JP6243286B2 - Digital / analog conversion circuit and optical transmitter - Google Patents

Description

  The present invention relates to a digital / analog conversion circuit and an optical transmitter, for example, a digital / analog conversion circuit mounted on an optical transmitter that generates an optical signal by pulse amplitude modulation.

  In recent years, with an increase in communication traffic, an increase in capacity of an optical communication network using optical fibers has been demanded. In an optical communication network, an optical transmitter including an optical modulator is used to realize high-speed and large-capacity communication. As a typical optical modulator, a Mach-Zehnder (hereinafter referred to as “MZ”) modulator or an electro-absorption (hereinafter referred to as “EA”) modulator is known.

  In recent years, an optical transmitter is equipped with a digital signal processing circuit (Digital Signal Processor (hereinafter sometimes referred to as “DSP”)) in order to generate an advanced modulation waveform by an MZ modulator, an EA modulator, or the like. Yes. Specifically, the optical transmitter performs digital signal processing on a signal to be transmitted by a DSP, and then performs digital signal processing by a digital / analog conversion circuit (hereinafter sometimes referred to as “D / A conversion circuit”). The converted signal is converted into an analog signal and input as a drive voltage for the optical modulator. Thereby, the optical transmitter generates an optical signal by modulating the light intensity according to the signal to be transmitted by the optical modulator, and transmits the optical signal to the optical fiber. By performing digital signal processing by the DSP in this way, multi-level optical modulation, which has been difficult in conventional optical transmission by on-off modulation, can be performed.

  In recent years, in order to perform multi-level modulation at a higher bit rate, not only phase modulation but also multi-level pulse amplitude modulation (hereinafter sometimes referred to as “PAM”) is used for optical modulation. Optical transmitters to perform are being studied. However, optical modulators such as MZ modulators and EA modulators have non-linear transfer characteristics, and thus it is necessary to devise a method for driving the optical modulator. For example, the characteristic (transfer characteristic) of the optical output with respect to the driving voltage of a general MZ modulator becomes a non-linear characteristic like a sine curve as shown in FIG. Therefore, in order to obtain the maximum optical output amplitude, not only the linear region 500 but also the nonlinear region 501 must be used.

  However, when the MZ modulator is driven to the non-linear region 501, not only the DSP signal processing but also the characteristics of the D / A conversion circuit that converts the digital signal processed signal into an analog signal becomes a problem. Specifically, a general D / A conversion circuit has, for example, a circuit configuration shown in FIG. 14, and has linear input / output characteristics (characteristics of an analog signal output with respect to an input digital signal) as shown in FIG. ). Therefore, when a D / A conversion circuit having linear input / output characteristics is used for an optical transmitter, the optical output is distorted in the nonlinear region 501 of the MZ modulator, and the digital signal output from the DSP as shown in FIG. The optical output of the optical transmitter cannot be controlled linearly (equally spaced) with respect to changes in each bit of the signal.

  For example, Patent Document 1 discloses a conventional technique for compensating for optical output distortion in a nonlinear region of an optical modulator. In Patent Document 1, an optical modulator is provided by applying an appropriate weight to the current value of each constant current source provided in the D / A conversion circuit to make the input / output characteristics of the D / A conversion circuit nonlinear. A technique for compensating for the distortion of the optical output in the nonlinear region is disclosed. Further, the D / A conversion circuit described in Patent Document 1 uses a decoder to convert an N-bit binary signal input from the DSP by a decoder to a 2N-1 thermometer code in order to increase the accuracy of compensation of nonlinearity of the optical output. The total current amount of the constant current source is adjusted based on the converted thermometer code.

JP 2012-195978 A

  However, the D / A conversion circuit described in Patent Document 1 has a problem that when the number of digital inputs increases, the circuit scale of a decoder that converts a binary signal into a thermometer code increases exponentially. The increase in the circuit scale of the D / A conversion circuit causes an increase in the cost of the optical transmitter, and becomes a problem particularly when the multi-level PAM such as 4-value or 8-value is advanced in the future.

  The present invention has been made in view of the above problems, and an object of the present invention is to realize a desired input / output characteristic in a D / A conversion circuit while suppressing the circuit scale.

  Another object of the present invention is to further increase the accuracy of the optical transmitter while suppressing the cost.

