JP2007103985A - Crystal oscillator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an oscillation circuit for a highly stable crystal oscillator employing an SC-cut crystal oscillator that selectively oscillates the C-mode or the B-mode. <P>SOLUTION: The Colpitts oscillator provided with a transistor, the SC-cut crystal oscillator, and a series circuit comprising first and second capacitors is configured such that a series circuit comprising third and fourth capacitors is connected between the emitter of the transistor and ground, a parallel circuit comprising an inductor and a fifth capacitor is connected between a connecting point of the first and second capacitors and a connecting point of the third and fourth capacitors, a series circuit comprising a sixth capacitor, a varactor diode, and a seventh capacitor is connected in parallel with the fifth capacitor, one terminal of a first resistor is connected to the cathode of the varactor diode, the other terminal of the first resistor is connected to a control voltage terminal, one terminal of a second resistor is connected to the anode of the varactor diode, and the other terminal of the second resistor is connected to ground. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a piezoelectric oscillator, and more particularly to a crystal oscillator that selectively oscillates a C mode or a B mode in a crystal oscillator using an SC cut crystal resonator.

Piezoelectric oscillators have excellent frequency accuracy, frequency temperature characteristics, frequency aging characteristics, and the like, so that they are used in many fields from mobile radio base stations to measuring instruments.
SC-cut quartz crystal with two rotation cuts in the piezoelectric vibrator used in the piezoelectric oscillator (a crystal using a crystal plate cut out by rotating about 22 degrees around the Z axis and further rotating about 34 degrees around the X axis) If a crystal resonator) or an IT-cut crystal resonator is used, a highly stable crystal oscillator superior in stress sensitivity characteristics and thermal shock characteristics can be obtained compared to the case of using an AT-cut crystal resonator, and has been put into practical use. It was.

FIG. 5 is a diagram showing the resonance characteristics of the SC cut crystal resonator, where the horizontal axis represents frequency and the vertical axis represents reactance. As is clear from the figure, in addition to the thickness shear vibration mode (C mode) of the main vibration, there are a thickness torsion mode (B mode) and a thickness longitudinal mode (A mode) as vibration modes having higher frequencies.
Therefore, vibrations in B mode and A mode other than the C mode cause various problems as spurious components (unnecessary vibration components) when the oscillator is configured. In particular, the frequency f2 of the B mode adjacent to the main mode C mode is only about 9 to 10% away from the frequency f1 of the C mode, which causes a frequency jump phenomenon in which the oscillation frequency changes from f1 to f2. It was.

Accordingly, a crystal oscillator that solves the above problem is disclosed in Japanese Patent Laid-Open No. 9-153740. FIG. 6 shows a Colpitts-type crystal oscillation circuit disclosed in this publication. One end of a crystal resonator Y1 is connected to the base of a collector-grounded transistor Tr as an oscillation amplifier, and the other end is connected via a variable capacitor Cv. To ground. A series circuit of divided capacitors C1 and C2 is connected between the base and collector of the transistor Tr, and a crystal resonator Y2 is inserted and connected between the midpoint (divided point) of the series circuit of C1 and C2 and the emitter. . In the figure, Vc is a power source, R2 and R3 are bleeder resistors, R1 is a feedback resistor (load resistor), and Vo is an output terminal of the oscillator. For example, an AT-cut crystal resonator is used as the crystal resonator Y2, and the series resonance frequency thereof is set to substantially coincide with the oscillation frequency due to the main vibration (C mode).

  Here, a Colpitts type oscillation circuit often used in a crystal oscillator will be briefly described. FIG. 7 shows a Colpitts type oscillation circuit, which has an arrangement in which an inductive element is connected between the collector and base of the transistor Tr, and a capacitive element is connected between the base and emitter and between the collector and emitter. . As an inductive element between the collector and the base of the transistor Tr, a crystal resonator Y1 inserted between the base and the ground is used. A resistor R1 is connected between the emitter and the ground, a series connection circuit of the capacitors C1 and C2 is connected between the base and the ground, and a midpoint of connection between the capacitors C1 and C2 and the emitter are connected to form a Colpitts type oscillation circuit. Configure.

