JP2009094621A - Spread spectrum clock generator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a spread spectrum clock generator capable of reducing the difficulties in designing the clock generator. <P>SOLUTION: The spread spectrum clock generator 1 for generating a clock signal CLKO subjected to spread spectrum is provided with a frequency divider 14 for outputting a frequency-division signal, by dividing the clock signal CLKO at a predetermined frequency division ratio; a phase comparator 10 for outputting a phase difference signals Sp, having changed level in response to the phase difference between a reference clock signal CLKR to be inputted and a frequency division signal CLKF; an oscillation control signal generator 11 for generating an oscillation control signal Sc, corresponding to the phase difference signal Sp; a voltage-controlled oscillator 12 having an oscillation portion that supplies a current corresponding to the level of the oscillation control signal Sc to the oscillation portion, that oscillates the oscillation portion at an oscillation frequency corresponding to the level of the oscillation control signal Sc, and that outputs a clock signal CLKO; and a modulated current generator 15 for generating a modulated current Im for making a current to be supplied to the oscillation portion increase or decrease and supplying the generated current to the voltage-controlled oscillator 12. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a spread spectrum clock generator that generates a spread spectrum clock signal by minutely changing the frequency in order to reduce electromagnetic radiation.

  In recent years, due to advances in semiconductor manufacturing technology, the frequency at which semiconductor elements can operate has become increasingly higher. For example, the operation clock frequency of a CPU (central processing unit) widely used in personal computers has already reached several GHz from about 10 MHz at the beginning of development.

  Due to the improvement of the operating frequency of semiconductor elements, the problem of electromagnetic radiation has become so large that it cannot be ignored in electronic devices using semiconductor elements. In particular, as the frequency increases, the wavelength of the high-frequency signal becomes shorter, and when the connection circuit or the wiring length inside the board is on the same order as the wavelength of the high-frequency signal, the connection part such as the wiring inside the board functions as an antenna. Therefore, there is a disadvantage that the electromagnetic radiation to the surroundings increases rapidly.

  In order to reduce such electromagnetic radiation, measures such as reducing leakage of electromagnetic waves to the surroundings by electromagnetic shielding (shielding) have been conventionally taken. However, miniaturization and weight reduction are required in portable devices and the like, and there are cases where a shield for reducing electromagnetic radiation cannot be sufficiently applied.

  Therefore, a spread spectrum clock generator (Spread Spectrum Clock Generator) has been proposed that generates a spread spectrum clock signal by minutely changing the frequency, realizes spread spectrum of the clock signal, and reduces electromagnetic radiation. (For example, see Patent Documents 1 and 2)

  As a spread spectrum clock generator of this type, Patent Document 1 discloses a circuit system in which a modulation signal input from the outside is added by a voltage adder to slightly change the oscillation frequency of a voltage controlled oscillator (VCO). It is disclosed. Patent Document 2 discloses a circuit system in which the oscillation frequency of the voltage controlled oscillator is minutely changed by periodically changing the frequency division ratio N of the frequency divider.

  The circuit system for adding the voltage of the external modulation signal in the voltage adder has, for example, the configuration shown in FIG. 4A and is output from the voltage controlled oscillator (VCO) 103 by the phase comparator (PFD) 100. The phase difference between the divided clock signal CLKF obtained by dividing and feeding back the clock signal CLKO by the 1 / N divider (1 / N Div) 105 and the reference clock signal CLKR generated by the reference clock generator (not shown) is detected. Then, a signal corresponding to this phase difference is input to the voltage adder 102 via the loop filter (LPF) 101.

  The voltage adder 102 is supplied with a modulation signal CLKM generated by the modulation signal generator 106 that causes the voltage to slightly fluctuate in a triangular waveform at a predetermined period (for example, 100 kHz), and the modulation signal CLKM is input to the loop filter. The voltage is added to the output signal 101 and applied to the voltage controlled oscillator (VCO) 103. As a result, a spread spectrum clock signal CLKO as shown in FIG. 4B is generated. The clock signal CLKO is amplified by the buffer (buf) 104 and output as the clock signal CLK.

  Further, the circuit system for periodically changing the frequency division ratio N of the 1 / N frequency divider has a configuration shown in FIG. 5, for example, and the phase comparator 110 causes the voltage controlled oscillator (VCO) 112 to A phase difference between a divided signal CLKF obtained by dividing the output clock signal CLKO by a 1 / N divider (1 / N Div) 114 and fed back, and a reference clock signal CLKR generated by a reference clock generator (not shown). Is detected. Then, a signal corresponding to the phase difference is input to the voltage controlled oscillator (VCO) 112 via the loop filter 111, and a clock signal CLKO is generated.

