Piezoelectric oscillator

Abstract

To obtain desired voltage-frequency characteristics even in a case where a piezoelectric oscillator is configured by using a variable capacitance element with non-linear voltage-capacity characteristics.SOLUTION: A piezoelectric oscillator comprises: a piezoelectric vibrator that comprises a first terminal and a second terminal; a first variable capacitance element whose cathode is connected with the first terminal of the piezoelectric vibrator; a second variable capacitance element whose cathode is connected with the second terminal of the piezoelectric vibrator; a DC power supply whose voltage value can be varied; a first resistor whose one end is connected with a connection point between the cathode of the first variable capacitance element and the first terminal of the piezoelectric vibrator and whose the other end is connected with the DC power supply; a transistor whose gate is connected with a positive electrode side terminal of the DC power supply, whose drain is connected with a power supply different from the DC power supply, and whose source is connected with a second resistor; the second resistor whose one end is connected with the source of the transistor and whose the other end is grounded; and a third resistor whose one end is connected with a connection point between the source of the transistor and the second resistor and whose the other end is connected with a connection point between the cathode of the second variable capacitance element and the second terminal of the piezoelectric vibrator.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a piezoelectric oscillator.

Background Art

[0002] Conventionally, there has been a voltage-controlled variable oscillator (VCO: Voltage Controlled Oscillator; hereinafter also referred to as a piezoelectric oscillator) such as a VCXO (Voltage Controlled Xtal Oscillator) using an on-chip variable capacitance element. As a variable capacitance element used in such a piezoelectric oscillator, for example, a variable capacitance diode (varicap) with a stepped junction as described in Patent Document 1 is known.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] When a piezoelectric oscillator is configured using such a variable capacitance element with a stepped junction, the combined capacitance of the variable capacitance element changes rapidly in a low voltage region, and the combined capacitance of the variable capacitance element levels off in a high voltage region. That is, when a variable capacitance element with a stepped junction is used, the voltage-capacitance characteristic is not linear. When a variable capacitance element with a non-linear voltage-capacitance characteristic is used, there is a problem that the voltage-frequency characteristic of the piezoelectric oscillator does not become linear and the desired characteristic cannot be obtained.

[0005] The present invention has been made in view of the above circumstances, and an object thereof is to provide a piezoelectric oscillator capable of obtaining a desired voltage-frequency characteristic even when a piezoelectric oscillator is configured using a variable capacitance element with a non-linear voltage-capacitance characteristic. [Means for solving the problem]

[0006] A piezoelectric oscillator according to one aspect of the present invention comprises a piezoelectric vibrator having a first terminal and a second terminal, and a cathode connected to the first terminal of the piezoelectric vibrator. The anode is grounded. Ta It is a variable capacitance diode. A first variable capacitance element and a cathode connected to the second terminal of the piezoelectric vibrator The anode is grounded. Ta It is a variable capacitance diode. The device comprises a second variable capacitance element, a DC voltage source, a first resistor with one end connected to the connection point between the cathode of the first variable capacitance element and the first terminal of the piezoelectric vibrator, and the other end connected to the DC voltage source, a transistor with its gate connected to the positive terminal of the DC voltage source, its drain connected to a power source different from the DC voltage source, and its source connected to the second resistor, a second resistor with one end connected to the source of the transistor and the other end grounded, and a third resistor with one end connected to the connection point between the source of the transistor and the second resistor, and the other end connected to the connection point between the cathode of the second variable capacitance element and the second terminal of the piezoelectric vibrator.

[0007] In a piezoelectric oscillator according to one aspect of the present invention, the transistor is an n-channel type MOS-FET.

[0008] In a piezoelectric oscillator according to one aspect of the present invention, both the first variable capacitance element and the second variable capacitance element are variable capacitance diodes.

[0009] In a piezoelectric oscillator according to one aspect of the present invention, the first variable capacitance element and the second variable capacitance element have equivalent electrical characteristics.