In the Colpitts type oscillation circuit, since the power source Vc and the ground are generally short-circuited by a bypass capacitor in terms of high frequency, a crystal resonator Y1 used as an inductive element is inserted between the collector and base in an equivalent circuit. It will be done. Further, since the midpoint between the capacitors C1 and C2 is connected to the emitter, the capacitor C1 is inserted between the base and emitter of the transistor Tr, and the capacitor C2 is inserted between the collector and emitter. It will act as capacitive. The resistors R2 and R3 are bleeder resistors and set a base bias voltage.
Further, in the Colpitts type crystal oscillation circuit, the amplification degree as seen from the both ends of the crystal resonator Y1, that is, the so-called negative resistance R (Ω) is inversely proportional to the capacitances C1 and C2 and the square of the frequency ω ² . It is known to be proportional to the collector current. That is, as the frequency increases, the absolute value of the negative resistance R (Ω) increases, reaches a peak value at a predetermined frequency, and then decreases as the frequency increases.

  FIG. 8 is a diagram showing negative resistance characteristics (frequency-negative resistance). The negative resistance (solid line) A of the oscillation circuit shown in FIG. 6 is compared with that of a general Colpitts oscillation circuit for comparison. It is the figure which showed negative resistance (broken line) b. As is well known, the larger the negative resistance value, the easier it is to oscillate. However, for reasons such as current consumption and suppression of unwanted vibration, it is recommended to set the value to several times the effective resistance of the crystal unit. It is common. Here, the effective resistance is a resistance exhibited by the crystal resonator at the resonance frequency of each mode.

  The negative resistance characteristic A is obtained by inserting the crystal resonator Y2 between the middle point of the series circuit of the divided capacitors C1 and C2 and the emitter of the transistor as compared with the negative resistance B of a general Colpitts circuit. The feedback circuit has frequency selectivity, and the negative resistance characteristic is narrow. The C-mode frequency (10.0 MHz) of the main vibration becomes a sufficiently large negative resistance, and the B-mode frequency (10 .9 MHz) indicates that the negative resistance is small or exhibits a positive resistance, and as a result, it cannot be oscillated at the frequency of the B mode.

  FIG. 9 shows a modified embodiment disclosed in the above-mentioned publication, which is an oscillation circuit in which the crystal resonator Y2 of FIG. 6 is replaced with a series circuit of an inductance L and a capacitance Ct, and the inductance L and the capacitance Ct. Is set to coincide with the frequency of the main vibration C mode. The negative resistance characteristic of this oscillation circuit is a frequency characteristic that is somewhat wider than the curve A as shown by the one-dot chain line C in FIG. 8, and exhibits a sufficiently large negative resistance at the frequency of the main vibration C mode. It is described in the above publication that the value becomes smaller at the frequency of the mode, and oscillation is not possible in the mode.

Japanese Unexamined Patent Publication No. 2000-295037 discloses a crystal oscillator that solves the jump problem between the C mode and the B mode. FIG. 10 is a circuit diagram showing the configuration of a high stability crystal oscillator. One end of the crystal resonator Y1 is connected to the base of a transistor Tr for oscillation amplification, and the other end is grounded via a variable capacitor Cv. A series circuit of divided capacitors C1 and C2 is connected between the base and ground of the transistor Tr, a series circuit of divided capacitors C3 and C4 is connected between the emitter and ground, and a connection point (middle point) of the divided capacitors C1 and C2 And a series circuit of an inductance L1 and a variable reactance Z is connected between the connection points (middle points) of the divided capacitors C3 and C4. A feedback resistor R1 and a load resistor R4 are connected between the emitter and ground of the transistor Tr and between the power source Vc and the collector, respectively, and bleeder resistors R2 and R3 are respectively connected between the power source and the base of the transistor Tr and between the base and the ground. Connecting. The output terminal Vo is connected to the collector of the transistor Tr.

