JP2018164125A - Crystal oscillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a crystal oscillator capable of improving floor noise and noise close to carrier frequencies at the same time, thereby having good phase noise characteristics.SOLUTION: A crystal oscillator includes a Colpitts oscillation circuit. In the crystal oscillator, one end of a crystal resonator X is connected to a base of a transistor Tr1 for oscillation, while the other end thereof is connected, through an output coupling capacitor C4, to a transistor Tr2 for amplification serving as a buffer amplifier section; one end of an output impedance element R10 is connected between the other end of the crystal resonator X and the output coupling capacitor C4, while the other end thereof is grounded; and an output signal is obtained through the transistor Tr2 for amplification. The impedance value of the output impedance element R10 is equal to or more than three times the equivalent series resistance of the crystal resonator X, and the impedance value of the output coupling capacitor C4 is less than the impedance value of the output impedance element R10.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a crystal oscillator provided with a Colpitts circuit, and more particularly to a crystal oscillator capable of simultaneously improving floor noise and near-carrier noise and having good phase noise characteristics.

[Description of Prior Art: FIG. 9]
A configuration of a general Colpitts oscillator as a conventional crystal oscillator will be described with reference to FIG. FIG. 9 is a circuit diagram showing a configuration example of a general Colpitts oscillator.
As shown in FIG. 9, the conventional crystal oscillator includes a crystal resonator X and an oscillation transistor Tr that amplifies an oscillation frequency signal of the crystal resonator X.
The collector of the transistor Tr is connected to the power supply terminal 1, and the emitter is grounded via the resistor R14.

One end of the capacitor C11 is connected to a point between the base of the transistor Tr and the crystal unit X, one end of the capacitor C12 is connected to the other end of the capacitor C11, and the other end is grounded.
The emitter of the transistor Tr is connected to the output terminal 2 and to a point between the capacitors C11 and C12.

One end of the crystal resonator X connected to the base of the transistor Tr is connected to a connection point between bias resistors R11 and R12 connected in series between the power supply terminal 1 and the ground.
The other end of the crystal unit X is grounded via a capacitor C14.

Further, one end of the capacitor C13 is connected to a point between the collector of the transistor Tr and the power supply terminal 1, and the other end is grounded.
That is, the conventional crystal oscillator has a configuration in which an output is extracted from the transistor Tr side.

[Floor noise and near-carrier noise: Fig. 10]
In general, in the conventional crystal oscillator, the C / N is improved by increasing the drive current of the crystal resonator, and the white noise (floor noise) is reduced.
However, when the drive current is increased, the influence of nonlinear distortion also increases, so that the noise near the carrier deteriorates.

Here, the relationship between floor noise and carrier vicinity noise in the conventional crystal oscillator shown in FIG. 9 will be described with reference to FIG. FIG. 10 is an explanatory diagram showing the relationship between floor noise and near-carrier noise in a conventional crystal oscillator.
FIG. 10 shows the phase noise characteristics when the drive current of the crystal resonator is changed.

As shown in FIG. 10, when the drive current of the crystal resonator is small, the carrier vicinity noise corresponding to the region having a small offset frequency can be reduced, but the floor noise corresponding to the region having a large offset frequency cannot be reduced.
On the contrary, when the drive current of the crystal resonator is large, the floor noise can be reduced, but the near-carrier noise cannot be reduced.
Thus, the floor noise and the carrier vicinity noise are in a trade-off relationship.

[Another Conventional Crystal Oscillator (1): FIG. 11]
Therefore, in order to reduce signal distortion and reduce phase noise, an impedance element is provided in series between the crystal resonator and the ground terminal, and a crystal oscillator (another conventional crystal that extracts the signal voltage from the impedance element) is provided. Has been devised (Patent Document 1). Here, the output terminal is provided on the opposite side of the crystal resonator from the side connected to the oscillation transistor.

Another conventional crystal oscillator will be described with reference to FIG. FIG. 11 is a circuit diagram showing a configuration example of another conventional crystal oscillator.
As shown in FIG. 11, another conventional crystal oscillator has the same basic configuration as the conventional crystal oscillator shown in FIG. 9, but the position of the output terminal 2 is different. Components similar to those in FIG. 9 are denoted by the same reference numerals, and description thereof is omitted.
In another conventional crystal oscillator, the output terminal 2 is connected to the side of the crystal resonator X opposite to the side connected to the transistor Tr via a resistor R15.

