Abstract:
Disclosed herein is a signal output circuit for outputting a signal onto a transmission line having a given transmission characteristic, the signal output circuit including a drive circuit adapted to drive an input signal by a current; and an output resistor which is connected to an output stage of the drive circuit and capable of adjusting the output signal waveform according to its resistance, wherein the drive current of the drive circuit and the resistance of the output resistor are variable.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present invention contains subject matter related to Japanese Patent Application JP 2007-213018 filed in the Japan Patent Office on Aug. 17, 2007, the entire contents of which being incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The embodiment of the present invention relates to a signal output circuit, optical pickup and optical device for outputting a signal onto a transmission line having a given transmission characteristic. 
         [0004]    2. Description of the Related Art 
         [0005]    An optical disk pickup has to convey a signal to the signal processor at the subsequent stage via a relatively long flexible cable whose transmission characteristic varies from one product to another. 
         [0006]    Therefore, elaborately thought-out ideas are desired at the output end of the transmitting circuit (IC), including addition of an optimal damping resistor for the flexible cable. 
         [0007]    Signal transmission from the pickup to the IC at the subsequent stage has been optimized by adding a chip resistor to the output end of the IC incorporated in the pickup. 
         [0008]    Media such as CD and DVD in high speed mode, BD and HD-DVD desire a signal band of 100 MHz or higher. A signal deteriorates if transmitted via FPC (Flexible Printed Circuit), FFC (Flat Flexible Cable) or other flexible transmission line. 
         [0009]    Therefore, optical pickups available today have an external resistor (damping resistor) to adjust the transmission waveform, thus suppressing the resonance of the FPC and FFC and providing a more or less flat transmission characteristic at a frequency around 100 MHz. 
         [0010]      FIG. 1  is an equivalent circuit diagram of the signal transmission system in a typical optical pickup. 
         [0011]    In this circuit, a resistor Rd is externally added to the output side of the output circuit which is the transmitting IC of the optical pickup. 
         [0012]    The resistance of the resistor Rd is adjusted to be optimal for a transmission line  2  such as flexible cable and an IC  3  at the subsequent stage. 
         [0013]    This technique can ensure optimal transmission (suppress the peaking of the flexible transmission line) to a certain extent. 
       SUMMARY OF THE INVENTION 
       [0014]    However, the need for an external component in the signal transmission system leads to an increased component count and larger substrate area in the optical pickup. 
         [0015]    On the other hand, if load C is large, a large drive current is necessary. As a result, the signal transmission may not be optimized simply by adjusting the fixed output and resistor Rd. 
         [0016]    It is an aim of the embodiment of the present invention to provide a signal output circuit, optical pickup and optical device capable of adjusting the signal waveform to suit each of transmission lines having different transmission characteristics without any external resistor and, by extension, ensure optimal signal transmission tailored to the transmission characteristic of the transmission line. 
         [0017]    A first mode of the present invention is a signal output circuit adapted to output a signal onto a transmission line having a given transmission characteristic. The signal output circuit has a drive circuit and output resistor. The drive circuit drives an input signal by a current. The output resistor is connected to an output stage of the drive circuit and can adjust the output signal waveform according to its resistance. The drive current of the drive circuit and the resistance of the output resistor are variable. 
         [0018]    Preferably, the drive current of the drive circuit and the resistance of the output resistor can be controlled according to the transmission characteristic of the transmission line. 
         [0019]    Preferably, the drive circuit and output resistor are integrated in the same circuit. 
         [0020]    Preferably, the drive circuit has a plurality of push-pull output stages disposed in parallel for a signal input. One end of each of the output resistors is connected to the output side of each of the plurality of push-pull output stages. The other ends of the output resistors are connected to a common output terminal. 
         [0021]    Preferably, the output resistors of the drive circuit have different resistances. 
         [0022]    Preferably, the drive circuit has a current source in the input-side circuit of each of the push-pull output stages. The operating conditions of the current sources are controlled by drive signals. 
