JP2008252768A - Crystal oscillator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem that, in a conventional oscillation circuit, fixed large capacitance is required between voltage variable capacitance and a power supply voltage, thus hindering reduction in size, and large parasitic capacitance is inserted in parallel with the voltage variable capacitance, thus making it difficult to obtain a wide frequency variable range. <P>SOLUTION: A feedback resistance, an inverted amplification circuit, and a crystal oscillator are connected in parallel. Connection points of both an input and an output of the inverted amplification circuit are grounded via direct-current cut capacitance and load capacitance. Further, the connection points of both the input and the output of the direct-current cut capacitance and the load capacitance are connected via the voltage variable capacitance. Frequency adjustment is performed by applying a control signal between both ends of the voltage variable capacitance. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a crystal oscillator including a voltage variable capacitor as a part of a load capacitor and capable of frequency control by applying a control signal to the voltage variable capacitor.

  A crystal oscillator in which a crystal resonator and an oscillation circuit are housed in a single package is used as a signal source for a wide range of electronic devices due to its high stability and small size.

  Some crystal oscillators have a voltage variable capacitor as part of the load capacitance and have a mechanism for controlling the oscillation frequency by changing the control voltage applied to the voltage variable capacitor. Depending on the purpose of the control, the voltage controlled crystal oscillator (Hereinafter abbreviated as VCXO) and temperature compensated crystal oscillator (hereinafter abbreviated as TCXO).

  The VCXO is a type of oscillator in which a user applies a control voltage applied to a voltage variable capacitor from the outside, and is frequently used for applications such as clock recovery in wired / wireless communication. The TCXO is an oscillator that keeps the frequency of the output signal substantially constant over a wide temperature range by giving the voltage variable capacitor a control signal that compensates the frequency-temperature characteristics of the crystal unit. Many are used. An oscillator (VC-TCXO) having both of these characteristics has also been commercialized.

  Although it is well known that the oscillation frequency changes by changing the capacitance value of the load capacitance in a crystal oscillation circuit, VCXO and TCXO use a voltage variable capacitor as a part of this load capacitance, and control this voltage variable capacitance. The frequency is controlled by applying a voltage. As the voltage variable capacitor, there are a varicap diode, a MOS variable capacitor, and the like, all of which have a feature that the capacitance value changes due to a voltage difference between both ends of the element.

  Since the characteristics of the voltage variable capacitance are determined by the semiconductor process, there is a case where the voltage range in which the capacitance changes greatly does not match the voltage range used in the TCXO. In this case, there is a problem that a sufficient capacitance variable width cannot be obtained. It was.

In order to solve such a problem, a crystal oscillator has been proposed in which a voltage range applied to a variable capacitor can be set freely (see Patent Document 1). The basic configuration is shown in FIG. The feedback resistor 503, the inverting amplifier circuit 502, and the crystal resonator 501 are connected in parallel, and the connection points on both sides of the input / output are connected via the DC cut capacitors 504 and 505 and the voltage variable capacitors 512 and 513, respectively. The connection point is grounded via the stabilization capacitor 514.
Japanese Patent Laid-Open No. 11-17114

  In the circuit of FIG. 5, a voltage range that can be applied to the voltage variable capacitors 512 and 513 is obtained by adopting a configuration in which the control signal that has been applied only from one side of the conventional voltage variable capacitor can be applied from both sides of the variable capacitors 512 and 513. The applied voltage range can be selected in accordance with the optimum voltage of the voltage variable capacitor.

  In TCXO, in addition to the frequency deviation correction control signal due to temperature, room temperature frequency deviation correction may be required, and in addition, the VC-TCXO described above requires an external input frequency adjustment signal. In such applications that require a plurality of control signals, different control signals can be applied to the control signal inputs 1, 2, and 3. Therefore, there is an advantage that it is not necessary to provide a circuit for adding the plurality of control signals. is there.

  However, in the circuit of FIG. 5, it is theoretically possible to apply different control signals to the control signal inputs 1, 2, and 3, but in reality, if there is a large difference between the combined capacitance values on the input side and the output side, Since adverse effects such as unstable oscillation occur, the same control signal is generally applied to the control signal inputs 1 and 2. For example, a temperature correction signal is input to the control signal inputs 1 and 2, and an external input frequency adjustment signal and a combined signal of normal temperature frequency deviation correction are input to the control signal input 3.

