Oscillation circuit

Abstract

To provide an oscillation circuit capable of suppressing noise.SOLUTION: An oscillation circuit includes an inverter, a crystal unit connected in parallel to the inverter, a first load capacitance connected to an input side of the crystal unit, a second load capacitance connected to an output side of the crystal unit, and a band elimination filter connected in parallel to the inverter and having an elimination band including a resonant frequency of the crystal unit, and the band elimination filter is a twin-T notch filter.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an oscillator circuit. [Background technology]

[0002] The crystal oscillator circuits described in Patent Document 1 and Patent Document 2 each have a CMOS inverter connected between a power supply and ground. A series circuit including an output resistor and a crystal unit, and a DC feedback resistor are connected in parallel between the input terminal and the output terminal of the inverter. One terminal of the crystal unit is connected to one terminal of a first load capacitance, and one terminal of a second load capacitance is connected to the other terminal of the crystal unit. In Patent Document 1, the other terminals of the first and second load capacitances are connected to a power supply, and in Patent Document 2, the other terminals of the first and second load capacitances are connected to ground. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 54-061447 [Patent Document 2] Japanese Patent Application Publication No. 08-023230 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the crystal oscillation circuits described in Patent Documents 1 and 2 tend to generate large noise due to their configurations. [Means for solving the problem]

[0005] The oscillator circuit according to the present invention comprises an inverter; A quartz crystal unit connected in parallel to the inverter; A first load capacitance connected to an input side of the crystal unit; A second load capacitance connected to an output side of the crystal unit; a band elimination filter connected in parallel to the inverter and having a elimination band including a resonance frequency of the crystal oscillator. [Brief description of the drawings]

[0006] [Figure 1] 1 is a circuit diagram of an oscillator circuit according to a first embodiment. [Diagram 2] FIG. [Diagram 3] 1 is a graph showing the characteristics of a twin-T notch filter. [Figure 4] FIG. 11 is a circuit diagram of an oscillator circuit according to a second embodiment. [Diagram 5] 1 is a graph showing the characteristics of a twin-T notch filter. [Figure 6] FIG. 4 is a diagram showing the waveform of an oscillation signal. [Figure 7] FIG. 4 is a diagram showing the waveform of an oscillation signal. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0007] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An oscillator circuit according to the present invention will now be described in detail with reference to the embodiments shown in the accompanying drawings.

[0008] First Embodiment Fig. 1 is a circuit diagram of an oscillator circuit according to a first embodiment of the present invention. Fig. 2 is an exploded perspective view of an acceleration sensor. Fig. 3 is a graph showing the characteristics of a twin-T notch filter.

[0009] The oscillator circuit 100 shown in FIG. 1 is a Colpitts type oscillator circuit that oscillates at a predetermined frequency, and includes a crystal oscillator 110, an inverter 120, a feedback resistor 130, a limiting resistor 140, a first load capacitor 150, and a second load capacitor 160.

[0010] Of these elements, the crystal unit 110, the inverter 120, and the feedback resistor 130 are connected in parallel with each other. A limiting resistor 140 is connected between the output terminal of the inverter 120 and the input terminal of the crystal unit 110, a first load capacitance 150 is connected between the input terminal of the crystal unit 110 and ground, and a second load capacitance 160 is connected between the output terminal of the crystal unit 110 and ground. Note that the first load capacitance 150 and the second load capacitance 160 may be connected to a power supply VDD instead of to ground.

[0011] The crystal unit 110 is mounted on an acceleration sensor 200. Therefore, before specifically describing the oscillator circuit 100, the acceleration sensor 200 will be briefly described with reference to FIG. 2. For convenience of description, FIG. 2 illustrates an X-axis, a Y-axis, and a Z-axis that are perpendicular to each other. The acceleration sensor 200 is a sensor that detects acceleration in a direction along the Z-axis (hereinafter also referred to as the Z-axis direction), and has a package 300 and an acceleration sensor element 400 accommodated in the package 300, as shown in FIG. 2.

