JP2009105611A - Oscillation circuit and oscillator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an oscillation circuit and an oscillator in which negative resistance in an activation is sufficiently larger than that in a normal mode. <P>SOLUTION: The oscillation circuit for oscillating a vibrator includes: an inverter having one transistor; a feedback resistor connected to the inverter in parallel; capacitive elements provided at an input side and an output side of the inverter, respectively; a variable current source for feeding one between two types of current at different levels to the inverter; a timer circuit for counting a prescribed time at the activation of the power supply of the oscillation circuit; and a current control part for controlling the variable current source so as to feed the current at the larger level to the inverter until the prescribed time elapses in counting time by the timer circuit and feed the current at the smaller level after the prescribed time elapses between the two types current that can be fed by the variable current source. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an oscillation circuit and an oscillator that oscillate a vibrator.

  FIG. 15 is a circuit diagram showing a conventional oscillation circuit that oscillates a crystal resonator. The oscillation circuit shown in FIG. 15 includes an inverter formed of CMOS between terminals XT and XTB to which a crystal resonator (X′TAL) is connected. Further, load capacitors C110 and C111 are provided on the input side and output side of the inverter, respectively, and a feedback resistor R110 is provided in parallel with the inverter. In the oscillation circuit, the crystal resonator oscillates by an inverter to which a voltage is applied, and outputs a signal having a constant frequency from the output terminal XTB.

  FIG. 16 is also a circuit diagram showing a conventional oscillation circuit that oscillates a crystal resonator. The oscillation circuit shown in FIG. 16 includes an inverter composed of a MOS transistor M120 whose source is grounded between terminals XT and XTB to which a crystal resonator (X′TAL) is connected. A constant current I120 is supplied from a constant current source. Further, load capacitors C120 and C121 are provided at the gate and drain of the MOS transistor M120, respectively, and a feedback resistor R120 is provided between the gate and drain of the MOS transistor M120. In the oscillation circuit, the crystal oscillator oscillates by the MOS transistor M120 to which the constant current I120 is supplied, and a signal having a constant frequency is output from the output terminal XTB.

  FIG. 17 shows an equivalent circuit of the oscillation circuit, and FIG. 18 shows an equivalent circuit of the crystal resonator. As shown in FIG. 17, the equivalent circuit of the oscillation circuit is represented by a series circuit of a negative resistance -RL and a load capacitance CL. Further, as shown in FIG. 18, the equivalent circuit of the crystal resonator is shown as a parallel circuit of an L1-C1-R1 series circuit and a capacitance C0 as a dielectric of crystal.

  With recent miniaturization of electronic devices, a small oscillator is desired. However, when the size of the oscillator including the crystal resonator is reduced, the equivalent series capacitance C1 of the crystal resonator illustrated in FIG. 18 is decreased, and the startup time of the crystal resonator is increased.

  Hereinafter, the reason why the start-up characteristic of the crystal resonator is degraded when the equivalent series capacitance C1 of the crystal resonator is small will be described. The “Q value” is used as a value representing the sharpness of resonance of the resonance circuit of the crystal resonator. In the case of a crystal resonator or an electric circuit, a larger Q value is generally desirable in terms of circuit stability. However, the larger the Q value, the worse the response and the longer the startup time. The Q value is expressed by the following equation. Therefore, when the equivalent series capacitance C1 of the crystal resonator is reduced, the Q value is increased, so that the response of the crystal resonator is deteriorated and the startup time is lengthened.

Q = ω × L1 / R1 = 1 / (ω × C1 × R1)
(However, ω = √ (1 / (L1 × C1)), and L1, C1, and R1 are the equivalent series constants of the vibrator, respectively.)

  Therefore, a method of improving the deterioration of the starting characteristics of the crystal resonator on the oscillation circuit side can be considered. For example, if the negative resistance -RL is increased by reducing the load capacitance CL of the oscillation circuit shown in FIG. 18, the startup time is shortened. That is, an oscillation circuit with good starting characteristics can be obtained. For this reason, the oscillation circuit disclosed in Patent Document 1 includes a MOS variable capacitor, and the capacitance value of the MOS variable capacitor is changed between startup and steady state. In addition, the oscillation circuit disclosed in Patent Document 2 includes two capacitors having different capacities, and switches the capacities using switches at the time of startup and at the time of steady operation. Further, a method may be considered in which MOS transistors shown in FIG. 15 having different sizes are prepared, and the negative resistance -RL is increased by switching and using the MOS transistor at the time of start-up, thereby shortening the start-up time.

