JP2008103808A - Crystal oscillator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that a floor noise of a phase noise of a crystal oscillator is greatly influenced by a noise generated by a buffer circuit using the output voltage generated by a power circuit because of a voltage noise of the output voltage and the problem that a DC voltage drops in a method of inserting a filter lowernig the voltage noise and stable oscillation characteristics can not be secured. <P>SOLUTION: A voltage noise (floor noise) of the source voltage of a buffer circuit 105 is lowered without deteriorating oscillation characteristics of the crystal oscillator by connecting a low-pass filter 116 only between the output of the source circuit 102 and the buffer circuit 105, so that the phase noise (floor noise) of the crystal oscillator can eventually be reduced. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a crystal oscillator that uses TCXO (temperature control crystal oscillator) as a main field of application and realizes excellent phase noise performance and particularly reduces floor noise of 100 KHz detuning or more.

  In recent years, mobile phones are required to have high call quality, and even TCXO (temperature control crystal oscillator) is required to have better noise performance, particularly in a high detuning frequency band of 100 KHz or higher. Good phase noise performance is required.

  Hereinafter, a conventional crystal oscillator will be described.

  A conventional crystal oscillator of this type is configured as shown in an example of a circuit diagram in FIG. In FIG. 1, a power supply circuit 2 having an external power supply 1 as an input, an inverter 6 connected with the power supply circuit 2 as a power supply, a feedback resistor 7 connected between input and output terminals of the inverter 6, A fixed capacitor 8 connected to the input terminal, a fixed capacitor 9 connected to the output terminal of the inverter 6, a voltage variable capacitor 11 connected between the other terminal of the fixed capacitor 8 and the ground, A voltage variable capacitor 12 connected between the other terminal of the fixed capacitor 9 and the ground, a connection terminal of the fixed capacitor 8 and the voltage variable capacitor 11, and a connection of the fixed capacitor 9 and the voltage variable capacitor 12. A crystal oscillation circuit 4 composed of a crystal resonator 10 connected between the terminals and a control voltage generation circuit 3 for generating a control voltage to be applied to the voltage variable capacitors 11 and 12 via resistors 13 and 14; And before It receives the output of the crystal oscillator circuit 4, and a buffer circuit 5 in which the power supply circuit 2 is connected as a voltage source.

  The crystal oscillation circuit 4 shown in FIG. 1 is supplied with power by the power supply circuit 2 and oscillates by the crystal resonator 10, the inverter 6, the fixed capacitors 8 and 9, and the voltage variable capacitors 11 and 12. In this configuration, a loop is formed and an oscillation waveform is output.

  The operation of the crystal oscillator configured as described above will be described. When a voltage is applied from the external power supply 1, a certain power supply voltage is supplied from the power supply circuit 2 to the inverter 6 and the crystal resonator 10, thereby performing an oscillation operation. The oscillation amplitude oscillated in the crystal oscillation circuit 4 is amplified and output by the buffer circuit 5 driven by the power supply circuit 2. Further, a control voltage is applied from the control voltage generation circuit 3 to the voltage variable capacitors 11 and 12 through the resistors 13 and 14, and the load value of the oscillation circuit is changed by changing the capacitance value of the voltage variable capacitors 11 and 12. The capacitance CL can be changed and adjusted to a desired oscillation frequency f0.

As prior art document information related to the invention of this application, for example, Patent Document 1 is known.
Japanese Patent Laid-Open No. 11-59811

  In the crystal oscillator configured as described above, the voltage noise of the output voltage generated in the power supply circuit is dominant in the phase noise performance. In the crystal oscillation circuit and the buffer circuit using the output voltage as a power supply, the power supply of low voltage noise is used. Even if the circuit is designed, it is amplified with high sensitivity.Therefore, there is a problem that the 1 / f noise of the phase noise greatly affects the crystal oscillation circuit, and the floor noise of the phase noise greatly affects the buffer circuit. It was.

  As an effect of reducing the voltage noise, a method of inserting a filter circuit is conceivable. However, since the crystal oscillation circuit consumes a large current and a DC current flows constantly, the DC voltage drops. Therefore, the negative resistance necessary for ensuring stable oscillation characteristics is deteriorated, and the oscillation amplitude necessary for ensuring a desired frequency variable range is reduced, which greatly affects the oscillation characteristics.

  The present invention solves the above-mentioned conventional problems, and realizes good phase noise performance (floor) without deterioration of characteristics such as negative resistance and oscillation amplitude due to drop of the DC voltage of the crystal oscillation circuit. With the goal.

  In order to achieve the above object, by connecting a low-pass filter with a cutoff frequency of 100 KHz that drops the voltage noise floor to almost zero only between the output of the power supply circuit and the buffer circuit, the power supply voltage of the buffer circuit is reduced. Voltage noise (floor noise) is reduced, and as a result, phase noise (floor noise) of the crystal oscillator is reduced.