[0012] The package 300 has a base 310 having a recess 311 opening on the top surface, and a plate-shaped lid 320 joined to the top surface of the base 310 and closing the opening of the recess 311. An airtight space S is formed inside the package 300, and an acceleration sensor element 400 is housed in this airtight space S.

[0013] The acceleration sensor element 400 has a substrate structure 500 fixed to a base 310 , a quartz crystal unit 110 disposed on the substrate structure 500 , and a weight portion 600 disposed on the substrate structure 500 .

[0014] The substrate structure 500 is formed from a quartz substrate and has a flat plate shape along an XY plane perpendicular to the Z axis. The substrate structure 500 has a support portion 510, a movable portion 520 displaced in the Z axis direction relative to the support portion 510, and a joint portion 530 located between the support portion 510 and the movable portion 520, and is fixed to the base 310 at three points of the support portion 510. The movable portion 520 is supported at one end by the support portion 510 like a cantilever. The joint portion 530, which is thinner than the support portion 510 and the movable portion 520, is disposed between the support portion 510 and the movable portion 520. The joint portion 530 functions as a hinge, making it easier for the movable portion 520 to be displaced in the Z axis direction relative to the support portion 510.

[0015] The quartz crystal unit 110 is a double-ended tuning fork type resonator formed from a quartz crystal substrate. The quartz crystal unit 110 has two vibrating beams 111 and 112, a first base 113 terminating one end of the two vibrating beams 111 and 112, and a second base 114 terminating the other end. The quartz crystal unit 110 has the vibrating beams 111 and 112 arranged along the Y axis, joined to the movable part 520 at the first base 113, and joined to the support part 510 at the second base 114. The quartz crystal unit 110 has a pair of excitation electrodes (not shown) provided on the surfaces of the vibrating beams 111 and 112. When an AC voltage drive signal is applied between these excitation electrodes, the vibrating beams 111 and 112 flexurally vibrate in a direction along the X axis so as to repeatedly move away from and toward each other in the X axis direction.

[0016] Weight 600 is joined to the tip of movable part 520. By disposing weight 600 on movable part 520, the mass of movable part 520 increases. Therefore, movable part 520 becomes more easily displaced even by small acceleration, and the resolution of acceleration sensor 200 is improved.

[0017] Such an acceleration sensor 200 can detect acceleration in the Z-axis direction as follows. When acceleration in the Z-axis direction is applied to the acceleration sensor 200, the movable part 520 is displaced in the Z-axis direction relative to the support part 510 with the joint part 530 as a fulcrum. Then, this displacement applies tensile or compressive stress to the crystal unit 110, and the resonant frequency of the crystal unit 110 changes depending on the magnitude of the applied stress. Specifically, when acceleration is applied in the positive Z-axis direction, the movable part 520 is displaced in the negative Z-axis direction relative to the support part 510, which applies tensile stress to the crystal unit 110 and increases the resonant frequency of the crystal unit 110. Conversely, when acceleration is applied in the negative Z-axis direction, the movable part 520 is displaced in the positive Z-axis direction relative to the support part 510, which applies compressive stress to the crystal unit 110 and decreases the resonant frequency of the crystal unit 110. Therefore, the acceleration sensor 200 can detect acceleration in the Z-axis direction based on the change in the resonant frequency of the crystal unit 110.

[0018] The above is a brief description of the acceleration sensor 200. However, there is no particular limitation on the configuration of the acceleration sensor 200. Furthermore, there is no particular limitation on the configuration of the quartz crystal resonator 110.

[0019] Returning to the description of the oscillator circuit 100, the inverter 120 is connected between a power supply VDD and ground. The inverter 120 of this embodiment is a CMOS inverter. By using a CMOS inverter for the inverter 120, the gain can be easily increased. In addition, the oscillator circuit 100 consumes less power. However, the inverter 120 is not particularly limited as long as it can perform its function.

