JP2014064223A - Crystal oscillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve stable oscillation of a crystal oscillator with high frequency accuracy.SOLUTION: The crystal oscillator includes: a first crystal oscillator unit 10; a transistor 20 that amplifies an output signal of the first crystal oscillator unit 10; a second crystal oscillator unit 30 provided on a feedback path from the transistor 20 to the first crystal oscillator unit 10; a phase comparator 40 that outputs a phase comparison signal corresponding to a phase difference between a signal output by the transistor 20 and a reference signal; and a variable capacitance element 50 that is connected between the transistor 20 and the second crystal oscillator unit 30 and varies electrostatic capacitance according to a voltage of a phase comparison signal output by the phase comparator 40.

Description

  The present invention relates to a crystal oscillator.

  In a crystal oscillator that oscillates a crystal resonator using a transistor, by providing a filter crystal resonator in a feedback path between the emitter and base of the transistor, a frequency bandwidth having a sufficiently large negative resistance can be obtained. A method of limiting and preventing erroneous oscillation at a frequency other than a desired frequency is known (see, for example, Patent Document 1).

  FIG. 6 shows a configuration example of a conventional crystal oscillator 200. The crystal oscillator 200 includes a crystal resonator 210, a transistor 220, a crystal resonator 230, a capacitor 240, a resistor 250, a resistor 251, a capacitor 252, a capacitor 253, a resistor 254, and a resistor 255. FIG. 7 shows an example of the frequency characteristic of the negative resistance of the crystal oscillator 200.

  As shown in FIG. 6, in the crystal oscillator 200, a crystal resonator 230 is provided in a feedback path between the emitter and base of a transistor 220 that functions as an oscillation amplifier. Therefore, the impedance between the emitter and base of the transistor 220 becomes a large negative resistance only in the region near the series resonance frequency of the crystal unit 230.

  As a result, as shown in FIG. 7, the frequency characteristic of the negative resistance of the crystal oscillator 200 shows a large negative resistance in a narrow band near the series resonance frequency of the crystal resonator 230. The crystal oscillator 200 oscillates stably in a narrow frequency band with a large negative resistance, and can prevent spurious oscillation. For example, when the crystal unit 210 is an SC cut crystal unit, the provision of the crystal unit 230 can prevent erroneous oscillation in the B mode, which is an unnecessary mode.

JP-A-9-153740

  However, when the frequency band in which the negative resistance of the crystal oscillator 200 shows a large negative value is narrow as shown in FIG. 7, the difference in temperature characteristics between the crystal oscillator 210 and the crystal oscillator 230, or the crystal oscillator The frequency band in which the negative resistance shows a negative value due to the difference between the temperature at the position where 210 is mounted and the temperature at the position where crystal unit 230 is mounted deviates from the desired oscillation frequency of crystal unit 210 was there. As a result, even when the oscillation frequencies of the crystal unit 210 and the crystal unit 230 are substantially equal at a predetermined temperature, the oscillation frequency of the crystal unit 210 and the oscillation frequency of the crystal unit 230 are accompanied by a change in the external temperature. In this case, a deviation of about several hundreds of ppb occurs, and there is a problem that oscillation does not occur at a frequency to be oscillated.

  For example, when the oscillation frequency of the crystal oscillator 200 is 10 MHz and the frequency bandwidth in which the negative resistance indicates a negative value is 5 Hz, the oscillation frequency of the crystal resonator 210 and the oscillation frequency of the crystal resonator 230 are When a deviation of 500 ppb occurs between them, there is a problem that the crystal oscillator 200 does not oscillate at 10 MHz. As the frequency bandwidth in which the negative resistance shows a negative value becomes narrower, the influence of the difference in temperature characteristics between the crystal unit 210 and the crystal unit 230, the temperature of the crystal unit 210 and the temperature of the crystal unit 230, Therefore, it is difficult to stably oscillate with high frequency accuracy with the conventional crystal oscillator.

  Therefore, the present invention has been made in view of these points, and an object thereof is to suppress the influence of an external temperature in a crystal oscillator and stably oscillate with high frequency accuracy.

  In order to solve the above problems, in the first aspect of the present invention, a first crystal resonator, a transistor that amplifies an output signal of the first crystal resonator, and feedback from the transistor to the first crystal resonator. A second crystal unit provided on the road, a phase comparator that outputs a phase comparison signal corresponding to a phase difference between a signal output from the transistor and a reference signal, and the transistor and the second crystal unit And a variable capacitance element whose capacitance changes in accordance with the voltage of the phase comparison signal output from the phase comparator. Since the crystal oscillator has such a configuration, even when the oscillation frequency deviates from the frequency of the reference signal as the temperature changes, the oscillation frequency can be finely adjusted to approach the desired frequency.