  However, the configuration of Patent Document 1 has the following drawbacks. One is that a stabilizing capacitor 514 having a large capacitance value is required between the voltage variable capacitors 512 and 513 and GND. In order for the inverting amplifier circuit 502 to exhibit a sufficient amplification factor at the time of oscillation start-up, the oscillation loop needs to be grounded with a low impedance in terms of alternating current, and thus a large capacity is required as the stabilization capacitor 514. In recent years, VCXO and TCXO circuits have been integrated and contributed to the reduction in package size. However, the fact that such a large capacity must be built in the integrated circuit is a great burden for miniaturization. .

  In addition, in order to obtain a frequency variable range as large as possible in VCXO and TCXO, a circuit configuration in which the voltage variable capacitance is not easily affected by the parasitic capacitance is desired. In the circuit of FIG. Since a large parasitic capacitance such as the 501 connection terminal capacitance is inserted in parallel with the voltage variable capacitors 512 and 513, the variable variable width of the entire load capacitor is limited, and it is difficult to obtain a large variable frequency width. There is also.

  In order to solve the above problem, a crystal oscillator according to the present invention includes at least a crystal resonator and an oscillation circuit, and the oscillation circuit connects the crystal resonator, a feedback resistor, and an inverting amplifier circuit in parallel, and the inverting amplifier circuit. The connection points on both sides of the input / output are grounded via a DC cut capacitor and a load capacitor, respectively, and further, the connection point between the DC cut capacitor and the load capacitor on the input side of the inverting amplifier circuit, and the DC cut on the output side A connection point between the capacitor and the load capacitor is connected via a voltage variable capacitor, and the frequency adjustment is performed by applying a control signal to both ends of the voltage variable capacitor.

  As another configuration, it is composed of at least a crystal resonator and an oscillation circuit, and the oscillation circuit connects the crystal resonator, a feedback resistor and an inverting amplifier circuit in parallel, and connection points on both input and output sides of the previous inverting amplifier circuit. Are connected to each other through a DC cut capacitor and a load capacitor, and further, a connection point between the DC cut capacitor and the load capacitor on the input side of the inverting amplifier circuit, and a connection point between the DC cut capacitor and the load capacitor on the output side. Are connected via two voltage variable capacitors connected in series, the connection point of the two voltage variable capacitors connected in series, and the DC cut capacitor and the voltage variable on both input and output sides of the inverting amplifier circuit. It is characterized in that the frequency adjustment is performed by applying a control signal to three connection points with the capacitor.

  In the crystal oscillator of the present invention, since the voltage variable capacitor is connected in parallel with the crystal resonator, the parasitic capacitance is not inserted in parallel with the voltage variable capacitor, and the voltage variable capacitor is not easily affected by the parasitic capacitance. ing. For this reason, it is possible to obtain a large frequency variable width as compared with the conventional product.

  In addition, since the stabilization capacitor that required a large capacitance value is not necessary, the area of the integrated circuit can be reduced, and a product that is smaller than the conventional product can be provided.

  Hereinafter, the best mode for carrying out the present invention will be specifically described with reference to the drawings.

[First Embodiment: FIG. 1]
FIG. 1 is a circuit diagram showing a configuration of a first embodiment of a crystal oscillator according to the present invention. In the circuit of FIG. 1, a feedback resistor 103, an inverting amplifier circuit 102, and a crystal resonator 101 are connected in parallel, and connection points on both input and output sides of the inverting amplifier circuit 102 are connected to DC cut capacitors 104 and 105 and load capacitors 107 and 108, respectively. To ground. Further, a connection point between the input side DC cut capacitor 104 and the load capacitor 107 and a connection point between the output side DC cut capacitor 105 and the load capacitor 108 are connected via a voltage variable capacitor 106, and control is performed at both ends of the voltage variable capacitor 106. The frequency adjustment is performed by applying a signal through resistors 109 and 110. In order to secure the oscillation phase condition, a limiting resistor may be inserted between the output of the inverting amplifier circuit 102 and the crystal resonator 101.

  An actual VCXO or TCXO is configured by sealing the crystal unit 101 in FIG. 1 and an integrated circuit including the circuit in FIG. 1 in a package such as ceramic.

  A problem with the prior art is that a large stabilization capacitor is required between the voltage variable capacitor and GND, which makes it difficult to reduce the size. However, the crystal oscillator according to the present invention does not require the stabilization capacitor itself. Such a problem does not occur. Since the load capacitors 107 and 108 are not intended to keep the impedance low like the stabilization capacitors, a large capacitance value is not required unlike the stabilization capacitors.