[0010] In a piezoelectric oscillator according to one aspect of the present invention, the range of voltage applied by the DC voltage source is twice or more the operating threshold voltage of the transistor. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a piezoelectric oscillator that can obtain a desired voltage-frequency characteristic even when a piezoelectric oscillator is configured using a variable capacitance element whose voltage-capacitance characteristic is not linear. [Brief explanation of the drawing]

[0012] [Figure 1] This is a circuit diagram showing an example of the circuit configuration of a piezoelectric oscillator according to the embodiment. [Figure 2] This graph shows an example of the voltage-capacitance characteristics of a variable capacitance element in a piezoelectric oscillator according to the embodiment. [Figure 3] This graph shows an example of the voltage-frequency characteristics at both ends of the piezoelectric resonator of the piezoelectric oscillator according to the embodiment. [Figure 4] This graph shows an example of the voltage-frequency characteristics of a piezoelectric oscillator according to the embodiment. [Figure 5] This is a circuit diagram showing an example of a circuit configuration for a piezoelectric oscillator according to conventional technology. [Figure 6] This graph shows an example of the voltage-capacitance characteristics of a variable capacitance element in a conventional piezoelectric oscillator. [Figure 7] This graph shows an example of the voltage-frequency characteristics of a piezoelectric oscillator based on conventional technology. [Modes for carrying out the invention]

[0013] [Conventional technology] First, a conventional piezoelectric oscillator 9 will be described with reference to Figures 5 to 7. The conventional piezoelectric oscillator 9 outputs a high-frequency signal corresponding to the applied voltage. As an example, the piezoelectric oscillator 9 may be a VCXO (Voltage Controlled Crystal Oscillator) that performs voltage control using a variable capacitance element (variable capacitance diode or varicap).

[0014] Figure 5 is a circuit diagram showing an example of the circuit configuration of a conventional piezoelectric oscillator. The circuit configuration of piezoelectric oscillator 9 will be explained with reference to this figure. The piezoelectric oscillator 9 includes a piezoelectric vibrator 91, a DC power supply 92, a resistor 931, a resistor 932, an inverter 94, a resistor 95, a capacitor 96, a capacitor 97, a variable capacitance element 98, and a variable capacitance element 99.

[0015] The piezoelectric vibrator 91 oscillates when a predetermined voltage is applied. The piezoelectric vibrator 91 is, for example, a crystal oscillator. The DC power supply 92 is a DC voltage source that outputs DC power. The DC power supply 92 applies a predetermined voltage across both ends of the piezoelectric vibrator 91. The DC power supply 92 applies a voltage, for example, from 0 [V (volt)] to 3.3 [V]. The resistor 931 is connected between the positive terminal of the DC power supply 92 and one end of the piezoelectric vibrator 91, and the resistor 932 is connected between the positive terminal of the DC power supply 92 and the other end of the piezoelectric vibrator 91. The inverter 94 is connected to one end of the piezoelectric vibrator 91 via the capacitor 96 and to the other end of the piezoelectric vibrator 91 via the capacitor 97. The resistor 95 is a feedback resistor connected between the input terminal and the output terminal of the inverter 94. The cathode K of the variable capacitance element 98 is connected to the connection point of the piezoelectric vibrator 91, the resistor 932, and the capacitor 96. The cathode K of the variable capacitance element 99 is connected to the connection point of the piezoelectric vibrator 91, the resistor 931, and the capacitor 97. The anodes A of the variable capacitance element 98 and the variable capacitance element 99 are grounded.

[0016] FIG. 6 is a graph showing an example of the voltage-capacitance characteristics of the variable capacitance element included in the piezoelectric oscillator according to the prior art. An example of the voltage-capacitance characteristics of the variable capacitance element included in the piezoelectric oscillator 9 will be described while referring to this figure. In this figure, with the horizontal axis representing the voltage applied to the variable capacitance element and the vertical axis representing the capacitance of the variable capacitance element, an example of the voltage-capacitance characteristics of the variable capacitance element 98 or the variable capacitance element 99 included in the piezoelectric oscillator 9 is shown. In the following description, when the variable capacitance element 98 or the variable capacitance element 99 is not distinguished, it may be simply referred to as the variable capacitance element.

[0017] FIG. 6 shows the voltage-capacitance characteristics as curves C61 and C62, respectively, when the piezoelectric oscillator 9 uses two different variable capacitance elements. The variable capacitance element having the characteristics shown by curve C61 and the variable capacitance element having the characteristics shown by curve C62 have different impurity concentration distributions at the pn junction surface. Specifically, in the voltage characteristic C ∝ V^-n of the junction capacitance, curve C61 shows an example of the characteristics of a variable capacitance element using a step junction where n = 1 / 2, and curve C62 shows an example of the characteristics of a variable capacitance element using a super-step junction where n = 2.