[0020] The feedback resistor 130 is connected in parallel with the inverter 120. In the oscillation circuit 100, the feedback resistor 130 feeds back a signal from the output side of the inverter 120 to keep the crystal unit 110 oscillating. The limiting resistor 140 is connected between the output end of the inverter 120 and the input end of the crystal unit 110. In the oscillation circuit 100, the limiting resistor 140 controls the current flowing into the crystal unit 110, and can adjust the negative resistance and the drive level, and suppress abnormal oscillation and frequency fluctuation of the crystal unit 110. The first load capacitance 150 is connected between the input end of the crystal unit 110 and the ground, and the second load capacitance 160 is connected between the output end of the crystal unit 110 and the ground. In the oscillation circuit 100, the first load capacitance 150 and the second load capacitance 160 can adjust the negative resistance, the drive level, the oscillation frequency, and the like.

[0021] Next, the feedback resistor 130 will be described in detail. As shown in FIG. 1, the feedback resistor 130 is configured with a twin-T notch filter 131, which is a band-elimination filter. This makes it easy to configure the band-elimination filter. The twin-T notch filter 131 is configured with three resistors R1, R2, and R3, and three capacitors C1, C2, and C3. Specifically, two resistors R1 and R2 connected in series and two capacitors C2 and C3 connected in series are connected in parallel, a capacitor C1 is connected between the resistors R1 and R2 and the ground, and a resistor R3 is connected between the capacitors C2 and C3 and the ground.

[0022] According to such a twin T notch filter 131, a desired central attenuation frequency Freq can be obtained by adjusting the resistance values ​​of the resistors R1 to R3 and the capacitance values ​​of the capacitors C1 to C3. For example, when R1=R2=R, R3=R / 2, C2=C3=C, and C1=C2, the attenuation amount is maximized at Freq=1 / (2πRC). In other words, the twin T notch filter 131 can increase the impedance only near the central attenuation frequency Freq. On the other hand, in order to amplify the signal by the inverter 120, the feedback resistor 130 needs to have an impedance equal to or higher than the threshold value It.

[0023] Therefore, in the inverter 120, as shown in FIG. 3, only the signal of the frequency component included in the elimination band Sf, which is a frequency band equal to or higher than the threshold value It, is amplified. Therefore, as shown in the figure, in the oscillation circuit 100, the twin-T notch filter 131 is configured so that the elimination band Sf includes the resonant frequency fr of the crystal unit 110, preferably so that the resonant frequency fr is located at the center of the elimination band Sf. As a result, a signal in which only the signal of the resonant frequency fr and its nearby frequency components is amplified is input from the inverter 120 to the crystal unit 110 via the limiting resistor 140. Then, the signal output from the crystal unit 110 is input to the inverter 120 as a positive feedback. As described above, in the acceleration sensor 200, the resonant frequency fr of the crystal unit 110 fluctuates due to the acceleration received, so in this embodiment, the twin-T notch filter 131 is configured so that the entire range of the fluctuation width of the resonant frequency fr is included in the elimination band Sf.

[0024] In this way, by using the twin-T notch filter 131 as the feedback resistor 130, it is possible to amplify only the signals of the frequency components included in the rejection band Sf, so that noise can be suppressed to a lower level compared to the conventional case where the feedback resistor 130 is configured with a single resistor, resulting in an oscillator circuit 100 with excellent noise characteristics.

[0025] The oscillator circuit 100 has been described above. As described above, the oscillator circuit 100 includes an inverter 120, a crystal unit 110 connected in parallel to the inverter 120, a first load capacitance 150 connected to the input side of the crystal unit 110, a second load capacitance 160 connected to the output side of the crystal unit 110, and a twin-T notch filter 131 connected in parallel to the inverter 120 as a band elimination filter having an elimination band Sf including the resonance frequency fr of the crystal unit 110. According to the oscillator circuit 100 configured as described above, a signal in which only the signal of the frequency component included in the elimination band Sf is amplified by the inverter 120 is input to the crystal unit 110. Therefore, the noise can be suppressed to a low level, resulting in an oscillator circuit 100 with excellent noise characteristics.

