JP5055648B2 - Voltage controlled crystal oscillator - Google Patents

Description

  The present invention relates to a voltage controlled crystal oscillator that applies a frequency control voltage to control an oscillation frequency.

  In a conventional voltage controlled crystal oscillator (VCXO), for example, as shown in FIG. 7, one terminal of DC cut capacitors 103 and 104 is connected to an input terminal and an output terminal of an inverting amplifier 101, respectively. The other terminal is connected to the terminals on both sides of the crystal unit 102 (see Patent Document 1). In this patent document 1, the quartz crystal vibrator 102 is described as a piezoelectric vibrator. One terminal of each of the voltage variable capacitance elements 107 and 108 is connected to the other terminal of the DC cut capacitors 103 and 104 and each terminal on both sides of the crystal unit 102. The other terminals of the voltage variable capacitance elements 107 and 108 are connected to the ground potential (GND). The inverting amplifier 101 has a feedback resistor 109 connected in parallel between its input and output terminals. The frequency control voltage is input via a bias resistor 106 between a connection portion between one terminal of the crystal unit 102 and the other terminal of the DC cut capacitor 104 and one terminal of the voltage variable capacitance element 108. The voltage is input between the connection between the other terminal of the crystal unit 102 and the other terminal of the DC cut capacitor 103 and one terminal of the voltage variable capacitor 107 through the bias resistor 105. This voltage crystal oscillator has an improved frequency variable width as compared with the conventional one without increasing the circuit area.

  The crystal oscillation circuit constituting the voltage controlled crystal oscillator is one in which the crystal resonator forms a resonance loop with the load capacitance, and the inverting amplifier compensates for the loss in the resonance loop to continue the oscillation. . The resonance loop in the circuit of FIG. 7 is a loop connecting the crystal resonator 102, the DC cut capacitors 103 and 104, and the voltage variable capacitance elements 107 and 108, and this oscillation is obtained by adding a parasitic capacitance to the series connection of these capacitors. This is the load capacity in the circuit.

  The relationship between the frequency control voltage VC (V) and the frequency change Δf (ppm) of this voltage controlled crystal oscillator is shown by the characteristic line a in FIG. 2 and the characteristic line b in FIG. A characteristic line “a” in FIG. 2 indicates a relationship between a frequency control voltage and a frequency change amount of a voltage controlled crystal oscillator provided with a protection element (not shown) that is usually provided to protect an IC or a device from electrostatic discharge (ESD). As the frequency control voltage VC increases, the frequency change Δf increases in proportion to VC. However, when the frequency control voltage VC is near the power supply voltage (3 V in this example), the frequency control voltage VC to be applied is shown. There is a flat portion where the frequency does not change even if is changed. This is because when the frequency control voltage VC is increased, the capacitance value decreases due to the electrical characteristics of the voltage variable capacitance element, the oscillation amplitude increases, and the anode of the protection element (protection diode) on the VDD side has a voltage higher than the power supply voltage. This occurs because a forward current flows through the protection element and the voltage actually applied to the cathode side of the voltage variable capacitance element becomes lower than the frequency control voltage VC.

On the other hand, the characteristic line b shows the relationship between the frequency control voltage of the voltage controlled crystal oscillator and the amount of frequency change when the above-described protection element is not provided and the oscillation amplitude is already large when the frequency control voltage VC is around 0V. When the frequency control voltage VC is around 0 V, the voltage variable capacitance element acts as a protection element and a forward current flows, and the voltage applied to the cathode side of the voltage variable capacitance element is higher than the frequency control voltage VC. Even if the frequency control voltage VC to be changed is changed, there is a flat portion where the frequency does not change. The voltage controlled crystal oscillator needs to change the frequency accurately according to the frequency control voltage. That is, the characteristic lines shown in FIGS. 2 and 4 are preferably straight lines.
JP 2002-26660 A

  The present invention has been made under such circumstances, and provides a voltage controlled crystal oscillator that enables accurate voltage control of the oscillation frequency by ensuring the linearity of the voltage control characteristic of the oscillation frequency.