The digital / analog conversion circuit according to the present invention has N (N is an integer of 2 or more) input terminals for inputting an N-bit digital signal for each bit, and voltage generation for generating a voltage corresponding to the input current. , A first current circuit group including N current circuits provided corresponding to each input terminal, and a second current circuit group including current circuits provided corresponding to each combination of the input terminals And whether the current of the current circuit included in the first current circuit group and the second current circuit group is supplied to the voltage generator according to the logic level of the digital signal input to each of the input terminals. A control unit that controls whether or not each current circuit has a digital signal input to the input terminal at a first logic level, the input terminal in the first current circuit group. Current corresponding to Controlling the path, supplying the current of the current circuit to the voltage generator, and when the digital signal input to the input terminal is at the second logic level, the input terminal in the first current circuit group When the corresponding current circuit is controlled to stop the supply of the current of the current circuit to the voltage generation unit, and when the digital signals input to at least two of the input terminals are both at the first logic level, The current circuit corresponding to the combination of the input terminals in the second current circuit group is controlled, the current of the current circuit is supplied to the voltage generator, and the digital signals input to the at least two input terminals are When at least one is at the second logic level, the current circuit corresponding to the combination of the input terminals in the second current circuit group is controlled, and the current circuit before the current circuit is controlled. Characterized by stopping the supply to the voltage generator.

In the digital / analog conversion circuit, the input terminal includes a first digital input terminal and a second digital input terminal, and the first current circuit group is provided corresponding to the first digital input terminal. A second current circuit provided corresponding to the second digital input terminal, the second current circuit group includes a third current circuit, and the control unit includes a first control circuit. A circuit, a second control circuit, and a third control circuit, wherein the first control circuit is configured such that when the first digital signal input to the first digital input terminal is at the first logic level, Supplying the current of the first current circuit to the voltage generator, and stopping the supply of the current of the first current circuit to the voltage generator when the first digital signal is at a second logic level; The second control circuit is When the second digital signal input to the second digital input terminal is at the first logic level, the current of the second current circuit is supplied to the voltage generator, and the second digital signal is at the second logic level. The supply of the current of the second current circuit to the voltage generator is stopped, and the third control circuit determines that the first digital signal and the second digital signal are both at the first logic level. If the current of the third current circuit is supplied to the voltage generator and at least one of the first digital signal and the second digital signal is at the second logic level, the third current The supply of the circuit current to the voltage generation unit may be stopped.

In the digital / analog conversion circuit, the input terminal includes a first digital input terminal, a second digital input terminal, and a third digital input terminal, and the first current circuit group includes the first digital input terminal. A first current circuit provided corresponding to the second digital input terminal, a second current circuit provided corresponding to the second digital input terminal, and a third current circuit provided corresponding to the third digital input terminal; The second current circuit group includes a fourth current circuit, a fifth current circuit, a sixth current circuit, and a seventh current circuit, and the control unit includes a first control circuit, a first current circuit, 2 control circuit, 3rd control circuit, 4th control circuit, 5th control circuit, 6th control circuit, and 7th control circuit, and said 1st control circuit is said 1st digital input terminal The first digital signal input to the When at a logic level, the current of the first current circuit is supplied to the voltage generator, and when the first digital signal is at a second logic level, the voltage generator of the current of the first current circuit The second control circuit is configured to supply the current of the second current circuit to the voltage generator when the second digital signal input to the second digital input terminal is at the first logic level. And when the second digital signal is at the second logic level, the supply of the current of the second current circuit to the voltage generator is stopped, and the third control circuit When the third digital signal input to the terminal is at the first logic level, the current of the third current circuit is supplied to the voltage generator, and the third digital signal is at the second logic level. The third current The supply of the current of the path to the voltage generator is stopped, and the fourth control circuit causes the fourth current to flow when both the first digital signal and the second digital signal are at the first logic level. Supplying a current of the circuit to the voltage generator, and when the fourth digital signal is at a second logic level, the supply of the current of the fourth current circuit to the voltage generator is stopped, and the fifth control The circuit supplies the current of the fifth current circuit to the voltage generator when the first digital signal and the third digital signal are both at the first logic level, and the first digital signal and the third digital signal When any one of the third digital signals is at the second logic level, the supply of the current of the fifth current circuit to the voltage generating unit is stopped, and the sixth control circuit is configured to output the second digital signal. And the third digital And the second digital signal are supplied to the voltage generator, and one of the second digital signal and the third digital signal is supplied to the second logic signal. When it is at the logic level, the supply of the current of the sixth current circuit to the voltage generator is stopped, and the seventh control circuit is configured to output the first digital signal, the second digital signal, and the third digital signal. When all the signals are at the first logic level, the current of the seventh current circuit is supplied to the voltage generator, and at least one of the first digital signal, the second digital signal, and the third digital signal is supplied. When one is at the second logic level, the supply of the current of the seventh current circuit to the voltage generator may be stopped.

In the digital / analog conversion circuit, the voltage generation unit may include an R-2R resistance ladder circuit.

An optical transmitter according to the present invention includes a light-emitting unit that includes a light-emitting element and outputs light of constant intensity, and a digital signal that performs digital signal processing according to a predetermined light modulation method on an input signal to be transmitted A processing device, the digital / analog conversion circuit for converting a signal processing result of the digital signal processing device into an analog signal, and an output from the light emitting unit according to a voltage level of the analog signal output from the digital / analog conversion circuit And an optical modulator for changing the intensity of the emitted light and outputting it.