         [0023]    Preferably, at least the on/off operations or current values of the current sources are controlled by the drive signals. 
         [0024]    An optical pickup according to a second mode of the present invention has a laser beam source, photoreceiving element and signal output circuit. The photoreceiving element converts a laser beam from the laser beam source or a beam returning from a recording medium into electric signal. The signal output circuit outputs the electric signal converted by the photoreceiving element onto a transmission line having a given transmission characteristic. The signal output circuit has a drive circuit and output resistor. The drive circuit drives an input signal by a current. The output resistor is connected to the output stage of the drive circuit and can adjust the output signal waveform according to its resistance. The drive current of the drive circuit and the resistance of the output resistor are variable. 
         [0025]    An optical device according to a third mode of the present invention has an optical recording medium and optical pickup. The optical pickup has a laser beam source, photoreceiving element and signal output circuit. The photoreceiving element converts a laser beam from the laser beam source or a beam returning from a recording medium into electric signal. The signal output circuit outputs the electric signal converted by the photoreceiving element onto a transmission line having a given transmission characteristic. The signal output circuit has a drive circuit and output resistor. The drive circuit drives an input signal by a current. The output resistor is connected to the output stage of the drive circuit and can adjust the output signal waveform according to its resistance. The drive current of the drive circuit and the resistance of the output resistor are variable. 
         [0026]    In the embodiment of the present invention, the optical pickup is controlled so that the resistance of the output resistor is increased, for example, if the resonance Q value of the transmission line is large. On the other hand, the optical pickup is controlled so that the current driving capability is increased, for example, if the current is insufficient. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]      FIG. 1  is an equivalent circuit diagram of the signal transmission system in a typical optical pickup; 
           [0028]      FIG. 2  is a diagram illustrating a configuration example of the major sections of an optical disk device as an information processing device according to an embodiment of the present invention; 
           [0029]      FIGS. 3A and 3B  are diagrams illustrating a configuration example of a disk tray combined with an optical pickup; 
           [0030]      FIG. 4  is a diagram illustrating a configuration example of the major sections of the optical pickup according to the present embodiment; 
           [0031]      FIG. 5  is a diagram illustrating an example of writing waveform; 
           [0032]      FIG. 6  is a diagram illustrating an example of writing waveform after adjustment according to the present embodiment; 
           [0033]      FIG. 7  is a circuit diagram illustrating a specific configuration example of a signal output circuit according to the present embodiment; and 
           [0034]      FIG. 8  is an equivalent circuit diagram of a system to which the embodiment of the present invention is applicable. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0035]    The embodiment of the present invention eliminates the need for external resistor by integrating the drive circuit and output resistor in the same circuit, thus providing an effective way of downsizing the optical pickup. 
         [0036]    Further, the embodiment of the present invention can adjust the signal waveform to suit each of transmission lines having different transmission characteristics without any external resistor and, by extension, ensure optimal signal transmission tailored to the transmission characteristic of the transmission line. 
         [0037]    The preferred embodiment of the present invention will be described below with reference to the accompanying drawings. 
         [0038]      FIG. 2  is a diagram illustrating a configuration example of the major sections of an optical disk device as an information processing device according to an embodiment of the present invention. 
         [0039]    An optical disk device  10  has an optical pickup (OP)  20  and AFE (analog front end) mounting substrate  30 . The optical pickup  20  is a movable section adapted to read a signal. The AFE mounting substrate  30  is a fixed section incorporating an AFE IC. The optical pickup  20  and AFE mounting substrate  30  are connected to a signal transmission line  40 . In  FIG. 2 , reference numeral  50  represents an optical disk. 
         [0040]    The transmission line  40  is formed with a flexible transmission line such as FPC (Flexible Printed Circuit) or FFC (Flat Flexible Cable). 