[0018] As shown by curve C61, the variable capacitance element using a step junction has a rapid change in capacitance in the region where the voltage is low, and the change in capacitance becomes small in the region where the voltage is high (in the following description, the fact that the change amount becomes small is also described as "reaching a plateau"). That is, the variable capacitance element using a step junction has a different change in capacitance with respect to the change in voltage according to the voltage. In other words, the variable capacitance element using a step junction does not have a linear voltage-capacitance characteristic.

[0019] On the other hand, as shown by curve C62, the variable capacitance element using a super-step junction has a substantially constant change amount of capacitance both in the region where the voltage is low and in the region where the voltage is high. Here, the range where the change amount of capacitance is substantially constant is a range where it can be considered that the change amount of capacitance with respect to the change in voltage does not change according to the voltage and is constant. That is, the variable capacitance element using a super-step junction does not depend on the voltage for the change amount of capacitance with respect to the change amount of voltage. In other words, the variable capacitance element using a step junction has a linear voltage-capacitance characteristic.

[0020] Note that the horizontal axis shown in FIG. 6 may be, for example, from 0 [V] to about 3.3 [V]. Also, the vertical axis shown in FIG. 6 may be, for example, from 0 [pF (picofarad)] to 10 [pF] or the like.

[0021] Figure 7 is a graph showing an example of the voltage-frequency characteristics of a conventional piezoelectric oscillator. Referring to this figure, an example of the voltage-frequency characteristics of a conventional piezoelectric oscillator 9 will be explained. In this figure, the horizontal axis represents the voltage applied to the variable capacitance element, and the vertical axis represents the oscillation frequency of the piezoelectric resonator 91, showing an example of the voltage-frequency characteristics of the piezoelectric oscillator 9. The oscillation frequency is given by f = 1 / 2π√LC ∝ 1 / √V^-n. The figure shows the voltage-capacitance characteristics of a variable capacitance element using a step junction with n=1 / 2 as curve C71, and the voltage-capacitance characteristics of a variable capacitance element using a superstep junction with n=2 as curve C62.

[0022] As shown in curve C71, a variable capacitance element using a step junction exhibits a rapid frequency change in the low-voltage region and a smaller frequency change in the high-voltage region. In other words, the amount of frequency change in response to voltage changes varies depending on the voltage. To put it another way, a variable capacitance element using a step junction has a non-linear voltage-frequency response.

[0023] On the other hand, as shown in curve C72, the variable capacitance element using a super-stepped junction exhibits a nearly constant change in frequency, regardless of whether the voltage is low or high. Here, the range in which the change in frequency is nearly constant is the range in which the change in frequency with respect to the change in voltage can be considered constant and unaffected by the voltage. In other words, the change in frequency with respect to the change in voltage of a variable capacitance element using a super-stepped junction is independent of the voltage. To put it another way, the voltage-frequency characteristics of a variable capacitance element using a stepped junction are linear.

[0024] [Embodiment] In this embodiment, the objective is to provide a piezoelectric oscillator 1 that can obtain desired voltage-frequency characteristics even when a piezoelectric oscillator is constructed using a variable capacitance element with a stepped junction having characteristics as shown by curve C61 or curve C71. In other words, the objective is to provide a piezoelectric oscillator that can obtain desired voltage-frequency characteristics even when a variable capacitance element with poor electrical characteristics is used.

[0025] Embodiments of the present invention will be described with reference to Figures 1 to 4. Figure 1 is a circuit diagram showing an example of the circuit configuration of a piezoelectric oscillator according to an embodiment. An example of the circuit configuration of piezoelectric oscillator 1 will be described with reference to this figure. Piezoelectric oscillator 1 differs from the conventional piezoelectric oscillator 9 in that different voltages are applied to the first variable capacitance element 18 and the second variable capacitance element 19, respectively.

[0026] Specifically, the piezoelectric oscillator 1 may be a VCXO that uses a variable capacitance element for voltage control. In the following description, the piezoelectric oscillator 1 will be described as a VCXO, but the piezoelectric oscillator 1 is not limited to this example. For example, the piezoelectric oscillator 1 may be an oscillator such as a VXO (Variable Crystal Oscillator).