  According to the present invention, in the D / A conversion circuit, it is possible to realize desired input / output characteristics while suppressing the circuit scale. Further, in the optical transmitter, it is possible to further increase the accuracy while suppressing the cost.

It is a figure which shows the structure of the optical transmitter which concerns on one embodiment of this invention. It is a figure which shows the structure of the D / A converter circuit which concerns on one embodiment of this invention. It is a figure which shows the 1st circuit structural example of a D / A conversion circuit. It is a figure which shows another circuit structural example of control circuit 141_0. It is a figure which shows the input-output relationship of D / A conversion circuit 1_1. It is a figure which shows the transfer characteristic in an EA modulator. It is a figure which shows the input / output characteristic of the D / A converter for PAM4 corresponding to an EA modulator. It is a figure which shows the transfer characteristic in an MZ modulator. It is a figure which shows the input / output characteristic of the D / A converter for PAM4 corresponding to a MZ modulator. It is a figure which shows the 2nd circuit structural example of a D / A conversion circuit. It is a figure which shows the input-output relationship of D / A conversion circuit 1_2. It is a figure which shows the 3rd circuit structural example of a D / A conversion circuit. It is a figure which shows the transfer characteristic of a general MZ modulator. It is a figure which shows the structure of the conventional general D / A conversion circuit. It is a figure which shows the input-output characteristic of the conventional general D / A conversion circuit. It is a figure which shows the characteristic of the optical output at the time of driving an MZ modulator with the conventional general D / A conversion circuit.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

≪Configuration of optical transmitter≫
FIG. 1 shows a configuration of an optical transmitter including a D / A conversion circuit according to Embodiment 1 of the present invention.
An optical transmitter 100 shown in the figure is an apparatus that converts an electrical signal input from the outside into an optical signal and outputs the optical signal to an optical network including optical fibers. The optical transmitter 100 includes, for example, a digital signal processing device 2, a digital / analog conversion circuit (hereinafter referred to as “D / A conversion circuit”) 1, a light emitting unit 3, and an optical modulator 4.

  The digital signal processing device 2 receives a signal (electrical signal) VIN to be transmitted and performs predetermined signal processing on the input signal VIN. Specifically, the digital signal processing device 2 performs signal processing for generating, for example, a digital signal for PAM from the input signal VIN, and the signal processing result is a digital signal of N (N is an integer of 2 or more) bits, for example. Output as. The digital signal processing device 2 is realized by a program processing device such as a DSP. Hereinafter, the N-bit signal output from the digital signal processing device 2 is referred to as “digital signals D0 to Dn”. Note that n is an integer of 1 or more.

The light emitting unit 3 includes a light emitting element such as a semiconductor laser (LD: Laser Diode) and outputs light having a constant intensity.
The optical modulator 4 outputs an optical signal (light intensity signal) OPOUT in which the intensity of the light is changed by applying external control to the light having a constant intensity output from the light emitting unit 3. This is a modulator. As the optical modulator 4, a modulator (MZ modulator or the like) that modulates light using an electro-optic effect of a dielectric material, or a modulator that modulates light using an electroabsorption effect of a semiconductor. (EA modulator etc.) can be illustrated.

  Specifically, the optical modulator 4 is configured to adjust the intensity of the light output from the light emitting unit 3 according to the voltage level of the drive voltage VOUT obtained by converting the digital signals D0 to Dn into analog signals by the D / A conversion circuit 1 described later. Change to output.

The D / A conversion circuit 1 converts the digital signals D0 to Dn output from the digital signal processing device 2 into analog signals. The analog signal is, for example, a voltage signal, and is input to the optical modulator 4 as the drive voltage VOUT as described above.
The D / A conversion circuit 1 is configured to be able to adjust an analog output characteristic (input / output characteristic) with respect to a digital input so as to correspond to an optical output characteristic (transfer characteristic) with respect to the drive voltage of the optical modulator 4. Hereinafter, the D / A conversion circuit 1 will be described in detail.

<< Outline of D / A Conversion Circuit 1 >>
FIG. 2 shows an internal configuration of the D / A conversion circuit 1. As shown in the figure, the D / A conversion circuit 1 includes input terminals IN0 to INn, a control unit 14, a first current circuit group 11, a second current circuit group 12, and a voltage generation unit 13.

  The input terminals IN0 to INn receive the digital signals D0 to Dn output from the digital signal processing device 2. The number of input terminals IN0 to INn corresponds to the number of digital signals D0 to Dn output from the digital signal processing device 2.

  In the following, for convenience of explanation, the first logic level is set to the low level and the second logic level is set to the high level as the logic levels of the digital signals D0 to Dn. However, the present invention is not limited to this. is not.

