JP2001085947A - Colpitts crystal oscillation circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a circuit suitably suppressing abnormal oscillation due to spurious radiation of an SC-cut crystal vibrator. SOLUTION: The Colpitts crystal oscillation circuit is a common emitter Colpitts crystal oscillation circuit where the emitter of a transistor(TR) 36 is connected to ground with a bypass capacitor for high frequencies. In this case, the bypass capacitor is replaced with a piezoelectric vibrator with a low Q. Thus, abnormal oscillation due to spurious radiation by the SC-cut crystal vibrator can be prevented and a negative resistance characteristic with a sufficiently higher resistance required to start the oscillation can be ensured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Colpitts crystal oscillation circuit, and more particularly to a Colpitts crystal oscillation circuit capable of preventing abnormal oscillation caused by spurious of an SC-cut crystal resonator and improving frequency stability.

[0002]

2. Description of the Related Art Crystal oscillators are widely used as reference frequency sources for various communication devices such as mobile communication devices, electronic devices, and OA devices. Among these, particularly in the field of communication base stations and the like where high frequency stability is required, an oven controlled crystal oscillator (OCXO) in which a crystal oscillator is placed in a thermostat to keep the temperature constant is used. I have. Conventionally, a Colpitts crystal oscillation circuit is generally used for such an oscillator. FIG. 2 shows a basic circuit configuration of the Colpitts crystal oscillation circuit. In the figure, reference numeral 1 denotes a transistor, a capacitor 2 between the base and the emitter of the transistor 1, a capacitor 3 between the emitter and the ground,
A series circuit of a crystal unit 4 and a frequency adjusting capacitor 5 is connected between the base and the ground.
Note that the resistors 6 to 8 are bias resistors, and the capacitor 9 is a bypass capacitor. The Colpitts crystal oscillation circuit having such a configuration is called a collector-grounded Colpitts crystal oscillation circuit because the collector side of the transistor 1 is configured to be grounded at a high frequency.

Conventionally, an AT-cut crystal resonator has been used as a crystal resonator connected to a Colpitts crystal oscillation circuit because of its excellent frequency stability. An excellent SC-cut quartz resonator has been put to practical use. However, the SC-cut quartz resonator is characterized by having extremely excellent frequency stability, but has a very large spurious component near the main vibration. For this reason, when the SC cut crystal resonator is connected to the Colpitts crystal oscillation circuit of the grounded collector type, there is a drawback that abnormal oscillation such as a frequency jump phenomenon occurs due to spurious.

As means for preventing abnormal oscillation caused by spuriousness of the SC-cut quartz resonator, for example, Japanese Patent Application Laid-Open
There is a technique as disclosed in 153740. This method is as shown in FIG. 3, in which a piezoelectric vibrator 11 is connected to the emitter of the transistor 10, and a capacitor 12 is provided between the other end of the piezoelectric vibrator 11 and the base of the transistor 10. A capacitor 13 is connected between the other end of the piezoelectric vibrator 11 and ground, and a series circuit of an SC-cut crystal vibrator 14 and a frequency adjusting capacitor 15 is connected between the base and ground of the transistor 10. I have. Note that the resistors 16 and 18 are bias resistors.

In the above configuration, the frequency of the oscillation circuit and the resonance frequency of the piezoelectric vibrator 11 are selected so as to match each other, so that the piezoelectric vibrator 11 functions as a filter. In the region, the negative resistance of the oscillation circuit is increased, and conversely, in the frequency region other than the main vibration, the negative resistance of the oscillation circuit is eliminated. By such an operation, it is possible to prevent abnormal oscillation caused by spurious of the SC-cut quartz resonator. However, even if such a circuit configuration is employed, there is a disadvantage that the equivalent resistance of the SC-cut quartz resonator may be increased due to manufacturing variations, and the oscillation may not be performed. In the above configuration, the piezoelectric vibrator 11 is in a state of series resonance in the frequency region of the main vibration, and is equivalent to operating in a state of the equivalent resistance of the piezoelectric vibrator 11. For this reason, in the frequency region of the main vibration, it is equivalent to a configuration in which a resistor is inserted between the emitter of the transistor and the capacitor 12, and
A resistor is inserted between the current source by the action of the transistor and the capacitor 13 in the oscillation loop. For this reason, when the oscillator is started, that is, during the process of growing the small signal, the small signal suffers a remarkable loss due to the equivalent resistance of the piezoelectric vibrator, and it is impossible to supply a sufficient current to the oscillation loop for the oscillation growth. That is, since the negative resistance at the time of starting oscillation is small, the above-described problem that oscillation does not occur when the equivalent resistance of the SC-cut crystal resonator is large has occurred.