Using the oscillation circuit of FIG. 10, the negative circuit resistance of the oscillation circuit was simulated by changing various circuit constants of the composite network including the divided capacitors C1, C2, C3, C4 and the inductance (L1 + Z). Capacitance C1 = 100 pF, C2 = C3 = C4 = 200 pF, Cv = 50 pF, resistance R1 = 1 kΩ, R2 = 220Ω, R3 = R4 = 10 kΩ, inductance (L1 + Z) = 1.3 μH to 1.5 μH In this case, the narrow band negative resistance characteristic shown in FIG. 11 was obtained. From the negative resistance characteristic of this figure, a sufficiently large negative resistance is obtained at the frequency of the main vibration (C mode), and the negative resistance is extremely small or positive at the frequency of the B mode (10.9 MHz to 11.0 MHz). It is clear that That is, it is proved that a circuit that oscillates only the main vibration (C mode) and completely suppresses the B mode oscillation is obtained. At this time, the series resonant frequency of the composite network substantially coincides with the C-mode frequency, and the parallel resonant frequency substantially coincides with the B-mode frequency.

  The crystal oscillation circuit shown in FIG. 12 is an example in which a parallel circuit of an inductance L1 and a capacitor C5 is used instead of the inductance (L1 + Z) of the crystal oscillation circuit shown in FIG. In this circuit configuration, the B mode oscillation is suppressed in accordance with the mode frequency. That is, a band elimination filter is formed by the inductance L1 and the capacitors C1, C2, C3, C4, and C5 in the oscillation loop, and the B mode frequency is blocked and oscillation at this frequency is prevented.

As is well known, in order to keep the oscillation frequency constant in a highly stable crystal oscillator, a thermistor is generally used as a temperature sensitive element in order to place the crystal resonator in a thermostatic chamber and to control the temperature of the thermostatic chamber to be constant. It was. However, Japanese Patent Laid-Open No. 5-243892 discloses a crystal oscillator that compensates the oscillation frequency by using the B mode of the SC cut crystal resonator, which has been suppressed as an unnecessary mode, as a temperature sensor. FIG. 13 is a block diagram of the highly stable crystal oscillator disclosed in this publication. The frequency temperature characteristic of the B mode changes almost linearly when the first-order temperature coefficient is about −30 ppm / ° C. That is, if the frequency of the crystal resonator is 10 MHz, it changes by -300 Hz with one change. This temperature-frequency characteristic is used to compensate the oscillation frequency.

  Dual mode oscillation circuit that oscillates both C and B modes is “Highly Stable and Low Phase-Noise Oven-Controlled Crystal Oscillators Using Dual-Mode Excitation” IEICE Trans. Fundamentals, Vol. E85-A, No2 February 2002 It is disclosed. FIG. 14 is a circuit showing a configuration of a dual mode oscillation circuit, which includes an oscillation circuit 1, an oscillation circuit 2, and a series connection circuit of an SC cut crystal resonator Xtal and a variable capacitance diode Cv. In the oscillation circuit 1, a resistor Re1 is connected between the emitter of the transistor Tr1 and ground (GND), a resistor Rb1 is connected between the base ground of Tr1, and a series connection circuit of a capacitor C1 and a capacitor C2 is further connected. A series connection circuit of the crystal resonator Xtal1 and the capacitor Ca1 is connected between the contact and the emitter of the capacitors C1 and C2. Resistors Rc1 and Ra1 are connected between the power supply Vc and the collector of Tr1 and between the bases of Vc and Tr1, respectively, and the output OUT1 is taken out from the emitter. One end of a series connection circuit of the SC cut crystal resonator Xtal and the variable capacitance diode Cv is connected to the base of the Tr1 through the capacitance Ci1, the other end is grounded, and the control voltage Vcnt is applied to the cathode of the variable capacitance diode Cv. .