In such a configuration, in order to increase the output signal (improve C / N (Carrier to Noise Ratio)), it is necessary to increase the impedance value of the impedance element.
However, when the impedance value is increased, the load of the oscillation circuit increases, the circuit Q decreases, and oscillation becomes unstable.

  In Patent Document 1, although the impedance value of the output impedance resistor R15 is made small, it is difficult to reduce the phase noise characteristic due to white noise (floor noise) because C / N cannot be improved.

[Another conventional crystal oscillator (2)]
Further, as another conventional crystal oscillator, there is a configuration in which a grounded base circuit is provided in the output section (see Patent Document 2, FIG. 2).
By providing a base ground circuit in the output unit, a large output signal can be extracted even if the output impedance is reduced.
However, in this configuration, the addition of an active element to the oscillation circuit deteriorates the phase noise characteristics.

[Model of phase noise]
In general, Leeson's model is often used as a method for evaluating phase noise characteristics from circuit characteristics of an oscillator (see Non-Patent Document 1).
Leeson's model shows that the larger the circuit Q (load Q) included in the equation, the better the phase noise characteristics.

[Load resistance and phase noise]
It is known that when a load resistance is added in series with a crystal resonator, the load Q of the crystal resonator is deteriorated.
However, even if such an additional resistor is provided, the circuit Q as an oscillation loop does not always deteriorate.

  That is, the impedance element is not directly related to the frequency-phase tilt value that is generally used as the definition of Q of the oscillation loop circuit. This means that even if an impedance element having a large impedance value is inserted into the circuit loop, it does not directly affect the deterioration of the phase noise (see Non-Patent Document 2).

[Related technologies]
As conventional techniques related to phase noise of a crystal oscillator, Japanese Utility Model Publication No. 58-173915 (Kinseki Co., Ltd., Patent Document 1), US Pat. No. 4,283,691 (Patent Document 2), DBLeeson: “A simple model of feedback oscillator noise spectrum ”, Proc. IEEE, Vol. 54, pp. 329-330 (1966) (Non-Patent Document 1), Sakamoto et al. , IEEJ Transactions C, Vol.135, No.1, pp.12-17 (2014) (Non-patent Document 2).

Non-Patent Document 1 describes a phase noise analysis model of a closed loop circuit including an amplifier circuit and a resonator.
Non-Patent Document 2 describes that the circuit Q (load Q) for determining whether the phase noise characteristic is good or bad is formulated by an expression determined by the value of the frequency-phase inclination.

Japanese Utility Model Publication No. 58-173915 US Pat. No. 4,283,691

D.B.Leeson: “A simple model of feedback oscillator noise spectrum”, Proc. IEEE, Vol. 54, pp. 329-330 (1966) Sakamoto et al. “Examination of near-carrier phase noise in oscillation circuit using multiple crystal units”, IEEJ Transactions C, Vol.135, No.1, pp.12-17 (2014)

  As described above, the conventional crystal oscillator has a trade-off relationship between the near-carrier noise and the floor noise. Therefore, it is impossible to reduce both of them, and it is possible to realize a crystal oscillator with good phase noise characteristics. There was a problem that it was difficult.

  The present invention has been made in view of the above circumstances, and an object thereof is to provide a crystal oscillator having good phase noise characteristics by simultaneously reducing near-carrier noise and floor noise.

  The present invention for solving the problems of the above conventional example is a crystal oscillator including a Colpitts oscillation circuit having a crystal resonator and an oscillation transistor, the crystal resonator including a side connected to the oscillation transistor; The buffer amplifier connected to the opposite side, the output terminal that outputs the output signal through the buffer amplifier, the output coupling capacitor connected between the crystal oscillator and the buffer amplifier, the crystal oscillator and the output An impedance element having one end connected to the coupling capacitor and the other end connected to the ground level. The impedance value of the impedance element is more than three times the equivalent series resistance value of the crystal resonator, and the output coupling The capacitor is characterized in that the impedance value is smaller than the impedance value of the impedance element.

  According to the present invention, in the crystal oscillator, the capacitance value of the output coupling capacitor is 100 pF or more.

  The present invention is also characterized in that, in the crystal oscillator, the negative resistance of the oscillation transistor is set to a value that enables the oscillation loop to oscillate in accordance with the load of the oscillation loop formed by the Colpitts circuit.