[0026] As described above, the band elimination filter is the twin-T notch filter 131. This simplifies the configuration of the band elimination filter.

[0027] As described above, the inverter 120 is a CMOS inverter. This makes it possible to easily increase the gain. In addition, the oscillator circuit 100 consumes less power.

[0028] As described above, the oscillator circuit 100 has a limiting resistor 140 connected between the output side of the inverter 120 and the crystal unit 110. This makes it possible to control the current flowing into the crystal unit 110, adjust the negative resistance and the drive level, and suppress abnormal oscillation and frequency fluctuation of the crystal unit 110.

[0029] <Second embodiment> Fig. 4 is a circuit diagram of an oscillation circuit according to a second embodiment. Fig. 5 is a graph showing the characteristics of a twin-T notch filter. Figs. 6 and 7 are diagrams showing waveforms of oscillation signals.

[0030] The oscillator circuit 100 according to this embodiment is similar to the oscillator circuit 100 according to the first embodiment, except that it further includes a first offset resistor 170 and a second offset resistor 180. In the following description, the oscillator circuit 100 according to this embodiment will be described with a focus on differences from the first embodiment, and descriptions of similar matters will be omitted. In addition, in each drawing of this embodiment, the same reference numerals are used for configurations similar to those of the above-mentioned embodiment.

[0031] As shown in FIG. 4, the oscillator circuit 100 of this embodiment further includes a first offset resistor 170 and a second offset resistor 180 connected to both ends of the twin T notch filter 131 in addition to the configuration of the first embodiment described above. The first offset resistor 170 is connected between the output side of the inverter 120 and the input side of the twin T notch filter 131, and the second offset resistor 180 is connected between the output side of the twin T notch filter 131 and the input side of the inverter 120. In this embodiment, the first offset resistor 170 and the second offset resistor 180 are set to the same resistance value. However, this is not limited to this, and they may have different resistance values.

[0032] Such first and second offset resistors 170, 180 have the function of offsetting the characteristics of the twin-T notch filter 131. Specifically, as shown in Fig. 5, as the resistance values ​​of the first and second offset resistors 170, 180 are increased, the characteristics of the twin-T notch filter 131 shifts upward. The elimination band Sf widens by the amount of the upward shift of the characteristics. In this way, by arranging the first and second offset resistors 170, 180 and adjusting the resistance values ​​of the first and second offset resistors 170, 180, the range of the elimination band Sf can be adjusted as desired, making noise design easier.

[0033] Moreover, the waveform of the oscillation signal output from the oscillation circuit 100 is determined by the total value of the phase noise and the amplitude noise. The larger the first and second offset resistors 170 and 180 are made to widen the rejection band Sf, the closer the oscillation signal becomes to a square wave, as shown in FIG. 6, and the smaller the rejection band Sf is made to, the closer the oscillation signal becomes to a sine wave, as shown in FIG. 7, and the smaller the phase noise is made. Therefore, by optimizing the first and second offset resistors 170 and 180, it is possible to easily adjust the waveform to match the characteristics of the frequency counter to be used, or to adjust the waveform so that the total value of the phase noise and the amplitude noise is minimized.

[0034] Furthermore, by arranging the first and second offset resistors 170 and 180, it is also possible to suppress overcurrent. Specifically, as shown in FIG. 5, in the twin T notch filter 131, the impedance decreases as the frequency moves away from the central attenuation frequency Freq. Therefore, when a signal having a frequency component far removed from the central attenuation frequency Freq is output from the inverter 120, the output side and the input side of the inverter 120 are short-circuited. Note that the "short-circuited" state includes a state in which the output signal of the inverter 120 is applied to the input side of the inverter 120, causing an overcurrent, which may lead to damage to the circuit. To address this problem, by arranging the first and second offset resistors 170 and 180, the current value input to the inverter 120 is limited, and the above-mentioned damage to the circuit can be effectively suppressed.