JP 2006-129594 A JP 2005-94147 A

  According to the method of adjusting the load capacity CL described above, there is an effect of shortening the start-up time, but the request regarding today's high start-up characteristics has not been sufficiently met. That is, the method of adjusting the load capacity CL does not sufficiently meet today's demand for shortening the startup time. Even in the method of adjusting the size of the MOS transistor, since the mutual conductance does not sufficiently increase due to the increase in parasitic capacitance accompanying the increase in the size of the MOS transistor, the effect of improving the deterioration of the starting characteristics is small. When the MOS transistor is increased in size, a large oscillating current can be supplied. The large oscillating current contributes to the improvement of the starting characteristics. However, since the oscillating current continues to flow even after starting, it is not preferable from the viewpoint of power consumption. . Therefore, an oscillation circuit and an oscillator with low power consumption in addition to good start-up characteristics are preferable.

  An object of the present invention is to provide an oscillation circuit and an oscillator having a sufficiently large negative resistance at the time of startup as compared with the negative resistance at the time of steady operation.

  The present invention is an oscillation circuit that oscillates a vibrator, and includes an inverter having one transistor, a feedback resistor connected in parallel with the inverter, and capacitors provided on the input side and the output side of the inverter, respectively. An element, a variable current source that supplies one of two types of currents having different levels to the inverter, a timer circuit that counts a predetermined time from the activation of the power supply of the oscillation circuit, and the variable current source that can be supplied Of the two types of current, the current having the higher level is supplied to the inverter until the count time by the timer circuit passes the predetermined time, and the current having the lower level is passed to the inverter after the predetermined time has passed. An oscillation circuit comprising: a current control unit that controls the variable current source to supply.

  In the oscillation circuit, the variable current source includes two current sources connected in parallel, and the current control unit includes a switch provided between one of the two current sources and the inverter. A switch control unit that controls on / off of the switch according to a count time by the timer circuit, and the switch control unit is configured to switch the switch until the count time by the timer circuit elapses the predetermined time. Is turned on, and after the predetermined time has elapsed, the switch is turned off.

  In the oscillation circuit, the variable current source includes one current source and a variable resistor, and the current control unit controls a resistance value of the variable resistor, and the variable current source supplies the inverter to the inverter. Change the current level.

  The present invention is an oscillation circuit that oscillates a vibrator, and includes an inverter, a feedback resistor connected in parallel with the inverter, a capacitive element provided on each of an input side and an output side of the inverter, and the 2 A variable capacitance element provided in series with the capacitance element between two capacitance elements, a timer circuit that counts a predetermined time from the activation of the power supply of the oscillation circuit, and an input / output of the inverter according to the count time by the timer circuit An oscillation circuit is provided that includes a capacitance control unit that controls a capacitance between the electrodes.

  In the oscillation circuit, the capacitance control unit is a switch that grounds the input / output of the inverter through the capacitance element by being turned on, and a switch control unit according to a count time by the timer circuit, The switch control unit turns on the switch until the count time by the timer circuit elapses the predetermined time, and switches the switch to the off state after the predetermined time elapses.

  The present invention is an oscillation circuit that oscillates a vibrator, and includes a first inverter having a first MOS transistor, and a second inverter having the first MOS transistor and the second MOS transistor constituting the CMOS. A feedback resistor connected in parallel with the first inverter and the second inverter, and capacitive elements provided on the input side and the output side of the first inverter and the second inverter, A current source that supplies current to the first inverter, a timer circuit that counts a predetermined time from the start-up of the power supply of the oscillation circuit, and the time until the count time by the timer circuit passes the predetermined time The first inverter and the second inverter are energized so that the second inverter is energized after the predetermined time has elapsed. Providing an oscillation circuit and a current control unit for controlling the inverter.