  Further, the present invention is characterized in that the crystal oscillator includes another impedance element having an impedance value smaller than that of the impedance element in series with the impedance element.

  Further, the present invention is characterized in that the crystal oscillator includes a reactance element having an impedance value at an oscillation frequency of three times or more of an equivalent series resistance value of the crystal resonator, instead of the impedance element.

  According to the present invention, there is provided a crystal oscillator including a Colpitts oscillation circuit having a crystal resonator and an oscillation transistor, the buffer amplifier unit being connected to the side opposite to the side connected to the oscillation transistor in the crystal resonator And an output terminal for outputting an output signal via the buffer amplifier section, an output coupling capacitor connected between the crystal oscillator and the buffer amplifier section, and one end connected between the crystal oscillator and the output coupling capacitor And an impedance element having the other end connected to the ground level, the impedance value of the impedance element being at least three times the equivalent series resistance value of the crystal resonator, and the impedance value of the output coupling capacitor being Since the crystal oscillator is smaller than the impedance value, the impedance element and output coupling capacitor As a result, the output level can be increased to reduce floor noise, and the impedance drive element can suppress the crystal drive current to suppress nonlinear distortion, thereby suppressing near-carrier noise and obtaining good phase noise characteristics. .

  In addition, according to the present invention, since the above-described crystal oscillator has the negative resistance of the oscillation transistor set to a value that enables the oscillation loop to oscillate according to the load of the oscillation loop formed by the Colpitts circuit, There is an effect that an oscillation operation can be performed.

  In addition, according to the present invention, since the above-described crystal oscillator includes another impedance element having an impedance value smaller than that of the impedance element in series with the impedance element, the output impedance value can be appropriately fine-tuned and optimized. There is an effect that it can be made to be a phase noise characteristic

It is a circuit diagram which shows the structure of this crystal oscillator. It is explanatory drawing which shows the drive current characteristic of the crystal oscillator of this crystal oscillator. It is explanatory drawing which shows the carrier vicinity noise characteristic of this crystal oscillator. It is explanatory drawing which shows the characteristic of the output level of this crystal oscillator. It is explanatory drawing which shows the floor noise characteristic of this crystal oscillator. It is explanatory drawing which shows the output characteristic of this crystal oscillator with respect to the capacitance value of the output coupling capacitor C4. It is explanatory drawing which shows the floor noise characteristic with respect to the capacitance value of the output coupling capacitor C4. FIG. 4 is a partial circuit diagram illustrating a configuration example in which an adjustment impedance element is provided in the present crystal oscillator. It is a circuit diagram which shows the structural example of a general Colpitts oscillator. It is explanatory drawing which shows the relationship between floor noise and carrier vicinity noise. It is a circuit diagram which shows the structural example of another conventional crystal oscillator.

Embodiments of the present invention will be described with reference to the drawings.
[Outline of the embodiment]
A crystal oscillator according to an embodiment of the present invention (the present crystal oscillator) includes a Colpitts oscillation circuit, one end of a crystal resonator is connected to an oscillation transistor, and the other end is connected to a buffer amplifier unit via an output coupling capacitor. One end of the output impedance element is connected to a point between the other end of the crystal unit and the output coupling capacitor, the other end is grounded, and an output signal is obtained via the buffer amplifier unit. The impedance value of the output impedance is 3 times or more of the equivalent series resistance of the crystal resonator, and the impedance value of the output coupling capacitor is smaller than the impedance value of the output impedance element. And by reducing the impedance value of the output coupling capacitor Increase the level while reducing the floor noise, by suppressing the crystal drive current, in which the carrier near the noise can be suppressed.

[Configuration of the present crystal oscillator: FIG. 1]
The configuration of the present crystal oscillator will be described with reference to FIG. FIG. 1 is a circuit diagram showing the configuration of the present crystal oscillator.
As shown in FIG. 1, the present crystal oscillator is a Colpitts oscillator, and includes a crystal resonator X, an oscillation transistor Tr1, and an amplification transistor Tr2. The amplifying transistor Tr2 corresponds to the buffer amplifier section in the claims.

The collector of the oscillation transistor Tr1 is connected to the power supply terminal 1 via a resistor R3, and the emitter is grounded via a resistor R4.
In addition, series-connected resistors R1 and R2 are provided in parallel to the oscillation transistor Tr1, and the base of the oscillation transistor Tr1 is connected to a point between the resistors R1 and R2 and further connected to one end of the crystal unit X. ing.