  The crystal oscillator may further include a low-pass filter that allows a frequency component lower than a predetermined frequency included in the phase comparison signal to pass. The capacitance of the variable capacitance element changes according to the voltage of the signal output from the low-pass filter. Since the low-pass filter blocks the high-frequency component of the phase comparison signal, the capacitance of the variable capacitance element changes gradually, so that generation of phase noise can be prevented.

  The crystal oscillator may further include an inductor connected between the variable capacitance element and the ground. Since the variable capacitance element is connected to the ground via the inductor, the voltage of the phase comparison signal can be applied to both ends of the variable capacitance element.

  The crystal oscillator further includes a thermostatic chamber that accommodates the first crystal resonator and the second crystal resonator, and the first crystal resonator and the second crystal resonator are provided at the same temperature in the thermostat. It may be. Since the crystal oscillator has such a configuration, the temperature of the first crystal resonator and the second crystal resonator are stabilized at the same temperature, so that the accuracy of the oscillation frequency can be further improved.

  According to the second aspect of the present invention, the first crystal unit, the transistor that amplifies the output signal of the first crystal unit, and the second crystal provided on the feedback path from the transistor to the first crystal unit. The vibrator has a thermostat that houses the first crystal vibrator and the second crystal vibrator, and the first crystal vibrator and the second crystal vibrator are provided at the same temperature in the thermostat. A crystal oscillator is provided. Since the crystal oscillator has such a configuration, the temperature of the first crystal unit and the second crystal unit are stabilized at the same temperature, so that the oscillation is stably performed with high frequency accuracy regardless of a change in the external temperature. Can be made.

  According to the crystal oscillator according to the present invention, it is possible to suppress the influence of the external temperature in the crystal oscillator and stably oscillate with high frequency accuracy.

1 shows a circuit configuration example of a crystal oscillator according to a first embodiment. 2 shows a configuration example of a phase comparator according to the first embodiment. The relationship between the oscillation signal, the reference signal, and the phase comparison signal when the frequency of the reference signal is equal to the frequency of the oscillation signal is shown. The relationship between the oscillation signal, the reference signal, and the phase comparison signal when the frequency of the oscillation signal is higher than the frequency of the reference signal is shown. The relationship between the oscillation signal, the reference signal, and the phase comparison signal when the frequency of the oscillation signal is lower than the frequency of the reference signal is shown. 4 shows an example of mounting a crystal resonator and a crystal resonator in the crystal oscillator according to the present embodiment. The other example of mounting of the crystal resonator and the crystal resonator in the crystal oscillator according to the present embodiment is shown. The structural example of the conventional crystal oscillator is shown. An example of the frequency characteristic of the negative resistance of a crystal oscillator is shown.

<First Embodiment>
[Fine adjustment of frequency with variable capacitance element]
FIG. 1 shows a circuit configuration example of a crystal oscillator 100 according to the first embodiment. The crystal oscillator 100 includes a crystal resonator 10, a transistor 20, a crystal resonator 30, a phase comparator 40, a variable capacitance element 50, a low-pass filter 60, an inductor 70, a resistor 81, a resistor 82, a capacitor 83, a capacitor 84, and a resistor. 85 and a resistor 86.

  The crystal unit 10 is, for example, an SC cut crystal unit. The transistor 20 amplifies the output signal of the crystal resonator 10. One end of the crystal unit 10 is connected to the ground, and the other end is connected to the base of the transistor 20. The base of the transistor 20 is connected to a connection point between a resistor 81 connected to a power source and a resistor 82 connected to the ground, and a bias voltage is applied.

  The emitter of the transistor 20 is connected to the resistor 86, the inductor 70, and the anode terminal of the variable capacitor 50. The terminal on the opposite side to the terminal connected to the transistor 20 in the resistor 86 and the inductor 70 is connected to the ground. The collector of the transistor 20 is connected to the power supply via the resistor 85 and outputs an oscillation signal. The crystal oscillator 10, the transistor 20, the resistor 81, the resistor 82, the capacitor 83, the capacitor 84, the resistor 85, and the resistor 86 constitute a Colpitts oscillation circuit.