  In addition, in the prior art, since a large parasitic capacitance such as an input / output capacitance of the inverting amplifier circuit or a crystal resonator connection terminal capacitance is inserted in parallel with the voltage variable capacitance, there is a problem that it is difficult to obtain a large frequency variable width. However, since the voltage variable capacitor 106 is connected in parallel with the crystal resonator 101 in the crystal oscillator of the present invention, the parasitic capacitance is not inserted in parallel with the voltage variable capacitor 106. The capacity variable width is not easily limited.

  In the present invention, the control voltage applied to both ends of the variable capacitor 106 can be flexibly selected because the control voltage can be applied from both sides of the voltage variable capacitor 106, as in the prior art. For example, as shown in FIG. 3, when the voltage change characteristic of the voltage variable capacitor is such that the voltage between the terminals spreads from minus to plus across the zero volt point, the variable capacitor can be applied only in one side. However, according to the present invention, it is possible to use the entire region where the capacitance is variable by setting the voltage range of the control signals 1 and 2 to an appropriate value.

  Since the capacitance value of the voltage variable capacitor at normal temperature is desirably near the center of the change range, the voltage of the control signal may be adjusted so that the voltage difference between the control signals 1 and 2 is in the vicinity thereof.

  In the case of VCXO, since the control signal is only an external input voltage, this external input voltage is input to either of the control signals 1 and 2 and the other is set to a fixed voltage. If the integrated circuit has a built-in memory and the fixed voltage value can be adjusted according to the value written in this memory, the fixed voltage value with the widest frequency variable width can be obtained by changing the memory value after the oscillator is completed. You can choose. Alternatively, in applications where the initial frequency accuracy is severe, the fixed voltage variable configuration using the memory may be used as the initial frequency deviation correction.

  In addition, the control direction of the voltage vs. frequency is reversed depending on which of the control signals 1 and 2 is applied with the external input voltage, but if this can be switched by the value written in the built-in memory, the control voltage can be increased. It can be used as either type of VCXO with frequency increase or control voltage increase and frequency decrease.

  In the case of TCXO, a control signal for correcting a frequency deviation due to temperature generated by another circuit (not shown) is input to one of the control signals 1 and 2. The temperature characteristic of an AT-cut quartz crystal resonator generally used for TCXO has a third-order term and a first-order term with respect to temperature. Therefore, for example, a third-order term correction signal is assigned to the control signal input 1 and a control signal input 2 is assigned 1 A configuration may be adopted in which a correction signal for the next term is input.

  In TCXO and VC-TCXO, in addition to the temperature frequency deviation correction signal, control signals such as room temperature frequency deviation correction and external input frequency adjustment may be required. Signal may be applied to any control signal input

  FIG. 4 shows a configuration of an N-type MOS variable capacitor as an example of the voltage variable capacitor. In this variable capacitor, an N-type rich layer 403 is formed in an annular shape in an N-well region 402 provided in a semiconductor P-type substrate 401, and an insulating film 405 and an electrode 406 are provided on the upper surface thereof. Since the two voltage application terminals 1 and 2 are independent of the potential of the semiconductor P-type substrate 401, it is possible to apply a voltage from both sides of the variable capacitor.

  In the case of a P-type MOS variable capacitor, the control voltage can be applied only from one side because the potential on one side is always the substrate potential in the normal configuration, but a P well region is further provided in the N well region. If a so-called triple well structure is formed in the MOS variable capacitor, a variable capacitor to which a control voltage can be applied from both sides can be obtained.

  In the case of a varicap diode which is another voltage variable capacitor, the anode side is always a P-type semiconductor. However, by adopting a triple well structure as described above, a variable capacitor capable of applying a voltage from both sides can be obtained.

  The above is a case of a semiconductor of a P-type substrate, but the case of a semiconductor of an N-type substrate is the same if the polarity is reversed.

  The above description describes the case where the present invention is used in combination with a crystal resonator, but it is of course possible to use it in combination with a piezoelectric resonator other than the crystal resonator.

[Second Embodiment: FIG. 2]
FIG. 2 is a circuit diagram showing a second embodiment of the temperature compensated crystal oscillator according to the present invention. In the description of FIG. 2, there are many portions in common with FIG. 1, so the description of these portions will be simplified.

  In the circuit shown in FIG. 2, a feedback resistor 203, an inverting amplifier circuit 202, and a crystal resonator 201 are connected in parallel, and connection points on both input and output sides of the inverting amplifier circuit 202 are connected to DC cut capacitors 204 and 205, and a load capacitor 207, respectively. And 208 to ground. Further, the connection point between the input side DC cut capacitor 204 and the load capacitor 207 and the connection point between the output side DC cut capacitor 205 and the load capacitor 208 are connected via two voltage variable capacitors 210 and 211 connected in series to change the voltage. A control signal is applied to three frequencies: a connection point between the capacitors 210 and 211, a connection point between the input side DC cut capacitor 204 and the voltage variable capacitor 210, and a connection point between the output side DC cut capacitor 205 and the voltage variable capacitor 211. It is characterized by making adjustments.