  Further, series-connected capacitors C1 and C2 are connected in parallel to the resistor R2, and the emitter of the oscillation transistor Tr1 is connected to a point between the capacitors C1 and C2 via the series-connected capacitor C3 and the coil L1. ing.

As a feature of the present crystal oscillator, an output impedance element R10 is connected in series between the other end of the crystal unit X and the ground level.
As will be described later, the impedance value of the output impedance element R10 is characterized in that it is at least three times the equivalent resistance of the crystal resonator.
A point between the other end of the crystal unit X and the output impedance element R10 is connected to the base of the amplifying transistor Tr2 via the output coupling capacitor C4.

A power supply terminal 1 is connected to the collector of the amplifying transistor Tr2, and a parallel circuit of a transformer 10 and a capacitor C5 is connected between the collector and the power supply terminal 1. An output terminal 2 is provided on the output side of the transformer 10.
The emitter of the amplifying transistor Tr2 is grounded via a resistor R7.
Further, a series circuit of resistors R5 and R6 is connected in parallel to the amplifying transistor Tr2, and a point between the resistors R5 and R6 is connected to the base of the amplifying transistor Tr2.

  In other words, the present crystal oscillator is provided with an output impedance element R10 having a large impedance value between the side of the crystal resonator X not connected to the oscillation transistor Tr1 and the ground terminal, and the crystal resonator X and the output impedance. In this configuration, an output signal is extracted from the element R10 via an output coupling capacitor C4 and an amplifying transistor Tr2 which is a buffer amplifier unit.

[Output impedance element]
Since the output impedance element R10 is included in the oscillation loop, it can be regarded not only as an impedance for generating an output signal voltage but also as a load of the oscillation loop.
In general, it is known that when the load of the oscillation loop is increased, the phase noise characteristics are adversely affected. However, as described above, the impedance element does not directly degrade the Q of the oscillation loop circuit.

Therefore, in this crystal oscillator, the impedance value of the output impedance element R10 is set to a sufficiently large value so as to be three times or more the equivalent series resistance of the crystal resonator so that a large output signal voltage can be obtained.
By increasing the output signal voltage, C / N can be improved and floor noise can be improved.

  Furthermore, the output impedance element R10 can reduce the drive current of the crystal resonator X by the function as a load of the oscillation loop, and can suppress nonlinear distortion of the crystal resonator X due to excessive crystal current.

  In other words, in this crystal oscillator, the output impedance element R10 is used not only as an output impedance but also as a nonlinear distortion suppression element, thereby reducing the drive current (crystal current) of the crystal unit X and suppressing nonlinear distortion. By doing so, the noise in the vicinity of the carrier is improved.

  In addition, a reactance element such as a capacitor can be used instead of the output impedance element R10. However, it is necessary to use an impedance value at the oscillation frequency that is three times or more the equivalent series resistance value of the crystal unit X. There is.

[Output coupling capacitor]
The output coupling capacitor C4 is an element that adjusts the strength of electrical coupling between the oscillation circuit unit and the buffer amplifier unit (amplifying transistor Tr2).
Generally, when the impedance of the output coupling capacitor is reduced (when the capacitance value is increased), the coupling between the oscillation unit and the buffer amplifier unit becomes stronger.
This increases the output level.

That is, in this crystal oscillator, the output level is increased by the action of the output coupling capacitor C4 in addition to the output impedance element R10, and floor noise is reduced.
In this crystal oscillator, an output coupling capacitor C4 having an impedance value smaller than the impedance value of the output impedance element R10 is used. This is because the amplifying transistor Tr2 is properly operated.

[Characteristics of this crystal oscillator by output impedance element: FIGS. 2 to 5]
In the present crystal oscillator, various characteristics when the resistance value of the output impedance element R10 is varied will be described.
[Crystal current characteristics: Fig. 2]
FIG. 2 is an explanatory diagram showing drive current characteristics of the crystal resonator of the present crystal oscillator.
As shown in FIG. 2, as the output impedance value increases, the drive current of the crystal resonator decreases.

[Near carrier noise characteristics: Fig. 3]
FIG. 3 is an explanatory diagram showing near-carrier noise characteristics of the present crystal oscillator.
As shown in FIG. 3, it can be seen that when the output impedance value is increased, the near-carrier noise is significantly reduced as the crystal current shown in FIG. 2 is reduced.