[0027] The piezoelectric oscillator 1 comprises a piezoelectric vibrator 11, a DC power supply 12, a first resistor 13, a transistor 21, a second resistor 22, a third resistor 23, an inverter 14, a resistor 15, a capacitor 16, a capacitor 17, a first variable capacitance element 18, and a second variable capacitance element 19.

[0028] The piezoelectric oscillator 1 oscillates when a voltage is applied. The piezoelectric oscillator 1 is, for example, a quartz crystal oscillator. The piezoelectric crystal oscillator 11 has a first terminal 111 and a second terminal 112. The first terminal 111 is also referred to as XT and the second terminal 112 as XTN.

[0029] The DC power supply 12 is a DC voltage source that outputs DC power. The DC power supply 12 may also be a constant voltage source capable of outputting a constant voltage regardless of the connected load. The DC power supply 12 has a positive terminal 121 and a negative terminal 122, and the negative terminal 122 is grounded.

[0030] The first resistor 13 has one end connected to the connection point (hereinafter referred to as connection point P1) between the cathode K of the first variable capacitance element 18, the first terminal 111 of the piezoelectric vibrator 11, and the capacitor 16, and the other end connected to the positive terminal 121 of the DC power supply 12.

[0031] The first variable capacitance element 18 may be a variable capacitance diode (varicap). The first variable capacitance element 18 comprises an anode A and a cathode K. The cathode K of the first variable capacitance element 18 is connected to the first terminal 111 of the piezoelectric vibrator 11 at connection point P1. The anode A of the first variable capacitance element 18 is grounded.

[0032] The second variable capacitance element 19 may be a variable capacitance diode (varicap). The second variable capacitance element 19 comprises an anode A and a cathode K. The cathode K of the second variable capacitance element 19 is connected to the second terminal 112 of the piezoelectric vibrator 11 at connection point P3. The anode A of the second variable capacitance element 19 is grounded.

[0033] Furthermore, the first variable capacitance element 18 and the second variable capacitance element 19 may have similar structures and therefore possess equivalent electrical characteristics. These electrical characteristics may include, for example, voltage-capacitance characteristics. The first variable capacitance element 18 and the second variable capacitance element 19 may have different structures and therefore have different electrical characteristics.

[0034] Transistor 21 controls the voltage applied to the second variable capacitance element 19 according to the output voltage of the DC power supply 12. Transistor 21 may be, for example, an n-channel type MOS-FET (Metal-Oxide-Semiconductor Field-Effect Transistor). Transistor 21 has its gate G connected to the connection point between the positive terminal of the DC power supply 12 and the first resistor 13, its drain D connected to the power supply 24, and its source S connected to the connection point between the second resistor 22 and the third resistor 23 (hereinafter referred to as connection point P2). Power supply 24 may be a different power supply from the DC power supply 12.

[0035] The second resistor 22 has one end connected to the source S of transistor 21 and the other end connected to ground. The third resistor 23 has one end connected to the connection point P2 between the source S of the transistor 21 and the second resistor 22, and the other end connected to the connection point (hereinafter referred to as connection point P3) between the cathode K of the second variable capacitance element 19 and the second terminal 112 of the piezoelectric vibrator 11.

[0036] The inverter 14 includes an input terminal 141 and an output terminal 142. The input terminal 141 is connected to the cathode K of the first variable capacitance element 18 via a capacitor 16. The output terminal 142 is connected to the cathode K of the second variable capacitance element 19 via a capacitor 17. The resistor 15 is a feedback resistor connected in parallel with the inverter 14 (i.e., one end to the input terminal 141 and the other end to the output terminal 142).

[0037] Next, the electrical characteristics of the piezoelectric oscillator 1 will be explained with reference to Figures 2 to 4. Note that the graphs shown in Figures 2 to 4 represent the results obtained from circuit simulations.

[0038] Figure 2 is a graph showing an example of the voltage-capacitance characteristics of the variable capacitance elements in the piezoelectric oscillator according to this embodiment. Referring to this figure, an example of the voltage-capacitance characteristics of the first variable capacitance element 18 and the second variable capacitance element 19 in the piezoelectric oscillator 1 will be described. In the following description, when the first variable capacitance element 18 and the second variable capacitance element 19 are not distinguished, they may simply be referred to as variable capacitance elements.