[0035] As described above, the oscillator circuit 100 of this embodiment has the first offset resistor 170 connected between the output side of the inverter 120 and the twin T notch filter 131, and the second offset resistor 180 connected between the twin T notch filter 131 and the input side of the inverter 120. In this way, by arranging the first and second offset resistors 170 and 180, the range of the elimination band Sf can be arbitrarily adjusted, making noise design easy. In addition, by optimizing the first and second offset resistors 170 and 180, it is possible to easily adjust the waveform according to the characteristics of the frequency counter used, or adjust the waveform so that the total value of the phase noise and the amplitude noise is minimized. In addition, the current value input to the inverter 120 can be limited, and damage to the circuit can be effectively suppressed.

[0036] The second embodiment as described above can also achieve the same effects as the first embodiment.

[0037] Although the oscillator circuit of the present invention has been described above based on the illustrated embodiment, the present invention is not limited thereto. The configuration of each part can be replaced with any configuration having a similar function. In addition, any other configuration may be added to the present invention. In addition, the present invention may be a combination of any two or more configurations of the above-mentioned embodiments.

[0038] In addition, in the above-described embodiment, the twin-T notch filter 131 is used as the band elimination filter, but this is not limited to this. For example, an active notch filter may be used, or a filter other than a notch filter may be used.

[0039] In addition, in the above-described embodiment, the configuration has one inverter 120 and one twin-T notch filter 131 connected in parallel thereto, but this is not limited to this, and the configuration may have, for example, a plurality of inverters 120 connected in series and a plurality of twin-T notch filters 131 connected in parallel to each inverter 120.

[0040] In the above-described embodiment, the crystal unit 110 is used as an acceleration sensor element for detecting acceleration, but the present invention is not limited to this, and may be used as an angular velocity sensor element for detecting angular velocity, or as an oscillation element for oscillating a signal of a predetermined frequency, or may be used for purposes other than these. [Explanation of symbols]

[0041] 100...oscillating circuit, 110...quartz crystal oscillator, 111...vibrating beam, 112...vibrating beam, 113...first base, 114...second base, 120...inverter, 130...feedback resistor, 131...twin-T notch filter, 140...limiting resistor, 150...first load capacitance, 160...second load capacitance, 170...first offset resistor, 180...second offset resistor, 200...accelerometer, 300 ...package, 310...base, 311...recess, 320...lid, 400...acceleration sensor element, 500...substrate structure, 510...support, 520...movable part, 530...joint, 600...weight, C1...capacitance, C2...capacitance, C3...capacitance, Freq...central attenuation frequency, It...threshold, R1...resistance, R2...resistance, R3...resistance, S...airtight space, Sf...rejection band, VDD...power supply, fr...resonance frequency

Claims

1. The inverter and A quartz crystal unit connected in parallel to the inverter; A first load capacitance connected to an input side of the crystal unit; A second load capacitance connected to an output side of the crystal unit; a band elimination filter connected in parallel to the inverter and having an elimination band including a resonant frequency of the crystal resonator.

2. 2. The oscillator circuit of claim 1, wherein the band elimination filter is a twin-T notch filter.

3. 2. The oscillator circuit of claim 1, further comprising: a first offset resistor connected between an output side of the inverter and the band-elimination filter; and a second offset resistor connected between the band-elimination filter and an input side of the inverter.

4. 2. The oscillator circuit of claim 1, wherein the inverter is a CMOS inverter.

5. 2. The oscillator circuit according to claim 1, further comprising a limiting resistor connected between the output side of the inverter and the crystal resonator.

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