The oscillation circuit 2 has the same configuration as that of the oscillation circuit 1 and is configured symmetrically with respect to the series connection circuit of the SC cut crystal resonator Xtal and the variable capacitance diode Cv. Then, by setting the resonance frequencies of the crystal resonators Xtal1 and Xtal2 to the C mode and B mode frequencies, respectively, the oscillation circuit 1 becomes the C mode oscillation circuit, the oscillation circuit 2 becomes the B mode oscillation circuit, and the output terminal OUT1. The quartz crystal resonator Xtal1 connected to 作用 acts as a filter that passes only the C-mode oscillation frequency, and the quartz-crystal resonator Xtal2 connected to the output terminal OUT2 acts as a filter that passes only the B-mode oscillation frequency.
JP-A-5-243892 JP-A-9-153740 JP 2000-295037 A `` Highly Stable and Low Phase-Noise Oven-Controled Crystal Oscillators Using Dual-Mode Excitation '' IEICE Trans. Fundamentals, Vol. E85-A, No2 February 2002

However, Patent Document 1 does not disclose a method for specifically oscillating the B mode, and the circuit diagram disclosed in Non-Patent Document 1 is complicated and has a problem of high cost.
The present invention has been made to solve this problem, and it is an object of the present invention to provide a low-cost oscillation circuit that easily oscillates the C mode and the B mode.

The invention of claim 1 of the piezoelectric oscillator according to the present invention is a Colpitts-type crystal oscillator comprising a transistor, an SC cut crystal resonator, and a series circuit of first and second capacitors, wherein the emitter of the transistor is grounded A series circuit of third and fourth capacitors is connected between the first and second capacitors, and an inductance and a fifth capacitor are connected between the connection points of the first and second capacitors and the third and fourth capacitors. A parallel circuit with a capacitor is connected, a series circuit of a sixth capacitor, a variable capacitance diode, and a seventh capacitor is connected in parallel with the fifth capacitor, and one of the first resistors is connected to the cathode of the variable capacitance diode. And one terminal of the second resistor connected to the anode of the variable capacitance diode, and the other terminal of the first resistor and the other terminal of the second resistor, Control voltage or fixed voltage It is characterized in that a crystal oscillator is configured by supplying only one of them.
According to a second aspect of the present invention, there is provided a Colpitts oscillator including a transistor, an SC cut crystal unit, and a series circuit of first and second capacitors. A series circuit of capacitors, and a series circuit of an inductance and a fifth capacitor is connected between a connection point of the first and second capacitors and a connection point of the third and fourth capacitors, A variable capacitance diode is connected in parallel to the fifth capacitor, one terminal of a first resistor is connected to the cathode of the variable capacitance diode, and one terminal of a second resistor is connected to the anode of the variable capacitance diode. A crystal oscillator is configured by exclusively supplying either a control voltage or a fixed voltage to the other terminal of the first resistor and the other terminal of the second resistor, respectively. And
The invention according to claim 3 is the crystal oscillator according to claim 1, wherein the fixed voltage is a ground voltage.

In the first aspect of the present invention, a parallel circuit of an inductance and a capacitor including a variable capacitance diode is inserted between the midpoint of the divided capacitors C1 and C2 and the midpoint of the divided capacitors C3 and C4. There is an advantage that the C mode or the B mode can be selectively oscillated by varying the band of the band elimination filter to be formed by the control voltage.
In the invention of claim 2 of the present invention, a series circuit of an inductance and a capacitor including a variable capacitance diode is inserted between the midpoint of the divided capacitors C1 and C2 and the midpoint of the divided capacitors C3 and C4. There is an advantage that the C mode or the B mode can be selectively oscillated by varying the band of the formed bandpass filter by the control voltage.
According to the third aspect of the present invention, the operation of the circuit is further stabilized by using one of the fixed voltages as the ground voltage in the first or second aspect.