  According to one aspect of the voltage controlled crystal oscillator of the present invention, an inverting amplifier, a crystal resonator, one terminal is connected to an input terminal of the inverting amplifier, and the other terminal is a first terminal of the crystal resonator. And a first DC cut capacitor connected to the output terminal of the inverting amplifier and a second DC cut capacitor connected to the second terminal of the crystal resonator. A control voltage input terminal; one terminal connected to the first terminal; the other terminal connected to a ground potential; and one terminal connected to the second terminal. One end of the second voltage variable capacitance element, the other terminal of which is connected to the ground potential, and the first terminal and one terminal of the first voltage variable capacitance element. A first bias resistor having an end connected to the control voltage input terminal A second bias resistor having one end connected between the second terminal and one terminal of the second voltage variable capacitance element and the other end connected to the control voltage input terminal; A first oscillation characteristic adjusting resistor is provided between one terminal of the first voltage variable capacitance element and a power supply potential, and a second oscillation characteristic is provided between the one terminal of the second voltage variable capacitance element and the power supply potential. An adjustment resistor is provided.

  The first and second oscillation characteristic adjusting resistors are composed of a plurality of divided resistors, and a switch connected in parallel to each of the divided resistors is provided between both terminals of the divided resistors. The resistance values of the first and second oscillation characteristic adjusting resistors may be controlled by the switch. The first and second bias resistors are composed of a plurality of divided resistors, and a switch connected in parallel to each of the divided resistors is provided between both terminals of the divided resistors, The resistance values of the first and second bias resistors may be controlled by a switch.

  The present invention can ensure the linearity of the voltage control characteristic of the oscillation frequency by inserting a resistor for adjusting the oscillation characteristic into the voltage controlled crystal oscillator, thereby enabling accurate voltage control of the oscillation frequency. .

  Hereinafter, embodiments of the invention will be described with reference to examples.

First, Embodiment 1 will be described with reference to FIGS.
FIG. 1 is a circuit diagram of a voltage controlled crystal oscillator for explaining this embodiment, and FIG. 2 is a characteristic diagram showing the relationship between the frequency variation and the frequency control voltage for explaining the oscillation characteristics of the voltage controlled crystal oscillator of FIG. . In the voltage controlled crystal oscillator of this embodiment, as shown in FIG. 1, one terminal of the first DC cut capacitor 4 is connected to the input terminal of the inverting amplifier 1, and one terminal of the second DC cut capacitor 3 is connected. Is connected to the output terminal of the inverting amplifier 1, the other terminal of the first DC cut capacitor 4 is connected to the first terminal of the crystal resonator 2, and the other terminal of the second DC cut capacitor 3 is crystal oscillation. The second terminal of the child 2 is connected. One terminal of the first voltage variable capacitance element 8 is connected to a connection portion (first connection portion) in which the other terminal of the first DC cut capacitor 4 and the first terminal of the crystal unit 2 are connected. Has been. One terminal of the second voltage variable capacitance element 7 is connected to a connection portion (second connection portion) in which the other terminal of the second DC cut capacitor 3 and the second terminal of the crystal resonator 2 are connected. Has been. The other terminals of the first voltage variable capacitor 8 and the second voltage variable capacitor 7 are connected to the ground potential (VSS). The inverting amplifier 1 has a feedback resistor 9 connected in parallel between its input and output terminals.

  The control voltage input terminal 12 to which the frequency control voltage is applied is connected to the first terminal of the crystal resonator 2 and the other terminal of the first DC cut capacitor 4 via the first bias resistor 6 ( A first connecting portion) and one terminal of the first voltage control variable capacitance element 8, and the second terminal of the crystal resonator 2 and the second terminal are connected via the second bias resistor 5. The DC cut capacitor 3 is connected between a connection portion (second connection portion) with the other terminal and one terminal of the second voltage variable capacitance element 7.

This voltage controlled crystal oscillator is different from the conventional one described above in that an oscillation characteristic adjusting resistor is incorporated. The oscillation characteristic adjusting resistors are connected in parallel to the first voltage variable capacitance element 8 and the second voltage variable capacitance element 7, respectively. The first oscillation characteristic adjusting resistor (R1) 11 has one end connected to one terminal (cathode) of the first voltage variable capacitance element 8 and the other end connected to the ground potential (VSS). The second oscillation characteristic adjusting resistor (R2) 10 has one end connected to one terminal (cathode) of the second voltage variable capacitance element 7 and the other end connected to the ground potential (VSS).