[0039] Note that while the horizontal axis in Figure 2 is shown in the range of 0 to 1, it could also be, for example, from 0[V] to approximately 3.3[V]. Similarly, while the vertical axis in Figure 2 is shown in the range of 0 to 2, it could also be, for example, from 0[pF] to 10[pF].

[0040] Curve C21 shows the voltage-capacitance characteristics for the first variable capacitance element 18. Curve C22 shows the voltage-capacitance characteristics for the second variable capacitance element 19. Curve C23 shows the voltage-capacitance characteristics for the combined capacitance of the first variable capacitance element 18 and the second variable capacitance element 19.

[0041] The characteristics shown in the figure illustrate an example where the operating threshold voltage VTN (i.e., the gate threshold voltage VGS(TH)) of transistor 21 is 0.5. Furthermore, both the first variable capacitance element 18 and the second variable capacitance element 19 are variable capacitance elements using a step junction.

[0042] The output voltage of the DC power supply 12 is applied to the first variable capacitance element 18 via the first resistor 13. Therefore, the voltage-capacitance characteristics of the first variable capacitance element 18 shown in curve C21 have the characteristics of a variable capacitance element using a step junction. That is, the voltage-capacitance characteristics of the first variable capacitance element 18 show a rapid change in capacitance in the low voltage region and a small change in capacitance in the high voltage region. In other words, curve C21 is not linear.

[0043] The voltage of the power supply 24 is applied to the second variable capacitance element 19 via the transistor 21 and the third resistor 23. The transistor 21 is off in the range where the output voltage of the DC power supply 12 is less than or equal to the operating threshold voltage VTN of the transistor 21 (e.g., 0 to 0.5). The cathode K of the second variable capacitance element 19 is grounded via the second resistor 22 and the third resistor 23. Therefore, the capacitance of the second variable capacitance element 19 is at its maximum value (e.g., 2.0) in the range where the output voltage of the DC power supply 12 is less than or equal to the operating threshold voltage VTN of the transistor 21.

[0044] When the output voltage of the DC power supply 12 exceeds the operating threshold voltage VTN of the transistor 21, the transistor 21 turns on, and the voltage of the power supply 24 is applied to the cathode K of the second variable capacitance element 19 via the third resistor 23. Therefore, within the range where the output voltage of the DC power supply 12 exceeds the operating threshold voltage VTN of the transistor 21, the capacitance of the second variable capacitance element 19 changes according to the output voltage of the DC power supply 12.

[0045] As shown in curve C23, the combined capacitance of the first variable capacitance element 18 and the second variable capacitance element 19 is a combination of the capacitance of the first variable capacitance element 18, which changes rapidly in the range where the output voltage of the DC power supply 12 is below the operating threshold voltage VTN of the transistor 21 (i.e., in the low voltage region), and the capacitance of the second variable capacitance element 19, which has a large and constant capacitance.

[0046] Furthermore, the capacitance of the first variable capacitance element 18, whose capacitance change plateaus in the range where the output voltage of the DC power supply 12 exceeds the operating threshold voltage VTN of the transistor 21 (i.e., the high voltage region), and the capacitance of the second variable capacitance element 19, whose capacitance changes rapidly, are combined.

[0047] Therefore, the combined capacitance of the first variable capacitance element 18 and the second variable capacitance element 19 changes even when the output voltage of the DC power supply 12 is below the operating threshold voltage VTN of the transistor 21, and does not plateau even when the output voltage of the DC power supply 12 exceeds the operating threshold voltage VTN of the transistor 21. Thus, a voltage-capacitance characteristic close to linear can be obtained for the combined capacitance of the first variable capacitance element 18 and the second variable capacitance element 19.

[0048] Furthermore, the voltage range applied by the DC power supply 12 is greater than the operating threshold voltage VTN of the transistor 21. More preferably, the voltage range applied by the DC power supply 12 may be twice or more the operating threshold voltage VTN of the transistor 21.

[0049] Figure 3 is a graph showing an example of the voltage-frequency characteristics at both ends of the piezoelectric resonator of the piezoelectric oscillator according to the embodiment. Referring to this figure, an example of the voltage-frequency characteristics at both ends of the piezoelectric resonator 11 of the piezoelectric oscillator 1 will be explained. Curve C31 shows the frequency characteristics of the voltage VXT at the first terminal 111. Curve C32 shows the frequency characteristics of the voltage VXTN at the second terminal 112.