The voltage generator 13 generates an output voltage VOUT according to an output current IOUT obtained by adding currents of current circuits of a first current circuit group 11 and a second current circuit group 12 described later. The voltage generation unit 13 is configured by a resistor circuit including, for example, a single resistor or a plurality of resistors, and converts the output current IOUT into the output voltage VOUT by the resistor. The output voltage VOUT is output from the output terminal OUT of the D / A conversion circuit 1 and is supplied to the optical modulator 4 as a drive voltage.
The output voltage VOUT may be input directly to the optical modulator 4 or may be input to the optical modulator 4 via a buffer circuit.

  The first current circuit group 11 includes N current circuits 110 </ b> _ <b> 0 to 110 </ b> _n (corresponding to “current circuit 110” when collectively referred to) provided corresponding to the input terminals IN <b> 0 to INn.

  The second current circuit group 12 is provided for each combination of input terminals IN0 to INn (digital signals D0 to Dn), and current circuits 120_0 to 120_m (collectively referred to as “current circuit 120”). including. Note that m is an integer of 1 or more.

  For example, when the D / A conversion circuit 1 has two input terminals IN0 and IN1, the combination of the input terminals IN0 and IN1 is only (IN0, IN1). Therefore, a current circuit 120_0 corresponding to the combination (IN0, IN1) is provided as a current circuit of the second current circuit group 12. Although details will be described later, supply and interruption of current to the voltage generation unit 13 of the current circuit 120_0 are controlled based on digital signals D0 and D1 input to the input terminals IN0 and IN1.

  For example, when the D / A conversion circuit 1 has three input terminals IN0, IN1, and IN2, the combinations of the input terminals IN0 to IN2 are (IN0, IN1), (IN0, IN2), (IN1, IN2). , And (IN0, IN1, IN2). Therefore, the second current circuit group 12 corresponds to the combination of the current circuit 120_0 corresponding to the combination of (IN0, IN1), the current circuit 120_1 corresponding to the combination of (IN0, IN2), and (IN1, IN2). It includes a current circuit 120_2 and a current circuit 120_3 corresponding to a combination of (IN0, IN1, IN2). Although details will be described later, supply and interruption of current to the voltage generation unit 13 of the current circuits 120_0 to 120_3 are controlled based on digital signals D0 to D2 input to input terminals corresponding to the respective current circuits 120_0 to 120_3. The

  Based on the digital signals D0 to Dn input to the input terminals IN0 to INn, the control unit 14 converts the currents of the current circuits 110 and 120 included in the first current circuit group 11 and the second current circuit group 12 into a voltage generation unit. 13 is controlled for each of the current circuits 110 and 120.

  Specifically, when the digital signal input to the input terminals IN0 to INn is at the first logic level (low level), the control unit 14 controls the current circuit 110 corresponding to the input terminal to control the current circuit. Is supplied to the voltage generator 13. On the other hand, when the digital signal input to the input terminals IN0 to INn is at the second logic level (high level), the control unit 14 controls the current circuit 110 corresponding to the input terminal to control the current of the current circuit. The supply to the voltage generator 13 is stopped.

  The control unit 14 also includes a current circuit corresponding to a combination of the input terminals in the second current circuit group 12 when both of the digital signals input to the two input terminals are at the first logic level (low level). 120 is controlled to supply the current of the current circuit to the voltage generator 13. On the other hand, when at least one of the digital signals input to the at least two input terminals is at the second logic level (high level), the current circuit 120 corresponding to the combination of the input terminals in the second current circuit group 12 is provided. And the supply of the current of the current circuit to the voltage generator 13 is stopped.

  According to the D / A conversion circuit 1 described above, the current values of the respective current circuits 110 and 120 are appropriately set so as to correspond to the transfer characteristics of the optical modulator 4, whereby a conventional D / A conversion circuit is provided. It is possible to generate a desired non-linear analog output (drive voltage VOUT) for a digital input (digital signals D0 to Dn) with a smaller circuit scale. Hereinafter, this will be described in more detail by exemplifying a specific circuit configuration of the D / A conversion circuit 1.

<< Specific Example 1 of D / A Conversion Circuit 1 >>
FIG. 3 is a diagram illustrating a first circuit configuration example of the D / A conversion circuit 1. The D / A conversion circuit 1_1 shown in the figure has a circuit configuration corresponding to four-valued PAM (PAM4).

  Specifically, the D / A conversion circuit 1_1 includes two input terminals IN0 and IN1 that input digital signals D0 and D1, respectively, as input terminals. The D / A conversion circuit 1_1 includes two current circuits 110_0 and 110_1 as the first current circuit 11 group, and one current circuit 120_0 as the second current circuit 12 group. Further, the D / A conversion circuit 1_1 includes control circuits 140_0, 140_1, and 141_0 as the control unit 14.

  The voltage generation unit 13 includes, for example, a resistor R0 connected between a power supply line to which the power supply voltage VCC is supplied and the output terminal OUT. As a result, the voltage VOUT (= VCC−R0 × IOUT) is output from the output terminal OUT.