  The relationship between the frequency control voltage VC (V) applied to the control voltage input terminal 12 of the voltage controlled crystal oscillator and the oscillation frequency variation Δf (ppm) is as shown in FIG. The vertical axis in FIG. 2 represents the oscillation frequency change amount Δf (ppm), and the horizontal axis represents the frequency control voltage VC (V). In this embodiment, for example, the first oscillation characteristic adjusting resistor 11 and the second oscillation characteristic adjusting resistor 10 are each 750 kΩ, and the first bias resistor 6 and the second bias resistor 5 are each 150 kΩ. It is. The characteristic line a in FIG. 2 shows the frequency characteristics of the conventional voltage controlled crystal oscillator, and the characteristic line A shows the first oscillation characteristic adjusting resistor 11 and the second oscillation characteristic adjusting resistor 10 in the voltage controlled crystal oscillator. The frequency characteristics of the voltage controlled crystal oscillator of this embodiment to which is added are shown.

  When the oscillation frequency is changed by the frequency control voltage, the conventional voltage controlled crystal oscillator increases the oscillation amplitude when the control voltage applied to the control voltage input terminal is increased, and the crystal oscillator is used to suppress ESD. A voltage higher than the power supply voltage is applied to the anode of the protection element (protection diode, not shown) on the power supply voltage (VDD) side provided at the connection terminal, and a forward current flows through the protection element, and the voltage variable capacitance element has a cathode side. Since the voltage actually applied is lower than the frequency control voltage VC, the frequency variable amount decreases when the applied frequency control voltage VC is near the power supply voltage, and the characteristic line a becomes flat. Therefore, in this embodiment of the present invention, by adding the oscillation characteristic adjusting resistor, the voltage applied to the voltage variable capacitance element can be reduced to a voltage equal to or lower than the voltage at which the VDD side protection diode is turned on. Linearity with respect to the control voltage is maintained.

  The characteristic of the frequency change due to the application of the control voltage is determined by the oscillation characteristic adjustment resistance value or the bias resistance / oscillation characteristic adjustment resistance ratio. For example, when the bias resistance is 150 kΩ and the oscillation characteristic adjusting resistance is 750 kΩ, the linearity characteristic indicated by the characteristic line A in FIG. If the resistance for adjusting the oscillation characteristics is smaller, for example, about 50 kΩ, the frequency change rate becomes small as shown by the characteristic line C in FIG. Thus, by appropriately selecting the oscillation characteristic adjusting resistance value according to the characteristics of the voltage controlled crystal oscillator, a voltage controlled crystal oscillator having desired characteristics can be obtained.

As described above, in this embodiment, by using the oscillation characteristic adjusting resistor connected in parallel to the voltage variable capacitance element, the linearity of the voltage control characteristic of the oscillation frequency can be ensured, and the accurate voltage control of the oscillation frequency can be ensured. Can be made possible.
The voltage controlled crystal oscillator shown in FIG. 1 has a structure in which a voltage variable capacitance element is provided on either side of the input / output of the inverting amplifier. However, the present invention can be configured with a fixed capacitor on either side. In this case, of course, an oscillation characteristic adjusting resistor may be provided only on the side where the voltage variable capacitance element is present.