[Output characteristics: Fig. 4]
FIG. 4 is an explanatory diagram showing the characteristics of the output level of the present crystal oscillator.
As shown in FIG. 4, as the output impedance value increases, the output level increases.

[Floor noise characteristics: Fig. 5]
FIG. 5 is an explanatory diagram showing the floor noise characteristics of the present crystal oscillator.
As shown in FIG. 5, when the output impedance value increases, the floor noise decreases with the increase of the output level shown in FIG.

  That is, as shown in FIGS. 3 and 5, it can be seen that in this crystal oscillator, both near-carrier noise and floor noise are sufficiently reduced in a region where the output impedance value is large (for example, 1500 ohms or more). .

As described above, the characteristic of improving the near-carrier noise and the floor noise at the same time is a characteristic that has not been seen in the conventional circuit configuration, and indicates that the phase noise characteristic of the present crystal oscillator is extremely good.
As shown in FIGS. 2 to 5, when an output impedance element having a small impedance value is provided, the effect of improving near-carrier noise and floor noise is not sufficient.

[Characteristics of this crystal oscillator with output coupling capacitor: Fig. 6 and Fig. 7]
Various characteristics when the capacitance value (impedance value) of the output coupling capacitor C4 is changed will be described.
[Output characteristics: Fig. 6]
FIG. 6 is an explanatory diagram showing the output characteristics of the present crystal resonator with respect to the capacitance value of the output coupling capacitor C4.
As shown in FIG. 6, when the capacitance value of the output coupling capacitor C4 increases (when the impedance value decreases), the output level increases. Incidentally, when the capacitance value exceeds 100 pF, the output level becomes almost flat.

[Floor noise characteristics: Fig. 7]
FIG. 7 is an explanatory diagram showing floor noise characteristics with respect to the capacitance value of the output coupling capacitor C4. In the figure, phase noise at an offset frequency of 10 kHz is shown.
In general, the floor noise does not greatly depend on the value of the output coupling capacitor. However, in this crystal oscillator, as shown in FIG. 7, when the capacitance value of the output coupling capacitor C4 increases (impedance value). Floor noise is reduced).

  In this crystal oscillator, the amplifying transistor Tr2 serving as the buffer amplifier unit also functions as a part of the oscillation loop, and the output coupling capacitor C4 not only adjusts the strength of coupling between the oscillation circuit unit and the buffer amplifier unit. This is considered to be because the impedance characteristics in the oscillation loop are also affected.

  That is, in the present crystal oscillator, the floor noise can be greatly reduced by the action of increasing the output level by the output coupling capacitor C4 in addition to the action of the output impedance element R10 shown in FIGS.

  In consideration of the characteristics shown in FIGS. 2 to 7, in the present crystal oscillator, the phase noise characteristics can be effectively obtained by increasing the impedance value of the output impedance element R10 and decreasing the impedance value of the output coupling capacitor C4. It can be seen that this can be improved.

  In this crystal oscillator, the impedance value of the output coupling capacitor C4 is smaller than the impedance value of the output impedance element R10 (more than three times the equivalent series resistance value of the crystal unit X), and in particular, the capacitance value is 100 pF or more. Is desirable.

  Furthermore, since the above-mentioned output impedance element R10 can suppress the near-carrier noise by the action of reducing the crystal drive current, the present crystal oscillator improves both the floor noise and the near-carrier-carrier noise, and has good phase noise characteristics. It can be said that.

[Oscillation condition]
In the present crystal oscillator, the output impedance element R10 having an impedance value more than three times the equivalent series resistance value of the crystal unit X is provided in the oscillation loop, so that the oscillation loop satisfies the oscillation condition with a margin. In order to achieve this, it must have a negative resistance commensurate with it.
Specifically, it is necessary to select the oscillation transistor Tr1 so as to obtain a sufficient negative resistance. On the other hand, noise is generated from the transistor which is an active element, and the phase noise characteristic is deteriorated.

  Therefore, the oscillation transistor Tr1 is appropriately selected and the impedance value of the output impedance element R10 is appropriately adjusted in consideration of the effect of reducing the crystal current by the output impedance element R10 and the generation of noise by the oscillation transistor Tr1. .

  As a result, the present crystal oscillator can perform stable oscillation operation and suppress the noise from the active element as much as possible, while sufficiently reducing the crystal current and optimizing the phase noise characteristics.