         [0041]    The transmission line  40  is configured, for example, so that a disk tray  60  and the optical pickup  20  are combined as illustrated in  FIGS. 3A and 3B . 
         [0042]    As a result, the transmission line  40  is longer by as much as the disk is opened. 
         [0043]      FIG. 4  is a diagram illustrating a configuration example of the major sections of the optical pickup according to the present embodiment.  FIG. 4  illustrates an auto power control (APC) system. 
         [0044]    The optical pickup  20  in  FIG. 4  has a laser beam source or a laser diode (hereinafter LD)  21 , laser diode driver (hereinafter LDD)  22 , polarized beam splitter  23  and power monitoring circuit (IC)  24 . 
         [0045]    Further, although not shown, the optical pickup  20  also has a photodiode adapted to convert the beam returning from the optical disk  50 , an optical recording medium, into electric signal. 
         [0046]    The power monitoring circuit  24  has a photoreceiving element or photodetector (hereinafter PD)  241  and signal output circuit  242 . 
         [0047]    The signal output circuit  242  includes a drive circuit (driver) DRV and output resistor R 0 . The driver DRV outputs an electric signal converted from the beam by the PD  241 . The output resistor R 0  is adapted to adjust the damping and connected to the output stage of the driver DRV. 
         [0048]    In the present embodiment, the signal output circuit  242  is configured so that the output resistor R 0  is formed (integrated) in the IC and so that the drive current of the driver DRV and the resistance of the output resistor can be controlled according to the transmission characteristic of the transmission line  40 . 
         [0049]    That is, in the present embodiment, the IC incorporating the signal output circuit  242  also incorporates a variable damping resistor. Alternatively, the IC incorporates, in the IC, a plurality of damping resistors which can be switched one from another. Further, the driving capability (bias current of the output circuit) can be varied. This ensures optimal signal transmission over the flexible transmission lines  40  having different transmission characteristics. 
         [0050]    Thus, incorporation of the damping resistor R 0  in the IC ensures reduced component count and mounting area. Further, controlling the output drive current makes it possible to respond to load variations of the flexible transmission line  40 . 
         [0051]    For example, the adjustment procedure varies depending on whether ringing in the signal waveform is caused by a large resonance Q value of the flexible transmission line  40  or insufficient current driving capability of the output of the signal output circuit (IC). 
         [0052]    If the flexible transmission line  40  has a large resonance Q value, the optical pickup is controlled so that the output resistance is increased. If the drive current is insufficient, the optical pickup is controlled so that the current driving capability is increased. 
         [0053]    Here, the reason why the output resistance and drive current have to be controlled will be described. 
         [0054]      FIG. 4  illustrates a laser APC system. The LD  21  is driven by the LDD  22 . The laser emission power of the LD  21  is determined by a drive current ILD caused to flow by the LDD  22 . 
         [0055]    Part of the laser beam emitted enters the PD  241  in the power monitoring circuit  24 . This beam is converted into voltage form before being output. 
         [0056]    The signal from the PD  241  is transmitted to the AFE mounting substrate  30  at the subsequent stage via the flexible transmission line  40 . 
         [0057]    An AFE (IC)  31  at the subsequent stage transmits a signal to the LDD  22 . This signal is adapted to adjust the laser power according to the output signal. 
         [0058]    The LDD  22  reflects the signal from the AFE  31  in the drive current ILD to adjust the laser power. The laser power is controlled to an arbitrary level set by this system. 
         [0059]    The signal transmitted over the transmission line  40  by this system differs between when data is read from the optical disk  50  and when data is written thereto. 
         [0060]    During reading, the LD  21  operates in DC mode. Therefore, high-speed signal transmission does not take place. As a result, the signal is hardly affected by the transmission characteristic of the flexible transmission line  40 . 
         [0061]    During writing, however, the LD  21  operates in pulsed mode. As a result, a high-speed signal is transmitted. 