  In the oscillation circuit, the current control unit includes a first switch provided on a path from a power source to the first inverter via the current source, and a power source to the second inverter. A second switch provided on the path; and a switch control unit that performs on / off control of the first switch and the second switch according to a count time by the timer circuit.

  The present invention provides an oscillator including a vibrator and the oscillation circuit that oscillates the vibrator.

  According to the oscillation circuit and the oscillator according to the present invention, since the negative resistance at the time of startup can be sufficiently increased as compared with the negative resistance at the time of steady operation, good startup characteristics can be obtained. In this way, the start-up characteristic can be prevented from being lowered on the oscillation circuit side due to the downsizing of the oscillator.

The load resistance -RL of the oscillation circuit that oscillates the vibrator is raised by the following method.
1. Increase the mutual conductance gm of the inverter.
2. Reduce input / output capacitive coupling (reduce signal return due to capacitance).
3. Reduce the load capacity.

  Regarding 1 above, the negative resistance -RL is determined by the circuit configuration and the constant of the transistor. For example, the negative resistance −RL of the oscillation circuit that does not include the feedback resistance illustrated in FIG. 1 is represented by “−RL = −gm / (ω2 × C1 × C2)”. As can be seen from the equation, when the mutual conductance gm is increased, the negative resistance increases. The mutual conductance gm can be expressed by the following equation from the current (oscillation current) I flowing through the transistor TR as an inverter. Therefore, when the current I flowing through the transistor TR is increased, the mutual conductance gm is increased and the negative resistance is increased.

gm = 1 / re = I / Vt
(Vt = kT / q = 26mV)

  In order to increase the gain of the inverter composed of one transistor shown in FIG. 16, the mutual conductance gm of the transistor is increased. In order to increase the mutual conductance gm, the current (oscillation current) flowing through the transistor may be increased. However, once the vibrator oscillates, a mutual conductance as large as that at the time of activation is not necessary in a steady state. That is, the steady-state oscillation current does not have to be as large as that at startup.

  Further, as described above, the size of the CMOS transistor is increased with respect to the oscillation circuit including the inverter constituted by the CMOS shown in FIG. However, if the size of the CMOS transistor is increased, the parasitic capacitance increases, so that the mutual conductance gm cannot be sufficiently increased. Therefore, in order to improve the start-up characteristic of an oscillation circuit including an inverter composed of CMOS, the inverter is switched to use an inverter composed of CMOS at the time of start-up and to use an inverter composed of one transistor at the time of steady operation.

  Next, regarding the above 2, the load capacitance connected to the input / output of the inverter affects the negative resistance of the oscillation circuit. A circuit in which the load capacitance shown in FIG. 2A is grounded, and a circuit in which the input terminal XT and the output terminal XTB are connected via the load capacitance shown in FIG. In the latter circuit (FIG. 2B) due to capacitive coupling, the signal is returned from the output terminal XTB to the input terminal XT, so that the load resistance is lowered. For this reason, the former circuit (FIG. 2 (a)) in which the load capacitance is grounded at the time of start-up is desirable, and the latter circuit (FIG. 2 (b)) in which the load capacitance is not grounded in the steady state is desirable.

  The method 3 is the method described in the background art.

  As described above, the method of increasing the load resistance -RL of the oscillation circuit includes not only a method of reducing the load capacitance but also a method of increasing the mutual conductance gm of the inverter (above 1) and a method of reducing the input / output capacitive coupling. (2).

<Method of increasing the mutual conductance gm of the inverter>
First, an embodiment of an oscillation circuit that utilizes the above-described method 1 and raises the mutual conductance gm of the inverter at the time of startup and makes the negative resistance at the time of startup larger than that at the normal time will be described with reference to the drawings.

(First embodiment)
FIG. 3 is a circuit diagram showing the concept of the oscillation circuit of the first embodiment that oscillates the crystal resonator. As shown in FIG. 3, the oscillation circuit of the first embodiment includes an inverter constituted by a MOS transistor M200 whose source is grounded between terminals XT and XTB to which a crystal resonator (X′TAL) is connected. The constant current is supplied from at least one of the two constant current sources to the drain of the MOS transistor M200. In addition, load capacitors C200 and C201 are provided at the gate and drain of the MOS transistor M200, respectively, and a feedback resistor R200 is provided between the gate and drain of the MOS transistor M200. In the oscillation circuit, the crystal resonator oscillates by an inverter to which a voltage is applied, and outputs a signal having a constant frequency from the terminal XTB.