Next, Embodiment 2 will be described with reference to FIGS.
FIG. 3 is a circuit diagram of the voltage controlled crystal oscillator for explaining this embodiment, and FIG. 4 is a characteristic diagram showing the relationship between the frequency variation and the frequency control voltage for explaining the oscillation characteristics of the voltage controlled crystal oscillator of FIG. . In the voltage controlled crystal oscillator of this embodiment, as shown in FIG. 3, one terminal of the first DC cut capacitor 24 is connected to the input terminal of the inverting amplifier 21, and one terminal of the second DC cut capacitor 23. Is connected to the output terminal of the inverting amplifier 21, the other terminal of the first DC cut capacitor 24 is connected to the first terminal of the crystal resonator 22, and the other terminal of the second DC cut capacitor 23 is the crystal vibration. The second terminal of the child 22 is connected. One terminal of the first voltage variable capacitance element 28 is connected to a connection portion (first connection portion) where the other terminal of the first DC cut capacitor 24 and the first terminal of the crystal resonator 22 are connected. It is connected. One terminal of the second voltage variable capacitance element 27 is connected to a connection portion (second connection portion) where the other terminal of the second DC cut capacitor 23 and the second terminal of the crystal resonator 22 are connected. It is connected. The other terminals of the first voltage variable capacitor 28 and the second voltage variable capacitor 27 are connected to the ground potential (VSS). The inverting amplifier 21 has a feedback resistor 29 connected in parallel between its input and output terminals.

  The control voltage input terminal 32 to which the frequency control voltage is applied is connected to the first terminal of the crystal resonator 22 and the other terminal of the first DC cut capacitor 24 via the first bias resistor 26 ( The first connection portion) and one terminal of the first voltage control variable capacitance element 28, and the second terminal of the crystal resonator 22 and the second voltage are connected via the second bias resistor 25. The DC cut capacitor 23 is connected between a connection portion (second connection portion) with the other terminal and one terminal of the second voltage variable capacitance element 27.

This voltage controlled crystal oscillator incorporates an oscillation characteristic adjusting resistor, but the connection configuration is different from that of the first embodiment. The first oscillation characteristic adjusting resistor (R3) 31 has one end connected to one terminal of the first voltage variable capacitor 28 and the other end connected to the power supply potential (VDD). The second oscillation characteristic adjusting resistor (R4) 30 has one end connected to one terminal of the second voltage variable capacitance element 27 and the other end connected to the power supply potential (VDD).

The relationship between the frequency control voltage VC (V) applied to the control voltage input terminal 32 of the voltage controlled crystal oscillator and the oscillation frequency variation Δf (ppm) is as shown by the characteristic line B in FIG. The vertical axis in FIG. 4 represents the oscillation frequency change Δf (ppm), and the horizontal axis represents the frequency control voltage VC (V). In this embodiment, for example, the first oscillation characteristic adjusting resistor 31 and the second oscillation characteristic adjusting resistor 30 are each 750 kΩ, and the first bias resistor 26 and the second bias resistor 25 are each 150 kΩ. It is. The characteristic line b in FIG. 4 shows the frequency characteristic of the conventional voltage controlled crystal oscillator.
The characteristic line b shows the relationship between the frequency control voltage and the amount of change in frequency when the oscillation voltage is already large when the frequency control voltage VC is near 0 V in the conventional voltage-controlled crystal oscillator without the above-described protection element. As shown in the figure, when the frequency control voltage VC is around 0 V, the voltage variable capacitance element acts as a protection element, a forward current flows, and the voltage applied to the cathode side of the voltage variable capacitance element becomes higher than the frequency control voltage VC. The frequency variable amount decreases and the characteristic line b becomes flat.
Therefore, according to the second embodiment of the present invention, the voltage applied to the voltage variable capacitance element is increased by adding the first oscillation characteristic adjusting resistor and the second oscillation characteristic adjusting resistor described above. The forward current no longer flows through the variable capacitance element, and linearity with respect to the applied control voltage with a variable amount of frequency can be ensured.

The characteristic of the frequency change due to the application of the control voltage is determined by the oscillation characteristic adjustment resistance value or the bias resistance / oscillation characteristic adjustment resistance ratio. For example, when the bias resistance is 150 kΩ and the oscillation characteristic adjusting resistance is 750 kΩ, the linearity characteristic shown by the characteristic line B in FIG. 4 is shown. If the oscillation characteristic adjusting resistor is further reduced, the frequency change rate is reduced. Thus, by appropriately selecting the oscillation characteristic adjusting resistance value according to the characteristics of the voltage controlled crystal oscillator, a voltage controlled crystal oscillator having desired characteristics can be obtained.