At this time, in the 1 / N frequency divider 114, the frequency division signal CLKF is changed by minutely changing the frequency division ratio N at a predetermined period (for example, 100 kHz) by the programmable control signal CONT. As a result, the voltage-controlled oscillator 112 generates a spread spectrum clock signal CLKO.
JP 2000-101424 A Japanese Patent Laid-Open No. 2003-332908

  However, since the voltage-controlled oscillator has very high sensitivity, the technique described in Patent Document 1 described above is designed for the voltage adder 102 on the assumption that the modulation signal CLKM to be added is a minute signal in units of several mV. It becomes necessary and the design is difficult.

  Further, in the technique described in Patent Document 2, it is necessary to accurately perform the timing for switching the frequency division ratio N of the 1 / N frequency divider 114 in order to suppress the occurrence of glitches. The design is difficult.

  The present invention has been made to solve such a problem, and an object of the present invention is to provide a spread spectrum clock generator capable of reducing the difficulty of design.

  In order to solve this problem, the invention according to claim 1 outputs a frequency-divided signal obtained by dividing the clock signal by a predetermined frequency division ratio in a spread spectrum clock generator for generating a spread spectrum clock signal. A frequency divider that outputs a phase difference signal whose level is changed according to a phase difference between the input reference clock signal and the frequency division signal, and an oscillation control signal according to the phase difference signal An oscillation control signal generator that generates a signal and an oscillation unit that oscillates at a frequency corresponding to the supplied current, supplies a current corresponding to the level of the oscillation control signal to the oscillation unit, and oscillates the oscillation unit A voltage-controlled oscillator that generates the clock signal, and a modulation current that generates and supplies a modulation current that increases or decreases a current supplied to the oscillation unit of the voltage-controlled oscillator Provided with a door.

  According to a second aspect of the present invention, in the spread spectrum clock generator according to the first aspect, the voltage controlled oscillator includes a plurality of inverter circuits connected in a chain as the oscillating unit, and the oscillation control signal A current corresponding to a level is supplied to each inverter circuit, and the modulation current generator increases or decreases the current supplied to each inverter circuit by the modulation current.

  The invention according to claim 3 is the spread spectrum clock generator according to claim 1 or 2, wherein the modulation current generator includes an operational amplifier, a feedback circuit, and a current distributor. An output node of the operational amplifier is input to the non-inverting node of the operational amplifier via the feedback circuit, a modulation voltage is input to the inverting node of the operational amplifier, and a modulation signal is output from the output of the operational amplifier to the current distributor. The modulation current according to the modulation signal is supplied from the current distributor to the voltage controlled oscillator.

  According to a fourth aspect of the present invention, in the spread spectrum clock generator according to the third aspect, the feedback circuit includes a variable resistor and a MOS transistor, and the gate of the MOS transistor is the gate of the operational amplifier. The MOS transistor is connected to an output node, and the drain of the MOS transistor is connected to the variable resistor and a non-inverting node of the operational amplifier, so that the resistance value of the variable resistor can be adjusted.

  The invention according to claim 5 is the spread spectrum clock generator according to claim 1 or 2, wherein the modulation current generator includes an operational amplifier, a feedback circuit, and a current distributor. An output node of the operational amplifier is input to the non-inverting node of the operational amplifier via the feedback circuit, and a reference voltage is input to the inverting node of the operational amplifier. The feedback circuit includes a variable resistor and a MOS transistor. The gate of the MOS transistor is connected to the output node of the operational amplifier, the drain of the MOS transistor is connected to the variable resistor and the non-inverting node of the operational amplifier, and the resistance value of the variable resistor varies. As a result, a modulation signal is output from the output of the operational amplifier to the current distributor, and the modulation signal is output from the current distributor. The modulation current and supplying to the voltage controlled oscillator in accordance with the.