         [0062]    The pulse signal fed to the power monitoring circuit  24  is fast. Therefore, the IC output signal thereof is affected by the transmission characteristic of the flexible transmission line  40 . 
         [0063]      FIG. 5  is a diagram illustrating an example of writing waveform. 
         [0064]    Depending on the transmission characteristic of the transmission line  40 , ringing may occur in the pulse waveform as illustrated in  FIG. 5 . This makes it difficult to accurately read the power level of the written portion (mark portion of the optical disk  50 ) or unwritten portion (space portion of the optical disk  50 ). 
         [0065]    This is the reason why the output resistance and drive current have to be controlled. 
         [0066]    Different adjustment procedures are used depending on whether ringing in the signal waveform is caused by a large resonance Q value of the flexible transmission line  40  or insufficient current driving capability of the IC output. 
         [0067]    If the flexible transmission line  40  has a large resonance Q value, the optical pickup is controlled so that the output resistance is increased. If the drive current is insufficient, the optical pickup is controlled so that the current driving capability is increased. 
         [0068]      FIG. 6  is a diagram illustrating an example of writing waveform after adjustment according to the present embodiment. 
         [0069]    As illustrated in  FIG. 6 , this adjustment provides a waveform less affected by the transmission characteristic of the flexible transmission line  40 . 
         [0070]    A description will be given next of a specific configuration example of the signal output circuit according to the present embodiment and its functionality. 
         [0071]      FIG. 7  is a circuit diagram illustrating a specific configuration example of the signal output circuit according to the present embodiment. 
         [0072]    The signal output circuit  242  has a plurality (two in the example of  FIG. 7 ) of output circuits  242 - 1  to  242 - n  (n=2 in  FIG. 7 ) for a signal input. 
         [0073]    The output circuit  242 - 1  includes pnp transistors P 11  and P 12 , npn transistors Q 11  and Q 12 , current sources I 11  and I 12  and output resistor R 1 . 
         [0074]    The transistor P 11  has its collector connected to a reference voltage which is, for example, a ground potential GND. The same transistor P 11  has its emitter connected to the emitter of the transistor Q 11  and one end of the output resistor R 1 . The same transistor P 11  has its base connected to the emitter of the transistor Q 12  and the current source I 11 . The output resistor R 1  has its output end connected to an output terminal TO of the signal output circuit  242 . Further, the current source I 11  is connected to the ground potential GND. 
         [0075]    The transistor Q 11  has its collector connected to a source potential VCC. The same transistor has its base connected to the emitter of the transistor P 12  and the current source I 12 . Further, the current source I 12  is connected to the source potential VCC. 
         [0076]    The transistor P 12  has its collector connected to the ground potential GND. The transistor Q 12  has its collector connected to the source potential VCC. The transistor P 12  and the transistor Q 12  have their bases connected commonly to the signal supply line of the PD  241  serving as a signal source. 
         [0077]    Of these components, the transistors P 11  and P 12 , transistors Q 11  and Q 12  and current sources I 11  and I 12  make up the driver DRV. 
         [0078]    Further, the transistors P 11  and Q 11  each form an emitter follower. The emitters of the transistors P 11  and Q 11  are connected together. The connection point of the emitters is connected to one end of the output resistor R 1  to form a so-called push-pull output stage PSPL 1 . 
         [0079]    Still further, the on/off operations or current values of the current sources I 11  and I 12  are controlled by control signals CTL 11  and CTL 12  from an unshown control system. 
         [0080]    The output circuit  242 - 2  includes pnp transistors P 21  and P 22 , npn transistors Q 21  and Q 22 , current sources I 21  and I 22  and output resistor R 2 . 
         [0081]    The transistor P 21  has its collector connected to a reference voltage which is, for example, the ground potential GND. The same transistor P 21  has its emitter connected to the emitter of the transistor Q 21  and one end of the output resistor R 2 . The same transistor P 21  has its base connected to the emitter of the transistor Q 22  and the current source I 21 . The output resistor R 2  has its output end connected to the output terminal TO of the signal output circuit  242 . Further, the current source I 21  is connected to the ground potential GND. 