As described above, in this embodiment, the linearity of the oscillation frequency voltage control characteristic can be ensured by using the oscillation characteristic adjusting resistor connected between one terminal of the voltage variable capacitance element and the power supply potential. Therefore, accurate voltage control of the oscillation frequency can be made possible.
The voltage controlled crystal oscillator shown in FIG. 3 has a structure in which a voltage variable capacitance element is provided on either input or output side of the inverting amplifier. However, in the present invention, a configuration in which one side is a fixed capacitor is also possible. In this case, of course, an oscillation characteristic adjusting resistor may be provided only on the side where the voltage variable capacitance element is present.

Next, Example 3 will be described with reference to FIG.
FIG. 5 is a partial circuit diagram including an oscillation characteristic adjusting resistor included in the basic configuration of the voltage controlled crystal oscillator according to this embodiment. This embodiment is a modification of the voltage controlled crystal oscillator of FIG. That is, the voltage controlled crystal oscillator of FIG. 5 is characterized in that the oscillation characteristic adjusting resistor is configured by a plurality of divided resistors and the resistance value is variable. This voltage controlled crystal oscillator includes a crystal resonator 40, an inverting amplifier 33, a DC cut capacitor 34, a control voltage input terminal 35, a bias resistor 36, an oscillation characteristic adjusting resistor 37, and a voltage variable capacitance element 39. A voltage variable capacitance element 39 is connected in series to the control voltage input terminal 35 via a bias resistor 36, and an oscillation characteristic adjusting resistor 37 is connected in parallel to the voltage variable capacitance element 39. The oscillation characteristic adjusting resistor 37 has one end connected to one terminal of the voltage variable capacitance element 39 and the other end grounded. The crystal resonator 40 is connected to the inverting amplifier 33 via the DC cut capacitor 34 and is connected to one terminal of the voltage variable capacitor 39.

  The oscillation characteristic adjusting resistor 37 includes, for example, divided resistors R5, R6, and R12 having a switch 38 and a divided resistor R11. The switch 38 includes switches SW1, SW2, SW3, and SW4. Switches SW2, SW3, and SW1 are connected in parallel to the respective divided resistors R5, R6, and R12, and the respective divided resistors are connected to and separated from the oscillation circuit by the on / off operation of these switches. It has become. Further, the switch SW4 is configured to remove the oscillation characteristic adjusting resistor 37 itself from the oscillation circuit by turning off the switch SW4, and this configuration is the conventional voltage controlled crystal oscillator shown in FIG.

As described above, in this embodiment, the oscillation characteristic adjusting resistor connected in parallel to the voltage variable capacitance element is constituted by a plurality of divided resistors, and the oscillation characteristic adjusting resistor is controlled by controlling the switch provided in each divided resistor. By making the value variable, it is possible to obtain a voltage control characteristic of a suitable oscillation frequency according to the characteristic of the crystal resonator in combination with the bias resistor.

Next, Embodiment 4 will be described with reference to FIG.
FIG. 6 is a partial circuit diagram including a bias resistor included in the basic configuration of the voltage controlled crystal oscillator according to this embodiment. This embodiment is a modification of the voltage controlled crystal oscillator of FIG. That is, the voltage controlled crystal oscillator of FIG. 6 is characterized in that the bias resistor is constituted by a plurality of divided resistors and the resistance value is variable. The voltage controlled crystal oscillator includes a crystal resonator 41, an inverting amplifier 43, a DC cut capacitor 44, a control voltage input terminal 45, a bias resistor 46, an oscillation characteristic adjusting resistor 47, and a voltage variable capacitor element 49. A voltage variable capacitance element 49 is connected in series to the control voltage input terminal 45 via a bias resistor 46, and an oscillation characteristic adjusting resistor 47 is connected in parallel to the voltage variable capacitance element 49. The oscillation characteristic adjusting resistor 47 has one end connected to one terminal of the voltage variable capacitance element 49 and the other end grounded. The crystal resonator 41 is connected to the inverting amplifier 43 via the DC cut capacitor 44 and is connected to one terminal of the voltage variable capacitance element 49.