FIG. 1 is a circuit diagram showing an embodiment of a high stability crystal oscillator according to the present invention, in which one end of a crystal resonator Y1 is connected to the base of a transistor Tr for oscillation amplification, and the other end is connected to a variable capacitor Cv. To ground. A series circuit of divided capacitors C1 and C2 is connected between the base and ground of the transistor Tr, and a series circuit of divided capacitors C3 and C4 is connected between the emitter and ground. A feedback resistor R1 and a load resistor R4 are connected between the emitter and ground of the transistor Tr and between the power source Vc and the collector, respectively, and between the power source and the base of the transistor Tr and between the base and ground, respectively. R2 and R3 are connected.

A parallel circuit of an inductance L1 and a capacitor C5 is connected between a connection point (middle point) of the divided capacitors C1 and C2 and a connection point (middle point) of the divided capacitors C3 and C4. Further, a series circuit of a capacitor C6, a variable capacitance diode D1, and a capacitance C7 is connected in parallel to the capacitance C5, one terminal of the resistor R5 is connected to the cathode of the variable capacitance diode D1, and the other terminal is connected to the control voltage Vcnt. Connecting. One terminal of the resistor R6 is connected to the anode of the variable capacitance diode D1, the other terminal is grounded, and the output terminal Vo is taken out from the collector of the transistor Tr, thereby constituting the dual-mode oscillation highly stable crystal oscillator according to the present invention. To do.

  FIG. 2 shows the negative resistance characteristics of the oscillation circuit shown in FIG. As the circuit conditions of FIG. 1, the frequency of the SC cut crystal resonator Y1 is 20 MHz, the resistors R1, R2, and R3 are 680Ω, 8 kΩ, 22 kΩ, and the capacitors C1, C2, C3, and C4 are 100 pF, 150 pF, 150 pF, respectively. The control voltage Vcnt is appropriately set so that the combined capacitance of the capacitors C5, C6, and C7 (C5, C6, and C7 are fixed values) and the variable capacitance diode D1 is 60 pF with 150 pF and the inductance L1 of 0.3 μH. Yes. As can be seen from FIG. 2, the peak of the negative resistance is about 20 MHz, and the crystal oscillator oscillates at the C mode frequency of 20 MHz. On the other hand, the negative resistance is almost zero at the B-mode frequency (21.8 MHz to 22 MHz) located about 9% to 10% higher than the C-mode frequency 20 MHz, and it can be seen that the B-mode does not oscillate.

FIG. 3 shows this negative resistance when the control voltage Vcnt applied to the variable capacitance diode D1 is changed. The other conditions are the same as those in the example of FIG. Here, the control voltage Vcnt is appropriately set so that the combined capacitance of the capacitors C5, C6, C7 and the variable capacitance diode D1 is 20 pF. From FIG. 3, the peak of the negative resistance is about 22 MHz, and the negative resistance at the frequency of 20 MHz in the C mode shows −22Ω. Therefore, it can be seen that oscillation does not occur in the C mode but only in the B mode.
As described above, according to the present invention, the capacitances C1, C2, C3, C4, C5, C6, C7 and the inductance L1 are fixed values, and the control voltage Vcnt of the variable capacitance diode D1 is appropriately set, so that One of the C mode and the B mode can be selectively oscillated by changing the band of the band elimination filter and changing the negative resistance characteristic of the oscillation circuit as a result.

  FIG. 4 is a circuit showing another embodiment of a high stability crystal oscillator capable of oscillating in both modes according to the present invention. The difference from FIG. A series circuit of an inductance L1 and a capacitor C5 is connected between the connection points of the capacitors C3 and C4. Further, a variable capacitor diode D1 is connected in parallel to the capacitor C5, and one end of a resistor R5 is connected to the cathode of the variable capacitor diode D1. And the other end to the control voltage Vcnt. The oscillation circuit is configured by connecting one end of the resistor R6 to the anode of the variable capacitance diode D1 and grounding the other end. The capacitors C1, C2, C3, C4, and C5, the inductance L1, and the variable capacitance diode D1 form a band pass filter in the oscillation loop, and have a negative resistance characteristic having a selection characteristic. That is, if the control voltage Vcnt is set to a predetermined V1, the C mode can be oscillated, and if the control voltage Vcnt is set to a predetermined V2, the B mode can be oscillated.