  Furthermore, the oscillation circuit of the present embodiment includes a timer circuit 201 that counts a predetermined time (for example, 500 μsec) from the start-up of the power supply Vcc of the oscillation circuit, and a MOS transistor that is connected from the power supply Vcc to one of two constant current sources. A switch SW200 provided on the path to M200 and a controller 203 that controls on / off of the switch SW200 according to the count time by the timer circuit 201 are provided. In the present embodiment, the level of the current supplied to the inverter is adjusted by on / off control of the switch SW200. That is, during a period until a predetermined time elapses after the activation of the power supply Vcc of the oscillation circuit (hereinafter referred to as “at the time of activation”), the oscillation is performed by combining the constant current I200 and the constant current I201 from the two current sources shown in FIG. After a current is supplied to the inverter and a predetermined time has elapsed since the start of the power supply Vcc of the oscillation circuit (hereinafter referred to as “steady time”), only the constant current I200 is supplied to the inverter.

  The predetermined time counted by the timer circuit 201 can be adjusted according to a time constant determined from the resistance and capacitance of the timer circuit 201. By changing the time constant, the timing for switching the oscillation current supplied to the inverter can be changed.

  FIG. 4 is a specific circuit diagram of the oscillation circuit shown in FIG. As shown in FIG. 4, the two constant current sources can be realized by a 1: 2 current mirror circuit. In the current mirror circuit formed of the MOS transistors M210, M211, and M212, when a current flows between the source and drain of the MOS transistor M210 whose drain is grounded via the resistor R210, the current mirror circuit causes a current to flow to the MOS transistor M211. I200 flows and current I201 flows through the MOS transistor M212. However, the source of the MOS transistor M212 is connected to the switch SW200 that is on / off controlled by the timer circuit 201 described above. Therefore, the current I201 flows through the MOS transistor M212 only when the switch SW200 is on.

  The current I200 flowing through the MOS transistor M211 and the current I201 flowing through the MOS transistor M212 are supplied to the inverter by the current mirror circuit. Therefore, the current I200 + I201 is supplied to the inverter by turning on the switch SW200 at the time of start-up, and the switch SW200 is switched off when a predetermined time has elapsed from the start-up of the power supply Vcc of the oscillation circuit. Supply only to the inverter.

  Note that the magnitudes of the currents I200 and I201 can be determined by the sizes of the MOS transistors M211 and M212. Further, a resistor may be provided between the MOS transistors M211 and M212 and the power supply Vcc, and the magnitudes of the currents I200 and I201 may be determined by the values of the resistors.

  As described above, if the oscillation current flowing through the inverter is large, the mutual conductance gm of the inverter is increased and the negative resistance of the oscillation circuit is increased. Therefore, the start-up time of the crystal resonator by the oscillation circuit of this embodiment is shortened. Therefore, it is possible to provide a small oscillator having a good start-up characteristic. Further, the oscillation current flowing through the inverter is large only at the time of startup, and the oscillation current in the steady state can be suppressed to a low level necessary for continuous oscillation of the crystal resonator, so that power consumption can be reduced.

  Note that the MOS transistors M200 and M210 to M212 of this embodiment may be bipolar transistors. When the bipolar transistor is considered to have the same area, a larger current than that of the MOS transistor can be flown, so that a large oscillation current can be flowed by using the MOS transistor M200 as a bipolar transistor.

  In this embodiment, one set of two constant current sources is provided, but a plurality of sets of constant current sources may be provided, and another set of constant current sources may be used depending on the oscillation frequency. For example, four constant current sources are provided, the first constant current source supplies current necessary for steady oscillation of 26 MHz, the second constant current source supplies current necessary for steady oscillation of 52 MHz, The third constant current source is set to supply a current necessary for oscillation at 26 MHz, and the fourth constant current source is set to supply a current necessary for oscillation at 52 MHz. In this case, when oscillating at 26 MHz, the second and fourth constant current sources are not operated, and the first and third constant current sources are operated as described above.