[0050] Note that while the horizontal axis in Figure 3 is shown in the range of 0 to 1, it could also be, for example, from 0[V] to approximately 3.3[V]. Similarly, while the vertical axis in Figure 3 is shown in the range of 0 to 1, it could also be, for example, from 0[MHz (megahertz)] to 8[MHz].

[0051] As shown in curve C31, as the voltage VXT at the first terminal 111 increases, the frequency also increases proportionally. On the other hand, as shown in curve C32, the voltage VXTN at the second terminal 112 is 0 in the voltage range from 0 to 0.3, and the frequency increases proportionally from the point where the voltage exceeds 0.3. In other words, the voltage VXTN rises with a lag behind the voltage VXT.

[0052] In other words, since the output voltage of the DC power supply 12 is directly applied to the first terminal 111 via the first resistor 13, the voltage VXT increases accordingly as the output voltage of the DC power supply 12 increases. On the other hand, no voltage is applied to the second terminal 112 until the transistor 21 is turned on. Therefore, the voltage VXTN rises later than the voltage VXT by the amount of the operating threshold voltage VTN of the transistor 21.

[0053] Figure 4 is a graph showing an example of the voltage-frequency characteristics of a piezoelectric oscillator according to the embodiment. Referring to this figure, an example of the voltage-frequency characteristics of the piezoelectric oscillator 1 will be explained. Curve C4 shows the voltage-frequency characteristics of the piezoelectric oscillator 1.

[0054] Note that while the horizontal axis in Figure 4 is shown in the range of 0 to 1, it could also be, for example, from 0[V] to approximately 3.3[V]. Similarly, while the vertical axis in Figure 4 is shown in the range of 1 to 2, it could also be, for example, from 8[MHz] to 16[MHz].

[0055] As shown in curve C4, the frequency changes sufficiently in the range where the output voltage of the DC power supply 12 is below the operating threshold voltage VTN of the transistor 21 (i.e., the low voltage region), and the frequency also changes sufficiently in the range where it exceeds the operating threshold voltage VTN (i.e., the high voltage region). In other words, the voltage-frequency characteristics of the piezoelectric oscillator 1 show that the frequency changes even in the low voltage region, and the amount of frequency change does not plateau even in the high voltage region.

[0056] [Summary of the effects of the embodiment] As described above, the piezoelectric oscillator 1 according to this embodiment comprises a piezoelectric resonator 11, a first variable capacitance element 18, a second variable capacitance element 19, a DC power supply (DC voltage source) 12, a first resistor 13, a transistor 21, a second resistor 22, and a third resistor 23. The piezoelectric oscillator 1, by including the transistor 21, makes the voltage applied to the first variable capacitance element 18 and the voltage applied to the second variable capacitance element 19 different. Specifically, the piezoelectric oscillator 1, by including the transistor 21, applies a voltage to the second variable capacitance element 19 later than to the first variable capacitance element 18. That is, the piezoelectric oscillator 1 ensures a change in capacitance by applying a voltage to the first variable capacitance element 18 in the low voltage region, and ensures a change in capacitance by starting to apply a voltage to the second variable capacitance element 19 in the high voltage region. Therefore, even when the voltage-capacitance characteristics of the first variable capacitance element 18 plateau in the high voltage region, the piezoelectric oscillator 1 can vary the combined capacitance of the first variable capacitance element 18 and the second variable capacitance element 19 by applying a voltage to the second variable capacitance element 19 via the transistor 21.

[0057] In other words, the piezoelectric oscillator 1 can obtain a linear voltage-frequency characteristic even when using the first variable capacitance element 18 and the second variable capacitance element 19, which have a stepped junction where the voltage-capacitance is not linear. Therefore, according to this embodiment, even when a piezoelectric oscillator is configured using a variable capacitance element with a stepped junction whose voltage-capacitance characteristics are not linear, the desired voltage-frequency characteristics can be obtained.

[0058] Furthermore, according to the embodiment described above, the transistor 21 is an n-channel type MOS-FET. Therefore, the piezoelectric oscillator 1 does not have a large power loss like when a bipolar transistor is used, and the time delay is small, so it can suitably obtain the desired voltage-frequency characteristics.