1 and 4, the anode side of the variable capacitance diode is grounded via a resistor, and a positive voltage is applied to the cathode side via a resistor to apply a reverse bias to the variable capacitance diode. The reverse bias may be applied by connecting the cathode side to a constant power source and applying a control voltage lower than the power source to the anode side. Alternatively, the power supply may be connected to the cathode through a resistor, and a control voltage lower than the voltage on the cathode side may be applied to the anode side.
In the above description, the variable capacitance diode is used. However, an electronic element whose capacitance value can be changed by changing the voltage may be used.

1 is a circuit showing an embodiment of a dual-mode oscillation highly stable crystal oscillator according to the present invention. This is the negative characteristic of the oscillation circuit of FIG. 1 when the control voltage Vcnt is set to V1. It is the oscillation circuit negative characteristic of FIG. 1 when the control voltage Vcnt is set to V2. It is a circuit which shows embodiment of the other both mode oscillation highly stable crystal oscillator based on this invention. This is a resonance characteristic of the SC cut crystal resonator. This is a highly stable crystal oscillator circuit that oscillates only the C mode. This is a circuit of a Colpitts crystal oscillator. It is a figure explaining the negative resistance of a highly stable crystal oscillator. This is a highly stable crystal oscillator circuit that oscillates only the C mode. This is a highly stable crystal oscillator circuit that oscillates only the C mode. It is a figure which shows the change of a negative resistance characteristic when the inductance L1 of the circuit of FIG. 10 is changed. This is a highly stable crystal oscillator circuit that oscillates only the C mode. It is a block diagram of the crystal oscillator which compensates the oscillation frequency of C mode by making B mode into a temperature sensor. This is a circuit of a C-mode and B-mode oscillation crystal oscillator.

Explanation of symbols

Y1 Quartz Crystal Tr Transistors R1, R2, R3, R4, R5, R6 Resistors C1, C2, C3, C4, C5, C6, C7 Capacitance Cv Variable Capacitance L1 Inductance D1 Variable Capacitance Diode
Vc Power supply voltage Vcnt Control voltage

Claims (3)

In a Colpitts-type crystal oscillator including a transistor, an SC-cut crystal resonator, and a series circuit of first and second capacitors,
A series circuit of third and fourth capacitors is connected between the emitter of the transistor and ground, and between the connection point of the first and second capacitors and the connection point of the third and fourth capacitors. , Connect a parallel circuit of the inductance and the fifth capacitor,
A series circuit of a sixth capacitor, a variable capacitance diode, and a seventh capacitor is connected in parallel to the fifth capacitor;
One terminal of a first resistor is connected to the cathode of the variable capacitance diode, and one terminal of a second resistor is connected to the anode of the variable capacitance diode, the other terminal of the first resistor; And a crystal oscillator, wherein either the control voltage or the fixed voltage is exclusively supplied to the other terminal of the second resistor. In a Colpitts oscillator including a transistor, an SC cut crystal resonator, and a series circuit of first and second capacitors,
A third and fourth capacitor series circuit is connected between the emitter of the transistor and ground, and between the connection point of the first and second capacitors and the connection point of the third and fourth capacitors. , Connecting a series circuit of an inductance and a fifth capacitor,
A variable capacitance diode is connected in parallel to the fifth capacitor,
One terminal of a first resistor is connected to the cathode of the variable capacitance diode, and one terminal of a second resistor is connected to the anode of the variable capacitance diode, the other terminal of the first resistor; And a crystal oscillator, wherein either the control voltage or the fixed voltage is exclusively supplied to the other terminal of the second resistor. The crystal oscillator according to claim 1, wherein the fixed voltage is a ground voltage.

Images