  In the present invention, in the phase noise generated in the crystal oscillator, the influence of the power supply voltage noise of the buffer circuit is dominant on the performance of the floor noise. (Floor) can be reduced.

  In addition, since the DC current is small in the buffer circuit, the power supply voltage after the low-pass filter connection is not greatly dropped, and the transient voltage response can be reduced by connecting a fixed capacitor between the ground potentials. . Further, since the low-pass filter is not connected to the power supply voltage system of the crystal oscillation circuit, there is no drop of the power supply voltage of the crystal oscillation circuit. From the above, there is no influence on various characteristics of the oscillation circuit, and stable oscillation characteristics can be maintained.

  Since the low-pass filter is intended to reduce the floor of power supply voltage noise, the constants of the constituent elements may be configured with a cutoff frequency of about 100 KHz or more. Can be realized in a sufficiently possible area.

(First embodiment)
Hereinafter, a first embodiment of the present invention will be described with reference to the drawings.

  FIG. 2 is a configuration diagram of the crystal oscillator according to the first embodiment of the present invention. FIG. 5 shows power supply voltage noise of the buffer circuit of the present invention. 2 and 5, a power supply circuit 102 having an external power supply 101 as an input, an inverter 106 connected with the power supply circuit 102 as a power supply, a feedback resistor 107 connected between input / output terminals of the inverter 106, A fixed capacitor 108 connected to the input terminal of the inverter 106, a fixed capacitor 109 connected to the output terminal of the inverter 106, and a voltage variable capacitor connected between the other terminal of the fixed capacitor 108 and the ground. 111, a voltage variable capacitor 112 connected between the other terminal of the fixed capacitor 109 and the ground, a connection terminal of the fixed capacitor 108 and the voltage variable capacitor 111, the fixed capacitor 109, and the voltage variable capacitor. A crystal oscillation circuit 104 including a crystal resonator 110 connected between the connection terminal 112 and the voltage variable capacitor 111; The control voltage generation circuit 103 that generates a control voltage to be applied to the resistor 112 through the resistors 113 and 114 and the output of the crystal oscillation circuit 104 are input, and the output of the power supply circuit 102 as a voltage source is about a cutoff frequency 403. A crystal oscillator comprising a 100 kHz resistor 117 and a buffer circuit 105 connected via a low pass filter 116 comprising a fixed capacitor 118.

  The operation of the crystal oscillator according to the embodiment configured as described above will be described below.

  When a power supply voltage is applied from the external power supply 101, a certain power supply voltage is supplied from the power supply circuit 102 to the inverter 106 and the crystal unit 110, thereby performing an oscillation operation. The oscillation amplitude oscillated in the crystal oscillation circuit 104 is amplified and output from the power supply circuit 102 by the buffer circuit 105 driven by a low noise voltage through the low-pass filter 116 having a cutoff frequency 403 of about 100 KHz. . Further, the control voltage is applied from the control voltage generation circuit 103 to the cathodes of the voltage variable capacitors 111 and 112 through the resistors 113 and 114, and the capacitance values of the voltage variable capacitors 111 and 112 are changed to change the load capacitance CL of the oscillation circuit. Can be adjusted to a desired oscillation frequency f0. Here, the fixed capacitor 118 constituting the low-pass filter 116 may be an external element without being incorporated in the IC.

  The voltage noise 401 at the output of the low-pass filter 116 is reduced by 3 dB with respect to the floor noise 402 at a cutoff frequency 403 of 100 KHz, and attenuates in accordance with the frequency characteristics of the primary low-pass filter in the subsequent detuning frequency band.

  Therefore, in the conventional method, the voltage noise of the buffer circuit voltage has a constant floor noise even in a high detuning frequency band (<100 KHz), whereas according to the embodiment of the present invention, the voltage noise is greatly reduced. As a result, the floor noise of the phase noise generated in the crystal oscillator can be greatly reduced.

(Second Embodiment)
Hereinafter, a second embodiment of the present invention will be described with reference to the drawings.