  The control circuit 140_0 controls the current circuit 110_0 and supplies the current of the current circuit 110_0 to the voltage generation unit 13 when the digital signal D0 input to the input terminal IN0 is at the first logic level (low level). . On the other hand, when the digital signal D0 is at the second logic level (high level), the control circuit 140_0 controls the current circuit 110_0 to stop supplying the current of the current circuit 110_0 to the voltage generator 13.

  Similarly, the control circuit 140_1 controls the current circuit 110_1 to supply the current of the current circuit 110_1 to the voltage generation unit 13 when the digital signal D1 input to the input terminal IN0 is at the first logic level. On the other hand, when the digital signal D1 is at the second logic level, the control circuit 140_1 stops supplying the current of the current circuit 110_1 to the voltage generation unit 13.

  The control circuits 140_0 and 140_1 are constituted by, for example, D-type flip-flop circuits (DFF), take in the input digital signals D0 and D1 in synchronization with the clock signal CLK, and invert the logic level of the taken-in signals. CN0 and CN1 are supplied to the corresponding current circuits 110_0 and 110_1, respectively. That is, the control signal CN0 becomes an inverted signal of the digital signal D0, and the control signal CN1 becomes an inverted signal of the digital signal D1.

  The control circuit 141_0 controls the current circuit 120_0 and supplies the current of the current circuit 120_0 to the voltage generator 13 when both the digital signal D0 and the digital signal D1 are at the first logic level (low level). On the other hand, when at least one of the digital signal D0 and the digital signal D1 is at the second logic level (high level), the control circuit 141_0 controls the current circuit 120_0 to the current voltage generator 13 of the current circuit 120_0. Stop supplying.

Specifically, the control circuit 141_0 outputs the control signal CN3 of the second logic level when both of the digital signals D0 and D1 are at the first logic level, and one of the digital signals D0 and D1 is the second signal. When it is at the logic level, it is composed of a logic circuit that outputs the control signal CN3 of the first logic level. As the logic circuit, for example, an AND circuit that performs a logical product operation of inverted signals (CN0, CN1) of digital signals D0, D1 as shown in FIG. 3, or digital signals D0, D1 as shown in FIG. For example, a NOR circuit or the like that performs a negative OR operation can be exemplified.
Note that the above-described control circuits 140_0, 140_1, and 141_0 are not limited to the above-described circuit configuration example as long as a desired signal can be generated.

  As shown in FIG. 3, the current circuit 110_0 includes a current source I0 and a switch element SW0 connected in series to the current source I0. The switch element SW0 is turned on when the control signal CN0 is at the second logic level (high level), and is turned off when the control signal CN0 is at the first logic level (low level). Thus, when the control signal CN0 is at the second logic level, the current of the current source I0 is supplied to the voltage generator 13, and when the control signal CN0 is at the first logic level, the voltage generator of the current of the current source I0. The supply to 13 is cut off.

  Similar to the current circuit 110_0, the current circuit 110_1 includes a current source I1 and a switch element SW1, and when the control signal CN1 is at the second logic level, the switch SW1 is turned on, and the control signal CN1 is at the first logic level. When it is, the supply and interruption of the current of the current source I1 to the voltage generator 13 are controlled.

  Similar to the current circuit 110_0, the current circuit 120_0 includes a current source I3 and a switch element SW3. When the control signal CN3 is at the second logic level, the switch SW3 is turned on, and the control signal CN3 is at the first logic level. By turning off when the current is, the supply and interruption of the current of the current source I3 to the voltage generator 13 are controlled.

  FIG. 5 shows the input / output relationship of the D / A conversion circuit 1_1. In the figure, I0 to I2 represent current values of the current sources I0 to I2.

  As shown in the figure, according to the D / A conversion circuit 1_1, by sequentially changing the digital inputs D0 and D1, the drive voltage VOUT has the change amounts a, b, and c with respect to the previous digital input. It changes sequentially. The output current IOUT changes in the order of “I0 + I1 + I2”, “I1”, “I0”, “0 (zero)” by sequentially changing the digital inputs D0 and D1 as shown in FIG. Change amounts a, b, and c are determined by the change amount of the output current IOUT. For example, the change amount a is “R0 × | I0 + I2 |”, the change amount b is “R0 × | I1-I0 |”, and the change amount c is “R0 × | I0 |”.

  Therefore, if the current values of the current sources I0 to I2 are appropriately adjusted, a desired non-linear output voltage VOUT can be generated for the digital inputs D0 and D1.

  Here, as an example, consider a case where an EA modulator having the transfer characteristic 200 shown in FIG. In this case, the amount of change a, b, c of the output voltage VOUT is set as shown in FIG. 7 by adjusting the current values of the current sources I0 to I2 so as to correspond to the transfer characteristics of the EA modulator.

  According to this, as shown in FIG. 7, it is possible to equalize the amount of change in the optical output OPOUT by the EA modulator when the values of the digital inputs D0 and D1 are sequentially changed.