  The crystal unit 30 is, for example, an SC cut crystal unit. The crystal resonator 30 is provided on the feedback path from the transistor 20 to the first crystal resonator 10. Specifically, the first terminal of the crystal unit 30 is connected to the emitter of the transistor 20 via the low-pass filter 60 and the variable capacitance element 50. The second terminal of the crystal unit 30 is connected to a connection point between a capacitor 83 connected to the base of the transistor 20 and a capacitor 84 connected to the ground. That is, the second terminal of the crystal unit 30 is connected to the base of the transistor 20 via the capacitor 83.

  The phase comparator 40 outputs a phase comparison signal corresponding to the phase difference between the oscillation signal output from the transistor 20 and the reference signal. Specifically, the phase comparator 40 acquires the oscillation signal output from the collector of the transistor 20 and the reference signal input from the outside, and the timing at which the logic value of the oscillation signal changes and the logic value of the reference signal are A phase comparison signal having a duty ratio corresponding to the difference from the changing timing is output.

  The reference signal is a signal generated by, for example, a temperature compensated oscillation circuit (TCXO) and maintained at a predetermined frequency over a wide temperature range. The crystal oscillator 100 may acquire a reference signal from an external temperature compensated oscillation circuit, and the crystal oscillator 100 may include a temperature compensated oscillation circuit.

  The variable capacitance element 50 is connected between the emitter of the transistor 20 and the second crystal unit 30 and changes the capacitance according to the voltage of the phase comparison signal. The variable capacitance element 50 is, for example, a varicap that changes the capacitance according to the voltage applied between the terminals. The capacitance of the variable capacitance element 50 changes in inverse proportion to the square root of the voltage applied between the terminals. That is, as the voltage applied between the terminals of the variable capacitance element 50 increases, the capacitance of the variable capacitance element 50 decreases, and as the voltage applied between the terminals of the variable capacitance element 50 decreases, the capacitance increases. The capacitance of the capacitive element 50 increases. When the capacitance of the variable capacitance element 50 increases, the frequency of the oscillation signal output from the transistor 20 decreases, and when the capacitance of the variable capacitance element 50 decreases, the frequency of the oscillation signal increases.

  The anode terminal of the variable capacitance element 50 is connected to the emitter of the transistor 20 and to the ground via the inductor 70. The cathode terminal of the variable capacitance element 50 is connected to the output terminal of the phase comparator 40 via the low-pass filter 60.

  The low-pass filter 60 has a resistor 61 and a capacitor 62. The resistor 61 has a first terminal connected to the output terminal of the phase comparator 40 and a second terminal connected to the first terminal of the capacitor 62. A second terminal of the capacitor 62 is connected to the ground. The low-pass filter 60 passes a frequency component lower than a predetermined frequency (hereinafter, cut-off frequency) included in the phase comparison signal and inputs the frequency component to the cathode terminal of the variable capacitance element 50. The cut-off frequency of the low-pass filter 60 is determined based on the size of the resistor 61 and the capacitor 62, and is selected according to the phase noise allowed for the oscillation signal.

  Since the inductor 70 has an impedance of approximately 0Ω at a frequency passing through the low-pass filter 60, the inductor 70 supplies a bias voltage equal to the ground voltage to the variable capacitance element 50. That is, the voltage of the anode terminal of the variable capacitance element 50 at the frequency of the phase comparison signal that has passed through the low-pass filter 60 becomes equal to the ground voltage. As a result, since the voltage obtained by smoothing the voltage of the phase comparison signal by the low-pass filter 60 is applied between the terminals of the variable capacitance element 50, the electrostatic capacitance of the variable capacitance element 50 is changed according to the voltage of the phase comparison signal. The capacitance changes, and the frequency of the oscillation signal output from the transistor 20 also changes.

  Specifically, when the duty ratio of the phase comparison signal is 50% and the voltage obtained by smoothing the voltage of the phase comparison signal is an intermediate voltage between the power supply voltage and the ground, the transistor 20 outputs. The frequency of the oscillation signal is maintained unchanged. On the other hand, when the frequency of the oscillation signal changes with the temperature change and the change timing of the oscillation signal with respect to the change timing of the reference signal changes, the duty ratio of the phase comparison signal changes from 50%, and the voltage of the phase comparison signal changes. The smoothed voltage changes from the intermediate voltage. As a result, the capacitance of the variable capacitance element 50 changes and the frequency of the oscillation signal output from the transistor 20 changes.