         [0082]    The transistor Q 21  has its collector connected to a source potential VCC. The same transistor has its base connected to the emitter of the transistor P 22  and the current source I 22 . Further, the current source I 22  is connected to the source potential VCC. 
         [0083]    The transistor P 22  has its collector connected to the ground potential GND. The transistor Q 22  has its collector connected to the source potential VCC. The same transistor Q 22  has its base connected commonly to the signal supply line of the PD  241  serving as a signal source. 
         [0084]    Of these components, the transistors P 21  and P 22 , transistors Q 21  and Q 22  and current sources I 21  and I 22  make up the driver DRV. 
         [0085]    Further, the transistors P 21  and Q 21  each form an emitter follower. The emitters of the transistors P 21  and. Q 21  are connected together. The connection point of the emitters is connected to one end of the output resistor R 2  to form a so-called push-pull output stage PSPL 2 . 
         [0086]    Still further, the on/off operations or current values of the current sources I 21  and I 22  are controlled respectively by control signals CTL 21  and CTL 22  from an unshown control system. 
         [0087]    As described above, the signal output circuit  242  shown in  FIG. 7  has the push-pull output stages PSPL 1  and PSPL 2  respectively for the output circuits  242 - 1  and  242 - 2 . The output resistors R 1  and R 2  different in resistance from each other are provided respectively at the outputs of the push-pull output stages PSPL 1  and PSPL 2 . The output ends of the output resistors R 1  and R 2  are connected to the output terminal TO of the signal output circuit  242 . 
         [0088]    Then, for example, during operation, the current sources of the desired output circuit, namely, the current sources I 11  and I 12  of a current Iref 1  flowing through the output circuit  242 - 1  or the current sources I 21  and I 22  of a current Iref 2  flowing through the output circuit  242 - 2 , are activated. At the same time, the current sources of the unused output circuit, namely, the current sources I 11  and I 12  of the output circuit  242 - 1  or the current sources I 21  and I 22  of the output circuit  242 - 2 , are kept inactive. 
         [0089]    Further, the current sources of both of the output circuits, namely, the current sources I 11  and I 12  of the current Iref 1  flowing through the output circuit  242 - 1  and the current sources I 21  and I 22  of the current Iref 2  flowing through the output circuit  242 - 2 , can be activated. 
         [0090]    In this case, the output resistance is equal to the combined parallel resistance of the output resistors R 1  and R 2 . 
         [0091]    Output drive currents Iout 1  and Iout 2  change as the currents Iref 1  and Iref 2  from the current sources I 11 , I 12 , I 21  and I 22  are controlled, thus changing the current driving capability and output impedance. 
         [0092]    As described above, the output drive currents Iout 1  and Iout 2  can be controlled to change by changing the currents Iref 1  and Iref 2 . 
         [0093]    The overall output resistance R 0  of the signal output circuit  242  can be found by the following formula from the resistances and the current of the push-pull output stages PSPL 1  and PSPL 2 . 
         [0000]        R   0   =R +( R in Q/hfeQ+Vt/I out)//( R in P/hfeP+Vt/I out) 
         [0094]    Here, Vt=kT/q, RinQ denotes the input impedance of the npn transistor Q and RinP the input impedance of the pnp transistor. 
         [0095]    The drive current Iout is determined by the ratio between the transistors P 12  and P 11  of the output circuit  242 - 1  and the transistors P 22  and P 21  of the output circuit  242 - 2 . The same current Iout is also determined by the ratio between the transistors Q 12  and Q 11  of the output circuit  242 - 1  and the transistors Q 22  and Q 21  of the output circuit  242 - 2 . 