  According to the first aspect of the present invention, a frequency divider that outputs a frequency-divided signal obtained by dividing the clock signal by a predetermined frequency dividing ratio, and a phase difference between the input reference clock signal and the frequency-divided signal A phase comparator that outputs a phase difference signal whose level has been changed, an oscillation control signal generator that generates an oscillation control signal according to the phase difference signal, and a current that corresponds to the level of the oscillation control signal is supplied to the oscillation unit And generating a modulation current for increasing or decreasing the current supplied to the oscillation unit of the voltage controlled oscillator in a clock generation block including a voltage controlled oscillator that oscillates the oscillation unit and generates a clock signal. A spread spectrum clock generator can be configured with a simple configuration that provides a modulation current generator to be supplied to the controlled oscillator, reducing the difficulty of circuit design and reducing design man-hours. Rukoto can. Further, since the basic frequency is determined by the clock generation block and the modulation degree is determined by the modulation current generator, each block can be designed independently, so that efficient design can be performed.

  According to the second aspect of the present invention, when a plurality of inverter circuits connected in a chain are used as the oscillation unit of the voltage controlled oscillator, the current supplied to each inverter circuit is increased or decreased in the modulation current generator. Thus, the spread spectrum clock generator can be configured with a simple configuration for supplying a modulation current to each inverter circuit.

  According to the invention described in claim 3, by using the operational amplifier, the design of the modulation current generator for generating the modulation current can be simplified and the circuit area can be reduced. Is possible.

  According to the invention described in claim 4, it is possible to adjust the amplitude of the modulation signal by making the resistance value of the feedback circuit adjustable, and to provide a spread spectrum clock generator having versatility. can do.

  Further, according to the invention described in claim 5, by using the operational amplifier, the design of the modulation current generator for generating the modulation current can be simplified and the circuit area can be reduced. Is possible. In addition, by changing the resistance value of the feedback circuit, a modulation signal having an arbitrary period and modulation waveform can be generated, and a spread spectrum clock generator having versatility can be provided.

(Schematic configuration of spread spectrum clock generator)
An outline configuration of an embodiment of a spread spectrum clock generator according to the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a schematic configuration of a spread spectrum clock generator in the present embodiment.

  The spread spectrum clock generator in this embodiment is a spread spectrum clock generator that generates a spread spectrum clock signal, and has a configuration similar to that of a general PLL circuit (phase-locked loop). And a modulation control block for modulating the clock signal generated by the clock generation block at a predetermined period.

  The clock generation block determines the basic frequency of the output clock signal, and the modulation control block determines the modulation factor. Thereby, in the spread spectrum clock generator in the present embodiment, each block can be designed independently, and an efficient design can be performed. Here, the fundamental frequency is in the range of 1 to 10 GHz, for example, and the modulation frequency is in the range of 30 kHz to 120 kHz, for example.

  As shown in FIG. 1A, the clock generation block includes a phase comparator (PFD) 10, an oscillation control signal generator (CP + LPF) 11, a voltage controlled oscillator (VCO) 12, and an output buffer (buf) 13. And a 1 / N frequency divider (1 / N Div) 14. The output buffer 13 is not necessary when the drive capability of the clock signal output from the voltage controlled oscillator 12 is sufficient.

  The phase comparator 10 is a phase difference whose level is changed in accordance with the phase difference between a reference clock signal CLKR output from a reference clock generator (not shown) and a divided signal CLKF output from the 1 / N frequency divider 14. The signal Sp is output. The frequency division signal CLKF is generated by dividing the clock signal CLKO by a predetermined frequency division ratio N in the 1 / N frequency divider 14.

  The oscillation control signal generator 11 has a function of converting the pulse signal output from the phase comparator 10 into a DC voltage signal, and includes a charge pump (CP) and a low-pass filter (LPF). In the oscillation control signal generator 11, the charge pump (CP) generates a charge current or a discharge current according to the input phase difference signal Sp, and charges / discharges the capacitor according to the charge current or the discharge current. The high-frequency component is removed by (LPF) to generate the oscillation control signal Sc. The oscillation control signal generator 11 is configured by a low-pass filter (LPF), and the oscillation control signal Sc is generated by removing the high-frequency component of the phase difference signal Sp input by the low-pass filter (LPF). Good.

  The voltage controlled oscillator 12 has an oscillating unit that oscillates at a frequency corresponding to the supplied current, supplies a current corresponding to the level of the oscillation control signal Sc to the oscillating unit, oscillates the oscillating unit, and generates an oscillation control signal. A clock signal CLKO having a frequency corresponding to the level of Sc is generated. The clock signal CLKO is driven by the output buffer 13 and output as the clock signal CLK.