  As another example, consider a case where an MZ modulator having the transfer characteristic 300 shown in FIG. In this case, the current values of the current sources I0 to I2 are adjusted to set the change amounts a, b, and c of the output voltage VOUT as shown in FIG. That is, since the nonlinear region of the transfer characteristic 300 is a symmetric characteristic, it is set so that c = a.

  According to this, as shown in FIG. 9, the amount of change in the optical output OPOUT by the MZ modulator when the values of the digital inputs D0 and D1 are sequentially changed can be made equal.

<< Specific Example 2 of D / A Conversion Circuit 1 >>
FIG. 10 is a diagram illustrating a second circuit configuration example of the D / A conversion circuit 1. The D / A conversion circuit 1_2 shown in the figure has a circuit configuration corresponding to 8-value PAM (PAM8).

  Specifically, the D / A conversion circuit 1_2 includes three input terminals IN0 to IN2 that input digital signals D0, D1, and D2 as input terminals. In addition, the D / A conversion circuit 1_2 includes three current circuits 110_0 to 110_2 as the first current circuit 11 group and four current circuits 120_0 to 120_3 as the second current circuit 12 group. Further, the D / A conversion circuit 1_2 includes control circuits 140_0 to 140_2 and 141_0 to 141_141_3 as the control unit 14. In the D / A conversion circuit 1_2, the same components as those in the D / A conversion circuit 1_1 are denoted by the same reference numerals, and detailed description thereof is omitted.

  The control circuit 140_2 is formed of a D-type flip-flop circuit (DFF), like the control circuits 140_0 and 140_1. Specifically, the control circuit 140_2 receives the digital signal D2 input to the input terminal IN2, and outputs an inverted signal of the acquired signal as the control signal CN2.

  The control circuit 141_1 outputs the control signal CN4 of the second logic level when both of the digital signals D0 and D2 are at the first logic level (low level), and one of the digital signals D0 and D1 is the second signal. When at the logic level, the control signal CN4 at the first logic level is output.

  The control circuit 141_2 outputs a control signal CN5 of the second logic level when both of the digital signals D1 and D2 are at the first logic level, and one of the digital signals D1 and D2 is at the second logic level. When it is, the control signal CN5 of the first logic level is output.

  Further, the control circuit 141_3 outputs the control signal CN6 of the second logic level when all of the digital signal D0, the digital signal D1, and the digital signal D2 are at the first logic level, and outputs the digital signal D0 and the digital signal D1. When at least one of the digital signals D2 is at the second logic level, the control signal CN6 at the first logic level is output.

  The control circuits 141_1 to 141_3 can be realized by an AND circuit, a NOR circuit, or the like, similarly to the control circuit 141_0.

  The current circuit 110_2 has a circuit configuration similar to that of the current circuits 110_0 and 110_1. Specifically, the current circuit 110_2 turns on the switch SW2 when the control signal CN2 is at the second logic level and turns off when the control signal CN2 is at the first logic level. The supply and interruption of the current of the source I2 is controlled.

  Further, the current circuits 120_1 to 120_3 have a circuit configuration similar to that of the current circuit 120_0. Specifically, in the current circuits 120_1 to 120_3, when the corresponding control signals CN4 to CN6 are at the second logic level (high level), the corresponding switches SW4 to SW6 are turned on, and the corresponding control signals CN4 to CN6 are turned on. By turning off when it is at the first logic level (low level), the supply and interruption of the currents of the current sources I4 to I6 to the voltage generator 13 are controlled.

  FIG. 11 shows the input / output relationship of the D / A conversion circuit 1_2. In the figure, I0 to I6 represent current values of the current sources I0 to I6.

  As shown in the figure, according to the D / A conversion circuit 1_2, by sequentially changing the digital inputs D0, D1, and D2, the drive voltage VOUT has a change amount a to g with respect to the previous digital input. It changes sequentially. The output current IOUT is obtained by changing the digital inputs D0 and D1 in order as shown in FIG. 10, so that “I0 + I1. Since it changes in the order of “I0” and “0 (zero)”, the driving voltages VOUT a to g are determined by the amount of change in the current IOUT, as in the D / A conversion circuit 1_1 for PAM4.

  Therefore, by appropriately adjusting the current values of the current sources I0 to I6, a desired non-linear output voltage VOUT is generated for the digital inputs D0, D1, and D2, similarly to the D / A conversion circuit 1_1 described above. Is possible. That is, the amount of change in the optical output OPOUT by the optical modulator 4 when the values of the digital inputs D0, D1, and D2 are sequentially changed can be made equal.

<< Specific Example 3 of D / A Conversion Circuit 1 >>
FIG. 12 is a diagram illustrating a third circuit configuration example of the D / A conversion circuit 1. The D / A conversion circuit 1_3 shown in the figure is different from the above-described D / A conversion circuit 1_1 corresponding to the PAM4 in that the circuit configuration of the voltage generation unit that generates the output voltage VOUT is different. This is similar to the D / A conversion circuit 1_1. In the D / A conversion circuit 1_3, the same components as those in the D / A conversion circuit 1_1 are denoted by the same reference numerals, and detailed description thereof is omitted.