In addition to the above-mentioned problem, in the conventional Colpitts circuit, an output is taken out from the emitter of the transistor, and although a signal having a relatively clean waveform can be taken out, the output of the transistor is relatively small. A slight distortion has occurred due to the nonlinearity. In order to obtain an oscillator having better phase noise characteristics, it is necessary to extract a clean sine wave, that is, a signal with high signal purity, from the oscillation circuit. In view of such circumstances, the inventor of the present application has conceived an oscillation circuit as shown in FIG. 4 as a circuit configuration in which a sine wave with better signal purity can be easily extracted.

In FIG. 1, reference numeral 19 denotes a transistor. A resistor 21 is connected between a power supply 20 and the base of the transistor 19, and a resistor 22 is connected between the base and the ground of the transistor 19. A resistor 23 and a coil 24 are connected in parallel between the collector of the transistor 19 and a resistor 25 and a bypass capacitor 26 are connected in parallel between the emitter and the ground to form a bias circuit. Next, a capacitor 27 is connected between the collector and the ground of the transistor 19, one end of the SC-cut crystal unit 28 is connected to the collector, and one end of the capacitance variable unit 29 is connected to the other end. A capacitor 30 is connected between the other end of the
1 and connected to the base of the transistor 19 to form an oscillation loop. Next, a coupling capacitor 32 is connected to a connection point between the capacitor 30 and the variable capacitance circuit 29, an input side of a buffer amplifier 33 is connected to the other end of the coupling capacitor 32, and an output side of the buffer amplifier 33 is connected to A coupling capacitor is connected, and the oscillation frequency of the oscillator is output from the other end of the capacitor. Note that the capacitor 35 in the figure is a bypass capacitor.

In such a configuration, the emitter of the transistor is grounded at a high frequency by a bypass capacitor 26, and is generally called a grounded emitter Colpitts oscillation circuit. As in the above configuration,
Since the oscillation output is taken out from one end of the capacitor 30, a sine wave signal with higher signal purity can be obtained. Even when an SC-cut crystal resonator is connected to such a grounded-emitter Colpitts oscillation circuit, abnormal oscillation due to spuriousness of the SC-cut crystal resonator occurs as in the case of the above-described grounded-collector type. In order to prevent this, a device on the circuit side is required. As in the case of the common-collector type described above, in order to cut off the point for supplying current to the oscillation loop in the spurious frequency region and operate to eliminate the negative resistance of the oscillation circuit, the collector of the transistor 19 and the capacitor 27 are required. (The point indicated by the symbol A in the figure) may be inserted between them.

[0009]

However, in such a configuration, although abnormal oscillation due to spurious of the SC-cut crystal resonator can be prevented, the negative resistance at the time of startup in the frequency domain of the main vibration is reduced. Causes a problem such as not oscillating when using an SC-cut crystal resonator having a large equivalent resistance.
This is the same as the case of the above-mentioned common collector type. The present invention
In view of the above-mentioned problem, in a Colpitts crystal oscillator circuit of a grounded emitter type, it is necessary to prevent abnormal oscillation caused by spurious of an SC-cut crystal resonator and to obtain a sufficient negative resistance at startup. With issues as
It is an object of the present invention to provide an oscillation circuit suitable for obtaining an oscillator having a higher frequency stability than a conventional oscillator, that is, an excellent phase noise characteristic.

[0010]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and an emitter of a transistor constituting an oscillation circuit using an SC-cut crystal resonator is connected to a high-frequency oscillator via a bypass capacitor. In the Colpitts crystal oscillation circuit grounded to the above, the bypass capacitor is replaced with a piezoelectric element having a resonance frequency substantially the same as the oscillation frequency of the oscillation circuit, wherein the piezoelectric element is the SC cut crystal. It is characterized by having a lower Q value than the vibrator.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail based on illustrated embodiments. FIG. 1 shows a common emitter type Colpitts crystal oscillation circuit showing an embodiment of the present invention. In the figure, reference numeral 36 denotes a transistor. A resistor 38 is connected between the power supply 37 and the base of the transistor 36, and a resistor 39 is connected between the base of the transistor 36 and the ground.
A resistor 4 is connected between the power supply 37 and the collector of the transistor 36.
0 and a coil 41 are connected in parallel, and a resistor 42 and a piezoelectric vibrator 43 which is a piezoelectric element are connected in parallel between the emitter and the ground of the transistor 36 to form a bias circuit. Next, the transistor 36 in the bias circuit
, A capacitor 44 is connected between the collector and ground, a collector of the transistor 36 is connected to one end of an SC-cut crystal resonator 45, and a variable capacitor 46 is connected to the other end of the SC-cut crystal resonator 45, and a variable capacitor is connected. A capacitor 47 is connected between the other end of the portion 46 and the ground to form an oscillation loop. Further, it is connected to the base of the transistor via a capacitor 48. Next, a coupling capacitor 49 is connected to a connection point between the capacitor 47 and the variable capacitance section 46, an input side of the buffer amplifier 50 is connected to the other end of the coupling capacitor 49, and an output side of the buffer amplifier 50 is The coupling capacitor 51 is connected, and the oscillation frequency of the oscillator is output from the other end of the capacitor 51. In the figure, reference numeral 52 denotes a bypass capacitor.