  For example, when the change timing of the oscillation signal with respect to the change timing of the reference signal becomes earlier and the duty ratio of the phase comparison signal becomes smaller than 50%, the voltage obtained by smoothing the voltage of the phase comparison signal becomes smaller than the intermediate voltage. The capacitance of 50 increases, and the frequency of the oscillation signal output from the transistor 20 decreases. Conversely, if the change timing of the oscillation signal with respect to the change timing of the reference signal is delayed and the duty ratio of the phase comparison signal becomes larger than 50%, the voltage obtained by smoothing the voltage of the phase comparison signal becomes larger than the intermediate voltage. The capacitance of the element 50 is reduced, and the frequency of the oscillation signal output from the transistor 20 is increased.

[Configuration Example of Phase Comparator 40]
FIG. 2 shows a configuration example of the phase comparator 40. 3A, 3B, and 3C show the relationship between the oscillation signal, the reference signal, and the phase comparison signal. The phase comparator 40 shown in FIG. 2 includes a JK flip-flop 41, a JK flip-flop 42, and a NAND element 43. The J terminals of the JK flip-flop 41 and the JK flip-flop 42 are connected to the power source, and the K terminal is connected to the ground. A reference signal is input to the clock terminal of the JK flip-flop 41, and an oscillation signal output from the transistor 20 is input to the clock terminal of the JK flip-flop 42. The output signal of the JK flip-flop 41 and the output signal of the JK flip-flop 42 are input to the NAND element 43, and the output signal of the NAND element 43 is connected to the clear terminals of the JK flip-flop 41 and JK flip-flop 42.

  FIG. 3A shows the relationship between the oscillation signal, the reference signal, and the phase comparison signal when the frequency of the reference signal is equal to the frequency of the oscillation signal. The output signal of the JK flip-flop 41 rises at the falling timing of the reference signal, and the output signal of the JK flip-flop 41 falls at the falling timing of the oscillation signal. As a result, the JK flip-flop 41 outputs a phase comparison signal having a pulse of length T / 2 corresponding to the time difference between the falling timing of the reference signal and the falling timing of the oscillation signal. That is, the duty ratio of the phase comparison signal is 50%.

  FIG. 3B shows an oscillation signal in a state where the frequency of the oscillation signal is higher than the frequency of the reference signal, and the falling timing of the oscillation signal with respect to the falling timing of the reference signal is earlier than the state shown in FIG. The relationship between a signal and a phase comparison signal is shown. The pulse width t of the phase comparison signal is smaller than T / 2, and the duty ratio is smaller than 50%. In this case, since the voltage of the signal obtained by smoothing the phase comparison signal is lower than the intermediate voltage, the capacitance of the variable capacitance element 50 is increased and the frequency of the oscillation signal is adjusted to be lower.

  3C shows an oscillation signal in a state where the frequency of the oscillation signal is lower than the frequency of the reference signal, and the falling timing of the oscillation signal with respect to the falling timing of the reference signal is slower than the state shown in FIG. The relationship between a signal and a phase comparison signal is shown. The pulse width t of the phase comparison signal is larger than T / 2, and the duty ratio is larger than 50%. In this case, since the voltage of the signal obtained by smoothing the phase comparison signal is higher than the intermediate voltage, the capacitance of the variable capacitance element 50 is reduced and the frequency of the oscillation signal is adjusted to be higher.

  As described above, according to the crystal oscillator 100 according to the present embodiment, when the frequency of the oscillation signal output from the emitter of the transistor 20 changes due to a temperature change, the capacitance of the variable capacitance element 50 changes in a direction to cancel the change. By doing so, it is possible to cancel the change in the frequency of the oscillation signal accompanying the temperature change. Therefore, even if there is a difference between the temperature characteristics of the crystal resonator 10 and the crystal resonator 30, the crystal oscillator 100 can oscillate stably with high frequency accuracy without spurious oscillation. it can. In particular, according to the crystal oscillator 100 according to the present embodiment, since the frequency can be finely adjusted in an analog manner using the variable capacitance element 50, a stable frequency can be obtained without generating phase noise such as frequency jitter. The oscillation signal can be output.