[0059] Furthermore, according to the embodiment described above, both the first variable capacitance element 18 and the second variable capacitance element 19 are variable capacitance diodes (varicaps). Therefore, according to this embodiment, the desired voltage-frequency characteristics can be obtained using a variable capacitance diode with a stepped junction whose voltage-capacitance characteristics are not linear, without having to spend money developing a super-stepped junction variable capacitance diode.

[0060] Furthermore, according to the above-described embodiment, the first variable capacitance element 18 and the second variable capacitance element 19 have similar structures and therefore possess equivalent electrical characteristics. Accordingly, according to this embodiment, the layout of the structure can be easily carried out.

[0061] Furthermore, according to the embodiment described above, the voltage range applied by the DC power supply 12 is at least twice the operating threshold voltage VTN of the transistor 21. In other words, the DC power supply 12 can be varied to a voltage at least twice the operating threshold voltage VTN of the transistor 21. Therefore, according to this embodiment, the capacitance of the second variable capacitance element 19 is combined at a voltage before the change in capacitance of the first variable capacitance element 18 plateaus. Consequently, the piezoelectric oscillator 1 can obtain a linear voltage-capacitance characteristic.

[0062] In the embodiment described above, a configuration was described in which voltage VXTN rises later than voltage VXT. However, a configuration in which voltage VXT rises later than voltage VXTN is also possible. In this case, the configuration may be changed by swapping the transistor 21, the second resistor 22, and the third resistor 23 with that of the first resistor 13. By replacing the configuration of transistor 21, second resistor 22, and third resistor 23 with that of the first resistor 13, the capacitance of the second variable capacitance element 19 changes when the output voltage of the DC power supply 12 is below the operating threshold voltage VTN of transistor 21, and when the output voltage of the DC power supply 12 exceeds the operating threshold voltage VTN of transistor 21, the capacitance of the variable first variable capacitance element 18 is combined, resulting in a linear voltage-capacitance characteristic and, consequently, a voltage-frequency characteristic.

[0063] Although embodiments for carrying out the present invention have been described above using examples, the present invention is not limited in any way to these embodiments, and various modifications and substitutions can be made without departing from the spirit of the present invention. [Explanation of Symbols]

[0064] 1...Piezoelectric oscillator, 11...Piezoelectric vibrator, 111...First terminal, 112...Second terminal, 12...DC power supply, 13...First resistor, 14...Inverter, 141...Input terminal, 142...Output terminal, 15...Resistor, 16...Capacitor, 17...Capacitor, 18...First variable capacitance element, 19...Second variable capacitance element, 21...Transistor, 22...Second resistor, 23...Third resistor, 24...Power supply, P1, P2, P3...Connection points

Claims

1. A piezoelectric vibrator having a first terminal and a second terminal, A first variable capacitance element is a variable capacitance diode whose cathode is connected to the first terminal of the piezoelectric vibrator and whose anode is grounded, A second variable capacitance element is a variable capacitance diode whose cathode is connected to the second terminal of the piezoelectric vibrator and whose anode is grounded, DC voltage source and A first resistor, one end of which is connected to the connection point between the cathode of the first variable capacitance element and the first terminal of the piezoelectric vibrator, and the other end of which is connected to the DC voltage source, A transistor whose gate is connected to the positive terminal of the DC voltage source, whose drain is connected to a power source different from the DC voltage source, and whose source is connected to a second resistor, A second resistor, one end of which is connected to the source of the transistor and the other end of which is grounded, A third resistor, one end of which is connected to the connection point between the source of the transistor and the second resistor, and the other end of which is connected to the connection point between the cathode of the second variable capacitance element and the second terminal of the piezoelectric vibrator. A piezoelectric oscillator equipped with the following features.

2. The transistor is an n-channel type MOS-FET. The piezoelectric oscillator according to claim 1.

3. Both the first variable capacitance element and the second variable capacitance element are variable capacitance diodes. A piezoelectric oscillator according to claim 1 or claim 2.

4. The first variable capacitance element and the second variable capacitance element have equivalent electrical characteristics. A piezoelectric oscillator according to any one of claims 1 to 3.

5. The voltage range applied by the DC voltage source is at least twice the operating threshold voltage of the transistor. A piezoelectric oscillator according to any one of claims 1 to 4.

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