  Specifically, the D / A conversion circuit 1_3 includes a voltage generation unit 23 instead of the voltage generation unit 13. The voltage generation unit 23 is configured by an R-2R type resistance ladder circuit. For example, the voltage generator 23 includes R1 connected between the node ND0 to which one end of the switch SW0 is connected and the power supply line VCC, a resistor R2 connected between the power supply line VCC and the output terminal OUT, A resistor R3 connected between the node ND0 and the output terminal OUT. When the resistance value of the resistor R1 is “R”, the resistance value of the resistor R2 is “2R”, and the resistance value of the resistor R3 is “R”.

  According to this, since the weights of the current values of the current sources I0 to I2 can be adjusted by the values of the resistors R1 to R3, the change amount a of the output voltage VOUT can be determined only by the current values of the current sources I0 to I2. , B, and c can be reduced as compared with the case of adjusting the current consumption of the entire D / A conversion circuit.

  As described above, the D / A conversion circuit 1 according to the present invention has been described with reference to a specific circuit configuration example. By appropriately setting the current values of the current circuits 110 and 120, the optical modulator Therefore, the input / output characteristics of the D / A conversion corresponding to the non-linear transfer characteristics of 4 can be easily realized.

  In addition, according to the D / A conversion circuits 1_1 to 1_3 (see FIGS. 3, 10, and 12) according to the specific examples 1 to 3 shown in the present embodiment, the D / A conversion described in the above-described Patent Document 1 is performed. Since a decoder or the like for generating a thermometer code such as an A conversion circuit is not required, the circuit scale (the number of logical operation elements) of the control circuit 14 for controlling the current circuits 110 and 120 is reduced. Is possible.

  That is, according to the D / A conversion circuit 1 of the present invention, nonlinear characteristics can be realized with high accuracy while suppressing the circuit scale. Accordingly, if the D / A conversion circuit 1 according to the present invention is applied to an optical transmitter as shown in FIG. 1, the optical modulator can be controlled with high accuracy, so that the optical output is suppressed while suppressing the distortion of the optical output. The width can be increased, and the accuracy of the optical transmitter can be increased while suppressing the cost.

  Although the invention made by the present inventors has been specifically described based on the embodiments, it is needless to say that the present invention is not limited thereto and can be variously modified without departing from the gist thereof. Yes.

  For example, as a specific example of the D / A conversion circuit 1, a circuit configuration corresponding to a 4-value PAM and an 8-value PAM has been illustrated. Various circuit configurations of the circuit 1 can be changed. For example, the number of current circuits 110 and 120 and control circuits 140 and 141 may be adjusted according to the number of input terminals D0 to Dn.

  Moreover, although the MZ modulator and the EA modulator are illustrated as the optical modulator 4 of the optical transmitter 100, the present invention is not limited to this, and other modulators can also be applied. In this case, the current values of the current sources constituting the current circuits 110 and 120 may be adjusted so as to correspond to the transfer characteristics of the applied modulator.

  12 illustrates the case where the R-2R type resistance ladder circuit is applied to the D / A conversion circuit for PAM4. However, the present invention is not limited to this, and the D / A conversion circuit for PAM8 shown in FIG. It is also possible to apply an R-2R resistance ladder circuit to the voltage generator 1_2. According to this, it becomes possible to reduce the current consumption of the entire D / A conversion circuit 1_2.

  DESCRIPTION OF SYMBOLS 100 ... Optical transmitter, 1 ... D / A conversion circuit, 2 ... Digital signal processing apparatus, 3 ... Light emission part, 4 ... Optical modulator, D0-Dn ... Digital signal, VIN ... Signal to be transmitted (electric signal), IN0 to INn: input terminal, IOUT: output current, VOUT: output voltage (drive voltage), 11: first current circuit group, 12: second current circuit group, VCC: power supply voltage, power supply line, 13, 23 ... voltage Generating unit, 14 ... control unit, 110_0 to 110_n, 120_0 to 120_m ... current circuit, SW0 to SW6 ... switch, I0 to I6 ... current source, 140_0 to 140_2, 141_0 to 141_3 ... control circuit, R0 to R3 ... resistance.