  Thus, the clock generation block has a configuration similar to that of a general PLL circuit.

  Next, the modulation control block will be described. This modulation control block includes a modulation current generator (Iref circuit) 15 as shown in FIG.

  The modulation current generator 15 includes an operational amplifier OP, PMOS transistors Ta and Tb, and a resistor Ra. The modulation current generator 15 generates a modulation current Im that increases or decreases the current supplied to the oscillation unit of the voltage controlled oscillator 12. It has a function of supplying to the controlled oscillator 12. As shown in FIG. 1B, the modulation current Im is a current that linearly and periodically changes in a range of current values I0 to I2 around a current value I1 in a triangular wave shape. The modulation current Im is, for example, a triangular wave current of about 30 kHz to 120 kHz.

  When the modulation current Im is supplied from the modulation current generator 15 to the voltage controlled oscillator 12 as described above, the frequency of the clock signal CLKO generated in the voltage controlled oscillator 12 becomes the basic frequency F1, as shown in FIG. Modulated between frequencies F0 to F2 at the center. In FIG. 1B, the vertical axis represents the frequency of the clock signal CLKO, and the horizontal axis represents the current value of the modulation current Im supplied from the modulation current generator 15.

  As a method of outputting the modulation current Im from the modulation current generator 15, a method of inputting a modulation voltage to the inverting node of the operational amplifier OP or a reference voltage having a constant value is input to the inverting node of the operational amplifier OP, and the resistor Ra There is a method of changing the resistance value. These methods will be described in detail later.

  As described above, in the spread spectrum clock generator 1 in the present embodiment, the modulation current generator 15 generates the current to be supplied to the oscillation unit of the voltage controlled oscillator 12 of the clock generation block having the same configuration as the conventional PLL circuit. The modulated clock signal can be generated simply by adding the modulated current Im, and the current value of the modulated current Im can be easily adjusted by the resistance value of the resistor Ra. Therefore, the difficulty of the circuit design can be reduced, and the design man-hour can be shortened.

  Further, as described above, the fundamental frequency is determined by the clock generation block, and the modulation degree is determined by the modulation current generator, so that each block can be designed independently and efficient design can be performed.

  Further, by using the operational amplifier, the design of the modulation current generator 15 for generating the modulation current Im can be simplified, and the circuit area can be reduced, so that the cost can be reduced.

  Hereinafter, several specific configurations of the voltage controlled oscillator 12 and the modulation current generator 15 will be specifically described with reference to the drawings.

(Specific Configuration 1 of Voltage Control Oscillator 12 and Modulation Current Generator 15)
First, the specific configuration 1 of the voltage controlled oscillator 12 and the modulation current generator 15 will be specifically described with reference to FIG. This specific configuration 1 is a configuration for realizing a method of inputting the modulation voltage Vh to the inverting node of the operational amplifier OP. FIG. 2 is a diagram showing a specific configuration of the voltage controlled oscillator and the modulation current generator in the present embodiment.

  As shown in FIG. 2, the voltage controlled oscillator 12 includes an oscillation unit 30 having a plurality of inverter circuits Inv1 to Invm connected in a chain, and currents Ic1 to Icm corresponding to the level of the oscillation control signal Sc. A current supply unit 31 including a plurality of PMOS transistors Tc1 to Tcm that supply the circuits Inv1 to Invm.

  The sources of the PMOS transistors Tc1 to Tcm are connected to the power supply potential, the drains are connected to the power supply input nodes of the inverter circuits Inv1 to Invm, and supply currents Ic1 to Icm to the inverter circuits Inv1 to Invm. The currents Ic1 to Icm are controlled by an oscillation control signal Sc input to the gates of the PMOS transistors Tc1 to Tcm. That is, currents Ic1 to Icm corresponding to the voltage level of the oscillation control signal Sc are supplied to the inverter circuits Inv1 to Invm. The PMOS transistors Tc1 to Tcm have the same chip size, and the currents Ic1 to Icm have the same current value.

  On the other hand, the modulation current generator 15 includes an operational amplifier OP, a feedback circuit 41, and a current distributor 42. The modulation current generator 15 modulates the current supplied to each inverter circuit Inv1 to Invm of the voltage controlled oscillator 12. Currents Im1 to Imm are generated and supplied to the inverter circuits Inv1 to Invm.