  As described in the first embodiment, since the circuit of FIG. 1 has less influence of the parasitic capacitance on the capacitance change width of the voltage variable capacitor 106 compared to the conventional example, a large frequency variable width can be obtained. Conventionally, only one voltage variable capacitor which has been used in two is used, so that a large amount cannot be obtained as an increase amount of the variable width.

  In the circuit of FIG. 2, the voltage variable capacitor 106 of the circuit of FIG. 1 is replaced with two voltage variable capacitors 210 and 211 connected in series, and the connection point between the two voltage variable capacitors 210 and 211 is also controlled. Since the configuration is such that a signal can be applied, it is possible to dramatically increase the frequency variable width while taking full advantage of the circuit of FIG.

  In the circuit of FIG. 2, it is possible to input three different control signals 1, 2, and 3. The conventional circuit shown in FIG. 5 also has three control signal inputs. However, in order to balance the load capacity on the input side and the output side, the same signal is usually applied to the two control signal inputs as described above. . In the configuration of the present invention, the capacitance values of the voltage variable capacitors 210 and 211 are shared as both input and output load capacitances, so there is no problem that the load capacitance balance between the input side and the output side is deteriorated as in the conventional example. It is possible to apply three separate control signals to VC-TCXO or the like which requires a plurality of control signal inputs.

  In the case of VCXO, a fixed voltage is input to the control voltage input 3 and an external input control signal is input to the control voltage inputs 1 and 2, or a fixed voltage is input to the control voltage inputs 1 and 2, and an external input control is applied to the control voltage input 3. It can be used as a signal. It is of course possible to apply the frequency adjustment mechanism and control direction selection mechanism using the built-in memory described in the first embodiment to the circuit of FIG.

  If the control voltage range of the voltage variable capacitor does not cross the zero point and moves only on one side, either positive or negative, the terminal of the control voltage 3 may be grounded.

  Since the crystal oscillator of the present invention can provide a larger frequency variable width than conventional products, it is possible to provide a higher-performance oscillator and eliminate the need for the stabilization capacitance that was required in the past. This makes it possible to provide products that are smaller than conventional products.

It is a basic block diagram of the crystal oscillator of this invention. It is another block diagram of the crystal oscillator of this invention. It is an example of the voltage-capacitance characteristic between terminals of a voltage variable capacitor. It is a structural example of a MOS variable capacitor. It is a basic block diagram of the conventional crystal oscillator.

Explanation of symbols

101, 201, 501: Crystal resonators 102, 202, 502: Inversion amplifier circuits 103, 203, 503: Feedback resistors 104, 105, 204, 205, 504, 505: DC cut capacitors 106, 210, 211, 512, 513 : Voltage variable capacitance 107, 108, 207, 208: Load capacitance 109, 110: Resistance 401: Semiconductor P type substrate 402: N well region 403: N type rich layer 405: Insulating film 406: Electrode 514: Stabilization capacitance

Claims (2)

In a crystal oscillator composed of at least a crystal resonator and an oscillation circuit,
The oscillation circuit connects the crystal resonator, a feedback resistor, and an inverting amplifier circuit in parallel, and grounds connection points on both input and output sides of the inverting amplifier circuit via a DC cut capacitor and a load capacitor, respectively.
Furthermore, the connection point between the DC cut capacitor and the load capacitor on the input side of the inverting amplifier circuit, and the connection point between the DC cut capacitor and the load capacitor on the output side are connected via a voltage variable capacitor,
A crystal oscillator, wherein a frequency is adjusted by applying a control signal to both ends of the voltage variable capacitor. In a crystal oscillator composed of at least a crystal resonator and an oscillation circuit,
The oscillation circuit is configured to connect the crystal resonator, the feedback resistor, and the inverting amplifier circuit in parallel, and connect the connection points on both the input and output sides of the inverting amplifier circuit through a DC cut capacitor and a load capacitor, respectively.
Further, the connection point between the DC cut capacitor and the load capacitor on the input side of the inverting amplifier circuit and the connection point between the DC cut capacitor and the load capacitor on the output side are connected via two voltage variable capacitors connected in series. And
Frequency adjustment is performed by applying a control signal to three connection points of the two voltage variable capacitors connected in series and a connection point between the DC cut capacitor and the voltage variable capacitor on both input and output sides of the inverting amplifier circuit. A crystal oscillator characterized by performing.

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