[Adjustment impedance element: Fig. 8]
A configuration for adjusting the impedance value of the output impedance will be described with reference to FIG. FIG. 8 is a partial circuit diagram showing a configuration example in which an adjustment impedance element is provided in the present crystal oscillator.
FIG. 8 shows a configuration in which an adjustment impedance element R20 is provided in series with the output impedance element R10 in the crystal oscillator shown in FIG. The components other than the adjustment impedance element R20 are the same as those in FIG. 1, and are not shown here.

The output impedance element R10 has a reference impedance value (more than three times the equivalent series resistance value of the crystal unit X), and the adjustment impedance element R20 has an impedance value smaller than that. It is an element.
By arranging a variable impedance element such as a variable resistor as the adjustment impedance element R20, the output impedance value can be finely adjusted.

[Effect of the embodiment]
The crystal oscillator according to the embodiment of the present invention includes a Colpitts oscillation circuit, one end of the crystal resonator X is connected to the base of the oscillation transistor Tr1, and the other end is grounded via the output impedance element R10. An amplification transistor Tr2 serving as a buffer amplifier section is connected between the other end of the crystal unit X and the output impedance element R10 via an output coupling capacitor C4, and an output signal is obtained via the amplification transistor Tr2. Yes, the impedance value of the output impedance element R10 is three times or more the equivalent series resistance of the crystal unit X, and the impedance value of the output coupling capacitor C4 is smaller than the impedance value of the output impedance element R10. Increase the impedance value of impedance element R10 sufficiently By making the impedance value of the output coupling capacitor C4 sufficiently small, the output level is increased to reduce the floor noise, and the load in the oscillation loop is used to suppress the crystal drive current, thereby suppressing the near-carrier noise. There is an effect that floor noise and near-carrier noise can be improved at the same time to obtain good phase noise characteristics.

  Further, according to the present crystal oscillator, since the oscillation transistor Tr1 having a sufficient negative resistance is provided according to the impedance value of the output impedance element R10 serving as a load in the oscillation loop, the oscillation loop can be stabilized. There is an effect of being able to oscillate.

  Further, according to the present crystal oscillator, since the adjustment impedance element R20 having a small impedance value is provided in series with the output impedance element R10, the output impedance value can be finely adjusted appropriately, and the optimum phase noise characteristic can be obtained. There is an effect that can be.

  The present invention is particularly suitable for a crystal oscillator that can improve floor noise and near-carrier noise at the same time and has good phase noise characteristics.

  DESCRIPTION OF SYMBOLS 1 ... Power supply terminal, 2 ... Output terminal, 10 ... Transformer, X ... Crystal resonator, Tr, Tr1 ... Oscillation transistor, Tr2 ... Amplification transistor, R10 ... Output impedance element, R20 ... Adjustment impedance element, R1, R2 , R3, R4, R5, R6, R7, R11, R12, R14, R15 ... resistors, C4 ... output coupling capacitors, C1, C2, C3, C5, C11, C12, C13, C14 ... capacitors, L1 ... coils

Claims (5)

A crystal oscillator including a Colpitts oscillation circuit having a crystal resonator and an oscillation transistor,
In the crystal unit, a buffer amplifier unit connected to a side opposite to the side connected to the oscillation transistor,
An output terminal for outputting an output signal via the buffer amplifier unit;
An output coupling capacitor connected between the crystal resonator and the buffer amplifier unit;
An impedance element having one end connected between the crystal resonator and the output coupling capacitor and the other end connected to a ground level,
The impedance value of the impedance element is at least three times the equivalent series resistance value of the crystal resonator,
A crystal oscillator, wherein an impedance value of the output coupling capacitor is smaller than an impedance value of the impedance element.   2. The crystal oscillator according to claim 1, wherein the capacitance value of the output coupling capacitor is 100 pF or more.   3. The crystal oscillator according to claim 1, wherein the negative resistance of the oscillation transistor is set to a value enabling the oscillation loop to oscillate in accordance with a load of the oscillation loop formed by a Colpitts circuit.   4. The crystal oscillator according to claim 1, further comprising another impedance element having an impedance value smaller than that of the impedance element in series with the impedance element.   5. The crystal oscillator according to claim 1, further comprising a reactance element having an impedance value at an oscillation frequency that is at least three times an equivalent series resistance value of the crystal resonator, instead of the impedance element.