[Accommodating crystal resonator 10 and crystal resonator 30 in a thermostatic chamber]
FIG. 4 shows a mounting example of the crystal resonator 10 and the crystal resonator 30 in the crystal oscillator 100. In FIG. 4, the crystal unit 10 and the crystal unit 30 are accommodated in a thermostatic chamber 90. A heater wire 91 is wound around the surface of the thermostat 90, and the temperature of the heater wire 91 is adjusted so that the temperature inside the thermostat 90 is within a predetermined range. For example, the crystal oscillator 100 includes a thermistor or a temperature sensor that detects the temperature inside or outside the thermostatic chamber 90, and adjusts the power supplied to the heater wire 91 according to the measured temperature.

  The crystal unit 10 and the crystal unit 30 are provided at positions where the temperatures in the thermostatic chamber 90 are substantially equal. For example, in the case where the thermostatic chamber 90 has a rectangular parallelepiped shape, the crystal unit 10 and the crystal unit are arranged at two positions that are axisymmetric with respect to a center line having the same distance from the opposite side of one side surface of the thermostat 90. 30 is provided.

  The terminals of the crystal unit 10 and the crystal unit 30 protrude outside the thermostatic chamber 90 and can be mounted on a printed circuit board. As an example, the transistor 20, the phase comparator 40, the variable capacitance element 50, the low-pass filter 60, the inductor 70, the resistor 81, the resistor 82, the capacitor 83, the capacitor 84, the resistor 85, and the resistor 86 in the crystal oscillator 100 are crystal oscillations. It is mounted on a printed circuit board to which the terminals of the child 10 and the crystal unit 30 are connected. At least part of the transistor 20, the phase comparator 40, the variable capacitance element 50, the low-pass filter 60, the inductor 70, the resistor 81, the resistor 82, the capacitor 83, the capacitor 84, the resistor 85, and the resistor 86 is the constant temperature bath 90. It may be provided inside.

  As described above, since the crystal resonator 10 and the crystal resonator 30 are provided at the same temperature in the thermostatic chamber 90, the difference in oscillation frequency due to the temperature difference between the crystal resonator 10 and the crystal resonator 30 does not occur. The crystal oscillator 100 can output an oscillation signal with higher frequency accuracy in a more stable manner.

  FIG. 5 shows another mounting example of the crystal resonator 10 and the crystal resonator 30 in the crystal oscillator 100. In the crystal oscillator 100 illustrated in FIG. 5, the positions of the crystal resonator 10 and the crystal resonator 30 are different from the positions in the crystal oscillator 100 illustrated in FIG. 4, and the shape of the crystal resonator 10 is the crystal illustrated in FIG. 4. It differs from the shape of the vibrator 10. The crystal resonator 10 shown in FIG. 5 has a piezoelectric element sealed in a package of, for example, HC-37 / U (MIL standard) or HC-40 / U (MIL standard). A piezoelectric element is enclosed in a 43 / U (MIL standard) package. The crystal resonator 10 is mounted on a substrate 11 having terminals 12a and terminals 12b, and is connected to other elements via the terminals 12a and terminals 12b.

  Other combinations of the positions and shapes of the crystal unit 10 and the crystal unit 30 may be used. For example, the crystal oscillator 100 includes an HC-43 / U (MIL standard) package in which a piezoelectric element is enclosed, and an HC-37 / U (MIL standard) or HC-40 / U (MIL standard). And a crystal resonator 30 in which a piezoelectric element is sealed, and a crystal in which a piezoelectric element is sealed in an HC-37 / U (MIL standard) or HC-40 / U (MIL standard) package. The vibrator 10 and the crystal vibrator 30 may be included.

  Thus, even when the crystal unit 10 and the crystal unit 30 have different shapes, by providing the crystal unit 10 and the crystal unit 30 at the same temperature in the thermostatic chamber 90, the crystal unit 10 Since there is no difference in oscillation frequency due to the temperature difference between 10 and the crystal unit 30, the crystal oscillator 100 can stably output an oscillation signal with high frequency accuracy.

<Second Embodiment>
In the above embodiment, as shown in FIG. 1, the crystal oscillator 100 includes the phase comparator 40 and the variable capacitance element 50, so that the frequency fluctuation of the oscillation signal output from the transistor 20 is suppressed, and the high frequency An accurate oscillation signal could be output stably. On the other hand, by accommodating the crystal resonator 10 and the crystal resonator 30 in the thermostatic chamber 90 shown in FIGS. 4 and 5, the crystal oscillator 100 is not provided with the phase comparator 40 and the variable capacitance element 50. It is also possible to stably output an oscillation signal with high frequency accuracy.