  The output node of the operational amplifier OP is input to the non-inverting node of the operational amplifier OP via the feedback circuit 41, the modulation voltage Vh is input to the inverting node of the operational amplifier OP, and the modulation signal Vm is output from the output node of the operational amplifier OP as a current distributor. 42 is output. Here, a voltage having a triangular waveform having a predetermined period (for example, 100 kHz) is input as the modulation voltage Vh.

  The feedback circuit 41 includes a PMOS transistor Ta and a variable resistor 50. The gate of the PMOS transistor Ta is connected to the output node of the operational amplifier OP, and the drain of the PMOS transistor Ta is connected to the variable resistor 50 and the non-inverting node of the operational amplifier OP. In the variable resistor 50, the resistors Ra1 to Ran connected in series and the switches SW1 to SWn are arranged in parallel, and these are connected between the ground potential with the non-inverting node of the operational amplifier OP. The

  The current distributor 42 includes a plurality of PMOS transistors Tb1 to Tbm. The modulation signal Vm is input to the gates of the PMOS transistors Tb1 to Tbm, and the modulation current Im1 corresponding to the voltage level of the modulation signal Vm. ˜Imm is supplied to the voltage controlled oscillator 12 from the drains of the PMOS transistors Tb1 to Tbm. The modulation currents Im1 to Imm are added to the currents Ic1 to Icm flowing from the drains of the PMOS transistors Tc1 to Tcm of the voltage controlled oscillator 12, and are supplied to the inverter circuits Inv1 to Invm as the currents Ih1 to Ihm. The PMOS transistors Tb1 to Tbm are configured with the same chip size, and the modulation currents Im1 to Imm have the same current value.

  As described above, in the specific configuration 1, when a plurality of inverter circuits Inv1 to Invm connected in a chain are used as the oscillation unit 30 of the voltage controlled oscillator 12, the modulation current generator 15 supplies the inverter circuits Inv1 to Invm. The spread spectrum clock generator 1 is configured by a simple configuration that supplies the modulation currents Im1 to Imm to the inverter circuits Inv1 to Invm so as to increase or decrease the currents Ih1 to Ihm, respectively.

  Moreover, by using the operational amplifier OP, the design of the modulation current generator 15 that generates the modulation currents Im1 to Imm can be simplified, and the circuit area can be reduced, so that the cost can be reduced. Since the modulation frequency is about 30 kHz to 120 kHz, the modulation currents Im1 to Imm can be easily taken out without worrying about the band of the operational amplifier OP.

  The variable resistor 50 includes a plurality of resistors Ra1 to Ran and a plurality of switches SW1 to SWn as described above, and these switches SW1 to SWn can be selected by an external control signal. The resistance value Rx is variable. Thereby, the amplitude width of the modulation signal Vm can be adjusted, and a spread spectrum clock generator having versatility can be provided.

(Specific configuration 2 of voltage controlled oscillator and modulation current generator)
Next, specific configuration 2 of the voltage controlled oscillator and the modulation current generator will be specifically described with reference to FIG. This specific configuration 2 is a configuration for realizing a method of changing the resistance value of the variable resistor 50 by inputting a constant reference voltage to the inversion node of the operational amplifier OP. The configuration of the voltage controlled oscillator 12 is as follows. It is the same as the specific configuration 1. FIG. 3 is a diagram showing another specific configuration of the voltage controlled oscillator and the modulation current generator in the present embodiment. In addition, about the same structure as the specific structure 1, it describes with the same code | symbol and abbreviate | omits description.

  In the specific configuration 1, the modulation current generator 15 inputs the modulation voltage Vh to the inverting input node of the operational amplifier OP. However, in the specific configuration 2, as shown in FIG. In the current generator 15 ′, a fixed reference voltage Vref is input to the inverting input node of the operational amplifier OP, and the modulation voltage Vh is input to the non-inverting input node of the operational amplifier OP.

  That is, in the modulation current generator 15 ′, the switches SW1 to SWn of the variable resistor 50 are controlled in order to input the modulation voltage Vh to the non-inverting input node of the operational amplifier OP, as shown in FIG. As described above, the resistance value of the variable resistor 50 is linearly changed in a triangular wave shape with a desired cycle (for example, 100 kHz) between the resistance values R0 to R2 with the predetermined resistance value R1 as a reference. As a result, a modulation signal having a triangular waveform is output from the output of the operational amplifier OP to the current distributor 42, and the modulation current Im corresponding to the modulation signal Vm is supplied from the current distributor 42 to the voltage controlled oscillator 12, as shown in FIG. As shown, the clock signal CLKO modulated between the frequencies F0 and F2 around the frequency F1 is output. As the reference voltage Vref, for example, a low voltage signal such as a band gap can be used.