Claims (5)

N (N is an integer of 2 or more) input terminals for inputting an N-bit digital signal for each bit;
A voltage generator that generates a voltage according to the input current;
A first current circuit group including N current circuits provided corresponding to each of the input terminals;
A second current circuit group including a current circuit provided corresponding to each combination of the input terminals;
Whether or not to supply the current of the current circuit included in the first current circuit group and the second current circuit group to the voltage generator according to the logic level of the digital signal input to each of the input terminals. And a control unit for controlling each current circuit ,
When the digital signal input to the input terminal is at the first logic level, the control unit controls a current circuit corresponding to the input terminal in the first current circuit group to obtain a current of the current circuit. When the digital signal supplied to the voltage generation unit and input to the input terminal is at the second logic level, the current circuit corresponding to the input terminal in the first current circuit group is controlled, and the current circuit The combination of the input terminals in the second current circuit group is stopped when the supply of the current to the voltage generator is stopped and both of the digital signals input to the input terminals are at the first logic level. The current circuit corresponding to the current circuit is supplied to the voltage generator, and at least one of the digital signals input to the at least two input terminals is supplied. If There is the second logic level, said controlling the current circuit corresponding to the combination of the input terminal of the second current circuits, stopping the supply to the voltage generator of current of the current circuit A digital / analog conversion circuit characterized by the above. The digital / analog converter circuit according to claim 1 ,
The input terminal includes a first digital input terminal and a second digital input terminal;
The first current circuit group includes:
A first current circuit provided corresponding to the first digital input terminal;
A second current circuit provided corresponding to the second digital input terminal,
The second current circuit group includes a third current circuit,
The control unit includes a first control circuit, a second control circuit, and a third control circuit,
The first control circuit supplies a current of the first current circuit to the voltage generator when the first digital signal input to the first digital input terminal is at the first logic level, and When one digital signal is at the second logic level, the supply of the current of the first current circuit to the voltage generator is stopped;
The second control circuit supplies a current of the second current circuit to the voltage generator when the second digital signal input to the second digital input terminal is at the first logic level, and When the two digital signals are at the second logic level, the supply of the current of the second current circuit to the voltage generator is stopped,
The third control circuit supplies the current of the third current circuit to the voltage generator when the first digital signal and the second digital signal are both at the first logic level, and A digital / analog converter circuit, wherein when at least one of the digital signal and the second digital signal is at the second logic level, the supply of the current of the third current circuit to the voltage generator is stopped. . The digital / analog converter circuit according to claim 1 ,
The input terminal includes a first digital input terminal, a second digital input terminal, and a third digital input terminal,
The first current circuit group includes a first current circuit provided corresponding to the first digital input terminal, a second current circuit provided corresponding to the second digital input terminal, and the third digital circuit. A third current circuit provided corresponding to the input terminal,
The second current circuit group includes a fourth current circuit, a fifth current circuit, a sixth current circuit, and a seventh current circuit,
The control unit includes a first control circuit, a second control circuit, a third control circuit, a fourth control circuit, a fifth control circuit, a sixth control circuit, and a seventh control circuit,
The first control circuit supplies a current of the first current circuit to the voltage generator when the first digital signal input to the first digital input terminal is at the first logic level, and When one digital signal is at the second logic level, the supply of the current of the first current circuit to the voltage generator is stopped;
The second control circuit supplies a current of the second current circuit to the voltage generator when the second digital signal input to the second digital input terminal is at a first logic level, and When the digital signal is at the second logic level, the supply of the current of the second current circuit to the voltage generator is stopped;
The third control circuit supplies a current of the third current circuit to the voltage generator when the third digital signal input to the third digital input terminal is at the first logic level, and When the three digital signals are at the second logic level, the supply of the current of the third current circuit to the voltage generator is stopped;
The fourth control circuit supplies a current of the fourth current circuit to the voltage generator when both the first digital signal and the second digital signal are at the first logic level, and When the digital signal is at the second logic level, the supply of the current of the fourth current circuit to the voltage generator is stopped;
When the first digital signal and the third digital signal are both at the first logic level, the fifth control circuit supplies the current of the fifth current circuit to the voltage generator, and When one of the digital signal and the third digital signal is at the second logic level, the supply of the current of the fifth current circuit to the voltage generator is stopped,
The sixth control circuit supplies a current of the sixth current circuit to the voltage generator when both the second digital signal and the third digital signal are at the first logic level, and When one of the digital signal and the third digital signal is at the second logic level, the supply of the current of the sixth current circuit to the voltage generator is stopped,
The seventh control circuit supplies the current of the seventh current circuit to the voltage generator when the first digital signal, the second digital signal, and the third digital signal are all at the first logic level. And when at least one of the first digital signal, the second digital signal, and the third digital signal is at the second logic level, the current of the seventh current circuit is supplied to the voltage generator. A digital / analog conversion circuit characterized by stopping the operation. The digital / analog converter circuit according to any one of claims 1 to 3 ,
The voltage generation unit includes an R-2R type resistance ladder circuit. A light-emitting unit that includes a light-emitting element and outputs light of constant intensity;
A digital signal processing device that performs digital signal processing in accordance with a predetermined optical modulation method on the input signal to be transmitted;
The digital / analog conversion circuit according to any one of claims 1 to 4 , which converts a signal processing result by the digital signal processing device into an analog signal;
An optical transmitter comprising: an optical modulator configured to change and output the intensity of light output from the light emitting unit in accordance with a voltage level of an analog signal output from the digital / analog conversion circuit.

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