  For example, by providing the crystal resonator 210 and the crystal resonator 230 in the crystal oscillator 200 shown in FIG. 6 at the same temperature in the thermostatic chamber 90, the temperature of the crystal resonator 210 and the temperature of the crystal resonator 230 are equal. Become. Therefore, even if there is a difference between the temperature characteristics of the crystal resonator 210 and the temperature characteristics of the crystal resonator 230, the crystal oscillation at the temperature at the position where the crystal resonator 210 and the crystal resonator 230 are accommodated. If the oscillation frequency of the child 210 and the oscillation frequency of the crystal resonator 230 are equal, the crystal oscillator 200 can output an oscillation signal with high frequency accuracy regardless of a change in the external temperature.

<Other embodiments>
The phase comparator 40 shown in FIG. 2 generates a phase comparison signal corresponding to the change timing of the reference signal and the oscillation signal by the JK flip-flop 41 and the JK flip-flops 42 and 43. May generate a phase comparison signal. For example, the phase comparator 40 may include an exclusive OR circuit, and a signal output from the exclusive OR circuit to which the reference signal and the oscillation signal are input may be output as the phase comparison signal.

  In the above embodiment, the configuration example in which the present invention is applied to the Colpitts type oscillation circuit has been described. However, even if the present invention is applied to the Hartley type oscillation circuit, it will be understood by those skilled in the art that the same effect can be obtained. it is obvious.

  In the above embodiment, the case where the crystal resonator 10 and the crystal resonator 30 are SC cut crystal resonators has been described. However, the crystal resonator 10 and the crystal resonator 30 are AT cut crystal resonators. Has the same effect. Furthermore, it will be apparent to those skilled in the art that even if the crystal oscillator 100 has an oscillation circuit that oscillates at the third-order overtone frequency of an AT-cut crystal resonator, the same effect can be obtained.

  As mentioned above, although this invention was demonstrated using embodiment, the technical scope of this invention is not limited to the range as described in the said embodiment. It will be apparent to those skilled in the art that various modifications or improvements can be added to the above-described embodiment. It is apparent from the scope of the claims that the embodiments added with such changes or improvements can be included in the technical scope of the present invention.

DESCRIPTION OF SYMBOLS 10 ... Crystal oscillator, 20 ... Transistor, 30 ... Crystal oscillator, 31 ... Substrate, 32a ... Terminal, 32b ... Terminal, 40 ... Phase comparator, 41. ..JK flip-flop, 42... JK flip-flop, 43... NAND element, 50... Variable capacitance element, 60. 70: Inductor, 81: Resistance, 82: Resistance, 83 ... Capacitor, 84 ... Capacitor, 85 ... Resistance, 86 ... Resistance, 90 ... Constant temperature bath, 91 ... Heater wire, 100 ... Crystal oscillator, 200 ... Crystal oscillator, 210 ... Crystal oscillator, 220 ... Transistor, 230 ... Crystal oscillator, 240 ... Capacitor, 250 ..Resistance, 251 ... Anti, 252 ... capacitor, 253 ... capacitor, 254 ... resistance, 255 ... resistance

Claims (5)

A first crystal unit;
A transistor for amplifying an output signal of the first crystal unit;
A second crystal unit provided on a feedback path from the transistor to the first crystal unit;
A phase comparator that outputs a phase comparison signal according to a phase difference between a signal output from the transistor and a reference signal;
A crystal oscillator comprising: a variable capacitance element connected between the transistor and the second crystal resonator and having a capacitance that changes according to a voltage of the phase comparison signal.   A low-pass filter that allows a frequency component lower than a predetermined frequency included in the phase comparison signal to pass; and the variable capacitance element has a capacitance according to a voltage of a signal output from the low-pass filter The crystal oscillator according to claim 1, wherein   The crystal oscillator according to claim 1, further comprising an inductor connected between the variable capacitance element and a ground. Further comprising a thermostatic chamber for accommodating the first crystal resonator and the second crystal resonator;
4. The crystal oscillator according to claim 1, wherein the first crystal unit and the second crystal unit are provided at positions where the temperatures in the thermostatic chamber are equal. 5. A first crystal unit;
A transistor for amplifying an output signal of the first crystal unit;
A second crystal unit provided on a feedback path from the transistor to the first crystal unit;
A thermostat housing the first crystal unit and the second crystal unit,
The first crystal unit and the second crystal unit are crystal oscillators provided at the same temperature in the thermostatic chamber.

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