In the above configuration, the resonance frequency of the piezoelectric vibrator 43 is substantially the same as the oscillation frequency of the oscillation circuit,
In addition, the Q value is selected so as to be smaller than the Q value of the SC-cut quartz resonator 45. With this selection, the piezoelectric vibrator 43 acts to ground the emitter of the transistor at a high frequency in the main vibration region of the SC-cut crystal resonator, and on the other hand, in the other region of the main vibration, simply acts as a number. Since the piezoelectric vibrator 43 functions as a capacitor of about pF, the piezoelectric vibrator 43 becomes high impedance and acts to eliminate the negative resistance of the oscillation circuit. Therefore, there is no occurrence of abnormal oscillation due to spurious of the SC-cut quartz resonator.

Next, in the above configuration of the present invention, since the collector stage of the transistor is directly connected to the oscillation loop, a small signal generated by the amplification stage by the transistor is supplied to the oscillation loop without loss. can do. Although the amplification factor of the amplifying portion slightly decreases due to the equivalent resistance of the piezoelectric vibrator 45, the oscillation signal does not become small enough to hinder the oscillation growth, so that the oscillation signal grows and reaches a steady state. That is, according to the present invention, since the negative resistance at the time of startup does not decrease as in the case of the conventional example, even when an SC-cut crystal resonator having an equivalent resistance large enough to be caused by manufacturing variations of the SC-cut crystal resonator is connected. Oscillating operation can be performed sufficiently.

In the present invention, the piezoelectric vibrator 43
Functions as a narrow band-pass filter, and the Q value of the piezoelectric vibrator 43 is selected to be smaller than that of the SC cut quartz vibrator 45. From this, the capacity variable section 46
, The oscillation frequency does not exceed the pass band of the piezoelectric vibrator 43 even when the oscillation frequency of the oscillation circuit is controlled to be greatly changed. As described above, since the frequency variable range can be widened, the present invention can be said to be a configuration particularly suitable for an oscillator with a voltage control function. Further, according to the present invention, since the piezoelectric vibrator 43 functions as a narrow band-pass filter, the influence of noise generated from a transistor or a power supply can be reduced. For this reason, the floor-level characteristic of the phase noise characteristic of the oscillation frequency can be further improved.

Although the above description has been made with reference to an example in which a piezoelectric vibrator is used as the piezoelectric element, the present invention is not limited to this, and any filter that exhibits a filter characteristic using series resonance can be used. , Any piezoelectric element may be used.

[0016]

According to the present invention, in a Colpitts crystal oscillator circuit of a grounded emitter type, abnormal oscillation caused by spuriousness of an SC cut crystal resonator is prevented by connecting a piezoelectric vibrator between an emitter and ground. . Further, since the negative resistance at the time of starting can be sufficiently ensured, S
Even when the equivalent resistance is large enough to be caused by manufacturing variations of the C-cut quartz resonator, a stable oscillation operation can be performed. According to the effect of the present invention, SC
It is possible to make full use of the frequency stability of the cut crystal resonator, and it is possible to provide an oscillator having more excellent frequency stability than a conventional oscillator.

[Brief description of the drawings]

FIG. 1 is an embodiment of a grounded emitter Colpitts crystal oscillation circuit of the present invention.

FIG. 2 is a basic circuit of a grounded collector Colpitts crystal oscillation circuit.

FIG. 3 shows a Colpitts crystal oscillation circuit of a common collector type in which abnormal oscillation of a conventional SC cut crystal resonator is suppressed.

FIG. 4 shows a conventional grounded emitter Colpitts crystal oscillation circuit.

[Explanation of symbols]

36 transistors, 37, 53 power supplies, 38, 39,
40, 42, 54, 56 resistance, 41 coil, 43 piezoelectric vibrator, 44, 47, 48, 49, 51, 52, 5
7,58 capacitor, 45 SC cut crystal oscillator,
46 variable capacitance section, 50 buffer amplifier, 46 variable capacitance diode

Claims (1)

[Claims] In a Colpitts crystal oscillation circuit in which an emitter of a transistor constituting an oscillation circuit using an SC-cut crystal resonator is grounded at a high frequency via a bypass capacitor, the bypass capacitor is connected to the oscillation frequency of the oscillation circuit. A piezoelectric element having substantially the same resonance frequency, wherein the piezoelectric element is
A Colpitts crystal oscillation circuit having a Q value lower than that of a C-cut crystal resonator.