  Thus, by varying the resistance value of the variable resistor 50 of the feedback circuit 41 as described above, it becomes possible to generate a modulation signal having an arbitrary period and modulation waveform, and to generate a spread spectrum clock having versatility. Can be provided. In particular, even when minute current control is required, appropriate scaling can be performed.

  Although several embodiments of the present invention have been described in detail with reference to the drawings, these are merely examples, and the present invention can be implemented in other forms that are variously modified and improved based on the knowledge of those skilled in the art. It is possible to implement.

It is a figure which shows schematic structure of the spread spectrum clock generator in this embodiment. It is a figure which shows the specific structure of the voltage controlled oscillator and modulation | alteration current generator in this embodiment. It is a figure which shows another specific structure of the voltage controlled oscillator and modulation | alteration current generator in this embodiment. It is a figure for demonstrating schematic structure of the conventional spread spectrum clock generator. It is a figure for demonstrating schematic structure of the conventional spread spectrum clock generator.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Spread spectrum clock generator 10 Phase comparator 11 Oscillation control signal generator 12 Voltage control oscillator 13 Output buffer 14 1 / N frequency divider 15 Modulation current generator 30 Oscillating unit 31 Current supply unit 41 Feedback circuit 42 Current divider 50 Variable resistor OP operational amplifier Ta, Tb, Tb1-Tbm, Tc1-Tcm PMOS transistor Inv1-Invm Inverter circuit Im, Im1-Imm Modulation current

Claims (5)

In a spread spectrum clock generator that generates a spread spectrum clock signal,
A frequency divider that outputs a divided signal obtained by dividing the clock signal by a predetermined dividing ratio;
A phase comparator that outputs a phase difference signal having a level changed according to a phase difference between the input reference clock signal and the divided signal;
An oscillation control signal generator for generating an oscillation control signal according to the phase difference signal;
Voltage control having an oscillating unit that oscillates at a frequency corresponding to the supplied current, supplying a current corresponding to the level of the oscillation control signal to the oscillating unit, and oscillating the oscillating unit to generate the clock signal An oscillator,
A spread spectrum clock generator comprising: a modulation current generator that generates a modulation current that increases or decreases a current supplied to an oscillation unit of the voltage controlled oscillator and supplies the modulation current to the voltage controlled oscillator. The voltage controlled oscillator has a plurality of inverter circuits chain-connected as the oscillating unit, and supplies a current corresponding to the level of the oscillation control signal to each inverter circuit,
The spread spectrum clock generator according to claim 1, wherein the modulation current generator increases or decreases a current supplied to each inverter circuit by the modulation current.   The modulation current generator includes an operational amplifier, a feedback circuit, and a current distributor, and an output node of the operational amplifier is input to a non-inverting node of the operational amplifier via the feedback circuit, and to an inverting node of the operational amplifier. A modulation voltage is input, a modulation signal is output from the output of the operational amplifier to the current distributor, and the modulation current corresponding to the modulation signal is supplied from the current distributor to the voltage controlled oscillator. The spread spectrum clock generator according to claim 1 or 2.   The feedback circuit includes a variable resistor and a MOS transistor, the gate of the MOS transistor is connected to the output node of the operational amplifier, and the drain of the MOS transistor is connected to the non-inverting node of the variable resistor and the operational amplifier. 4. The spread spectrum clock generator according to claim 3, wherein the spread spectrum clock generator is connected and the resistance value of the variable resistor can be adjusted.   The modulation current generator includes an operational amplifier, a feedback circuit, and a current distributor, and an output node of the operational amplifier is input to a non-inverting node of the operational amplifier via the feedback circuit, and to an inverting node of the operational amplifier. A reference voltage is input, the feedback circuit includes a variable resistor and a MOS transistor, a gate of the MOS transistor is connected to an output node of the operational amplifier, and a drain of the MOS transistor is the variable resistor And a modulation signal is output from the output of the operational amplifier to the current distributor by changing a resistance value of the variable resistor, and the modulation signal is output from the current distributor. The modulation current according to claim 1 is supplied to the voltage controlled oscillator. Kutoramu spread spectrum clock generator.

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