) based on the gate delay, which is far shorter than a delay time t 2  (about 170 nanoseconds) based on the timing of the clock CLK.  
         [0041]    In this way, by making the delay time t produced by the timing adjustment circuits T 1  and T 2  shorter, specifically one tenth or shorter, exclusively when the direction of the current flowing through the primary coil L 1  is inverted as compared with when the current is turned off, it is possible to reduce overshoots and undershoots while minimizing distortion, unnecessary delay, and other unwanted effects in the square wave appearing across the coil L 2  and thereby achieve smooth signal transmission with a minimum amount of jitter.  
         [0042]    In the embodiment described above, as the switching devices constituting the driving circuit for the coil L 1 , P-channel and N-channel MOS transistors are used. This helps make the range of the voltage applied to the coil L 1  as wide as the supply voltage permits and thereby maximize the range of the voltage appearing across the coil L 2 . However, it is also possible to use as those switching devices only N-channel MOS transistors. This helps keep the range of the voltage across the coil L 1  lower than the supply voltage and thus further reduce overshoots and undershoots that occur in the coil L 2 .  
         [0043]    The logic circuit  11  may be so configured as to yield only one output. In that case, the voltages applied to the gates of the individual transistors are produced by the use of an inverter circuit. The number of switching devices used may be two or more, and, as the switching devices, bipolar transistors may be used.  
         [0044]    In the terminal adapter  1  configured as described above, by adjusting the number of inverters provided on the output side of the NAND circuits NAND 1  and NAND 2  or other parameters, it is possible to adjust the length of the delay time easily. It is to be understood that the present invention may be carried out with any other circuit configuration than specifically described above in connection with the embodiment, as long as it operates in a similar manner. For example, amplifiers may be connected respectively to the gates of the transistors QH 1  and QH 2  constituting the output circuit  12  so that the output level is adapted to comply with the standard.