         [0096]    Letting the size (e.g., emitter area) of the transistors P 11  and P 12  be denoted by P 1 , the size (e.g., emitter area) of the transistors P 21  and P 22  by P 2 , the size (e.g., emitter area) of the transistors Q 11  and Q 12  by Q 1 , and the size (e.g., emitter area) of the transistors Q 21  and Q 22  by Q 2 , the drive current Iout can be found by the following formula when P 1 /P 2 =Q 1 /Q 2 : 
         [0000]        I out= P 1/ P 2× I ref= Q 1/ Q 2× I ref 
         [0097]    In the above configuration, we assume that the resistance of the output resistor R 1  of the output circuit  242 - 1  is set larger than that of the output resistor R 2  of the output circuit  242 - 2  and that the current driving capability of the output circuit  242 - 2  is set larger than that of the output circuit  242 - 1 . 
         [0098]    In such a configuration, if the flexible transmission line  40  has a large resonance Q value, the current sources I 11  and I 12  of the output circuit  242 - 1  are activated while keeping the current sources I 21  and I 22  of the output circuit  242 - 2  inactive so that the output resistance is increased. 
         [0099]    Further, if the drive current is insufficient, the current sources I 21  and I 22  of the output circuit  242 - 2  are activated while keeping the current sources I 11  and I 12  of the output circuit  242 - 1  inactive so that the current driving capability is increased. 
         [0100]    Still further, if the flexible transmission line  40  has a large resonance Q value and the drive current is insufficient, the current sources I 11  and I 12  of the output circuit  242 - 1  are activated, for example, so that the output resistance is increased. At the same time, the same sources I 11  and I 12  are controlled by the control signals CTL 11  and CTL 12  so that the current Iref 1  is increased, and the current sources I 21  and I 22  of the output circuit  242 - 2  are kept inactive. 
         [0101]    As described above, in the present embodiment, the signal output circuit  242  includes the drive circuit (driver) DRV and output resistor R 0 . The driver DRV outputs an electric signal converted from a laser beam by the PD  241 . The output resistor R 0  is adapted to adjust the damping and connected to the output stage of the driver DRV. The signal output circuit  242  is configured so that the output resistor R 0  is formed (integrated) in the IC and so that the drive current of the driver DRV and the resistance of the output resistor can be controlled according to the transmission characteristic of the transmission line  40 . This provides the advantageous effects described below. 
         [0102]    Integration (incorporation) of the output resistor in the same circuit (same IC) eliminates the need for an external resistor previously desired, thus providing an effective way of downsizing the pickup. 
         [0103]    Further, the embodiment of the present invention permits adjustment of the signal waveform to suit each of transmission lines having different transmission characteristics without any external resistor and, by extension, ensures optimal signal transmission tailored to the transmission characteristic of the transmission line. 
         [0104]    The embodiment of the present invention is effective for use in those systems operable to transmit signals over flexible or other cables and adjust the waveform with resistors, irrespective of whether an optical pickup is used. 
         [0105]    As described above, the embodiment of the present invention is applicable irrespective of whether an optical disk is used. 
         [0106]      FIG. 8  is an equivalent circuit diagram of a system to which the embodiment of the present invention is applicable. 
         [0107]    In a signal transmission system  300  shown in  FIG. 8 , a transmission line  320  having a given transmission characteristic is connected to a signal output circuit  310  incorporating the DRV with variable driving capability and an output resistor RD having a variable resistance. A receiving IC  330  is connected to the transmission line  320 . 
         [0108]    The embodiment of the present invention is effective for waveform adjustment when the transmission (output) resistance is low and the reception impedance is high in the signal transmission system  300 . 
         [0109]    The same advantageous effect can be achieved by lowering the reception input impedance. However, this technique leads to increased power consumption and other disadvantages. It is therefore more advantageous to use the system of the embodiment of the present invention. 
         [0110]    It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factor in so far as they are within the scope of the appended claims or the equivalents thereof.