  The bias resistor 46 includes, for example, divided resistors R8, R9, and R10 having a switch 48 and a divided resistor R13. The switch 48 includes switches SW5, SW6, and SW7. Switches SW5, SW6 and SW7 are connected in parallel to the respective divided resistors R8, R9 and R10, and the respective divided resistors are connected to and separated from the oscillation circuit by the on / off operation of these switches. It has become. For example, when the bias resistor 46 has R8 = 50Ω, R9 = 50Ω, R10 = 50Ω, and R12 = 50Ω, a variable resistance value of 50Ω to 200Ω can be obtained by combining the ON / OFF operation of the switch 48. .

  As described above, in this embodiment, the bias resistor is configured by a plurality of divided resistors, and the bias resistance value can be varied by controlling a switch provided in each divided resistor. In combination, it is possible to obtain voltage control characteristics with a suitable oscillation frequency according to the characteristics of the crystal resonator. The voltage-controlled crystal oscillator of this embodiment may be applied to the voltage-controlled crystal oscillator including the oscillation characteristic adjusting resistor configured by a plurality of division resistors and a plurality of switches shown in the third embodiment.

1 is a circuit diagram showing a basic configuration of a voltage controlled crystal oscillator according to Embodiment 1 of the present invention. The characteristic view which shows the relationship of the frequency variation-frequency control voltage explaining the oscillation characteristic of the voltage control crystal oscillator of FIG. FIG. 5 is a circuit diagram showing a basic configuration of a voltage controlled crystal oscillator according to a second embodiment of the invention. FIG. 4 is a characteristic diagram illustrating a relationship between a frequency change amount and a frequency control voltage for explaining oscillation characteristics of the voltage controlled crystal oscillator of FIG. 3. FIG. 10 is a partial circuit diagram including an oscillation characteristic adjusting resistor included in the basic configuration of the voltage controlled crystal oscillator according to the third embodiment of the present invention. FIG. 9 is a partial circuit diagram including a bias resistor included in the basic configuration of the voltage controlled crystal oscillator according to the fourth embodiment of the present invention. The circuit diagram which shows the basic composition of the conventional voltage control crystal oscillator.

Explanation of symbols

1, 2, 33, 43... Inverting amplifier 2, 22, 40, 41... Crystal resonator 3, 4, 23, 24, 34, 44... DC cut capacity 5, 6, 25, 26, 36, 46... Bias resistor 7, 8, 27, 28, 39, 49... Voltage variable capacitance element 9, 29... Feedback resistor 10, 11, 30, 31, 37, 47. Adjustment resistor 12, 32, 35, 45 ... Control voltage input terminal 38, 48 ... Switch

Claims (3)

An inverting amplifier, a crystal resonator, one terminal connected to the input terminal of the inverting amplifier, and the other terminal connected to the first terminal of the crystal resonator, A second DC cut capacitor having a terminal connected to the output terminal of the inverting amplifier and the other terminal connected to the second terminal of the crystal resonator, a control voltage input terminal, and one terminal being the first terminal A first voltage variable capacitor having a second terminal connected to the ground potential, a first terminal connected to the second terminal, and a second terminal connected to the ground potential. One of the first voltage variable capacitance element, the first terminal and one terminal of the first voltage variable capacitance element, and the other end connected to the control voltage input terminal. A bias resistor; the second terminal; and the second voltage variable capacitance element. And a second bias resistor having one end connected to the control voltage input terminal and the other end connected to the control voltage input terminal. One terminal of the first voltage variable capacitance element and a power supply potential A voltage controlled crystal oscillator characterized in that a first oscillation characteristic adjusting resistor is provided between them, and a second oscillation characteristic adjusting resistor is provided between one terminal of the second voltage variable capacitance element and a power supply potential. . The first and second oscillation characteristic adjusting resistors are composed of a plurality of divided resistors, and a switch connected in parallel to each of the divided resistors is provided between both terminals of the divided resistors. 2. The voltage controlled crystal oscillator according to claim 1 , wherein resistance values of the first and second oscillation characteristic adjusting resistors are controlled by the switch. The first and second bias resistors are composed of a plurality of divided resistors, and a switch connected in parallel to each of the divided resistors is provided between both terminals of the divided resistors, 3. The voltage controlled crystal oscillator according to claim 1, wherein resistance values of the first and second bias resistors are controlled by a switch.

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