  FIG. 3 is a block diagram of a crystal oscillator according to the second embodiment of the present invention. FIG. 5 shows power supply voltage noise of the buffer circuit of the present invention. 3 and 5, a power supply circuit 202 having an external power supply 201 as an input, an inverter 206 connected using the power supply circuit 202 as a power supply, a feedback resistor 207 connected between input and output terminals of the inverter 206, A fixed capacitor 208 connected to the input terminal of the inverter 206, a fixed capacitor 209 connected to the output terminal of the inverter 206, and a voltage variable capacitor connected between the other terminal of the fixed capacitor 208 and the ground. 211, a voltage variable capacitor 212 connected between the other terminal of the fixed capacitor 209 and the ground, a connection terminal of the fixed capacitor 208 and the voltage variable capacitor 211, the fixed capacitor 209, and the voltage variable capacitor. A crystal oscillation circuit 204 including a crystal resonator 210 connected between the connection terminal 212 and the voltage variable capacitor 211; The control voltage generation circuit 203 that generates a control voltage to be applied to the resistor 212 through the resistors 213 and 214 and the output of the crystal oscillation circuit 204 are input, and the output of the power supply circuit 202 as a voltage source is about a cutoff frequency 403. A crystal oscillator including a 100 KHz resistor 217, a fixed capacitor 218, and a buffer circuit 205 connected via a low-pass filter 216 including a switch element 219 connected in parallel to the resistor 217.

  The operation of the crystal oscillator according to the second embodiment configured as described above will be described below.

  When a power supply voltage is applied from the external power supply 201, a certain power supply voltage is supplied from the power supply circuit 202 to the inverter 206 and the crystal unit 210, thereby performing an oscillation operation. Here, the switch element 219 is short-circuited for a certain period after the power supply is applied, and the fixed capacitor 218 constituting the low-pass filter 216 is operated to be released after being sufficiently charged by the power supply circuit 202, whereby the buffer circuit The transient characteristic of the power supply voltage 205 is reduced.

  The oscillation amplitude oscillated in the crystal oscillation circuit 204 is amplified and output from the power supply circuit 202 by the buffer circuit 205 driven by a low noise voltage through the low-pass filter 216 having a cutoff frequency 403 of about 100 KHz. .

  Further, the control voltage is applied from the control voltage generation circuit 203 to the cathodes of the voltage variable capacitors 211 and 212 through the resistors 213 and 214, and the capacitance values of the voltage variable capacitors 211 and 212 are changed to change the load capacitance CL of the oscillation circuit. Can be adjusted to a desired oscillation frequency f0. Here, the fixed capacitor 218 constituting the low-pass filter 216 may be an external element without being incorporated in the IC.

  Further, the voltage noise 401 at the output of the low-pass filter 216 decreases by 3 dB with respect to the floor noise 402 at 100 KHz of the cutoff frequency 403, and attenuates according to the frequency characteristic of the primary low-pass filter in the subsequent detuning frequency band.

  Therefore, in the conventional method, the voltage noise of the buffer circuit voltage has a constant floor noise even in a high detuning frequency band (<100 KHz), whereas according to the embodiment of the present invention, the voltage noise is greatly reduced. As a result, the floor noise of the phase noise generated in the crystal oscillator can be greatly reduced.

  The present invention reduces the power supply voltage noise of the buffer circuit, which particularly affects the floor noise, among the phase noise characteristics of the crystal oscillator by a low-pass filter, thereby achieving good phase noise performance without deteriorating the oscillation characteristics of the crystal oscillator. Since the crystal oscillator can be realized, phase noise that can be said to be particularly important for improving the communication quality among the crystal oscillator performance can be reduced, so the utility value and possibility are very high.

Configuration diagram showing a crystal oscillator according to a conventional embodiment 1 is a configuration diagram showing a crystal oscillator according to a first embodiment of the present invention. The block diagram which shows the crystal oscillator based on the 2nd Embodiment of this invention Graph showing power supply voltage noise example of conventional circuit buffer circuit Graph showing power supply voltage noise example of new circuit buffer circuit

Explanation of symbols

1, 101, 201 External power supply 2, 102, 202 Power supply circuit 3, 103, 203 Control voltage generation circuit 4, 104, 204 Crystal oscillation circuit 5, 105, 205 Buffer circuit 6, 106, 206 Inverter 7, 107, 207 Feedback Resistor 8, 9, 108, 109, 118, 208, 209, 218 Fixed capacitance 10, 110, 210 Quartz crystal 11, 12, 111, 112, 211, 212 Voltage variable capacitance 13, 14, 113, 114, 117, 213, 214, 217 Resistor 15, 115, 215 Inverter 116, 216 Low-pass filter 301, 401 Buffer circuit power supply voltage noise 302, 402 Buffer circuit power supply voltage floor noise 403 Cut-off frequency 404 Buffer circuit power supply voltage noise at the cut-off frequency

Claims (2)

A crystal oscillation circuit that is connected to a power supply circuit as a voltage source and receives the output of the control voltage generation circuit, and a buffer circuit that is connected to the power supply circuit as a voltage source and receives the output of the crystal oscillation circuit. A crystal oscillator configured by connecting a low pass filter between the power supply circuit and the buffer circuit. 2. The crystal oscillator according to claim 1, comprising a low pass filter comprising a resistor and a fixed capacitor between a power supply circuit and a buffer circuit, and a switch element connected in parallel to the resistor.

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