(Second Embodiment)
FIG. 5 is a circuit diagram showing the concept of the oscillation circuit of the second embodiment for oscillating a crystal resonator. As shown in FIG. 5, the oscillation circuit of the second embodiment is substantially the same as the oscillation circuit of the first embodiment except for the constant current source and the switch. In FIG. 5, the same reference numerals are given to components common to FIG. 3.

  The oscillation circuit of the second embodiment includes a variable current source. The value of the current supplied by the variable current source is controlled according to the count time by the timer circuit 201. As a result, the level of current supplied from the variable current source to the inverter is adjusted.

  FIG. 6 is a specific circuit diagram of the oscillation circuit shown in FIG. As shown in FIG. 6, the current source can be realized by a 1: 1 current mirror circuit. In the current mirror circuit formed of the MOS transistors M213 and M214, when a current flows between the source and drain of the MOS transistor M213, a current I202 also flows through the MOS transistor M214 by the current mirror circuit. However, the source of the MOS transistor M213 is grounded via resistors 202 and R203 connected in parallel, and a switch SW201 is provided in series with the resistor R203.

  The switch SW201 is on / off controlled according to the count time by the timer circuit 201, as in the first embodiment. The resistance component on the source side of the MOS transistor M213 changes according to the on / off state of the switch SW201. That is, the resistance component on the source side of the MOS transistor M213 when the switch SW201 is in the off state is only the resistor R202. On the other hand, resistance components on the source side of the MOS transistor M213 when the switch SW201 is in the on state are resistors R202 and R203 connected in parallel. Therefore, the resistance component on the source side of the MOS transistor M213 becomes small when the switch SW201 is in the on state.

  The current flowing between the source and drain of the MOS transistor M213 is larger as the resistance component is smaller. The mirror current I202 flowing through the MOS transistor M214 is proportional to the current flowing through the MOS transistor M213. Also in this embodiment, in order to make the oscillation current flowing to the inverter at the time of startup larger than in the steady state, the switch SW201 is turned on until a predetermined time elapses after the power supply Vcc of the oscillation circuit starts up, and after the predetermined time has elapsed. Switches the switch SW201 to the off state.

  As described above, also in this embodiment, since the oscillation current supplied to the inverter is large at the time of startup, the mutual conductance gm of the inverter is increased and the negative resistance of the oscillation circuit is increased, so that the startup time of the crystal resonator is short. Become. Therefore, it is possible to provide a small oscillator having a good start-up characteristic. Further, the oscillation current flowing through the inverter is large only at the time of startup, and the oscillation current in the steady state can be suppressed to a low level necessary for continuous oscillation of the crystal resonator, so that power consumption can be reduced.

  In addition, the oscillation circuit of this embodiment has a smaller parasitic capacitance, which is one of the factors that degrade the oscillation characteristics, compared to the oscillation circuit of the first embodiment. The decrease in resistance is small.

  Note that the MOS transistors M200, M213, and M214 of this embodiment may be bipolar transistors. When the bipolar transistor is considered to have the same area, a larger current than that of the MOS transistor can be flown, so that a large oscillation current can be flowed by using the MOS transistor M200 as a bipolar transistor.

<Method of reducing input / output capacitive coupling>
Next, an embodiment of an oscillation circuit that utilizes the above-described method 2 and reduces the input / output capacitive coupling at the time of startup and increases the negative resistance at the time of startup as compared with the steady state will be described with reference to the drawings. .

(Third embodiment)
FIG. 7 is a circuit diagram showing the concept of the oscillation circuit of the third embodiment for oscillating a crystal resonator. As shown in FIG. 7, the oscillation circuit of the third embodiment includes an inverter 700 between terminals XT and XTB to which a crystal resonator (X′TAL) is connected. Further, load capacitors C700 and C701 are provided on the input side and the output side of the inverter 700, respectively, and a feedback resistor R700 is provided in parallel with the inverter 700. In the oscillation circuit, the crystal resonator is oscillated by the inverter 700 to which the power supply voltage Vcc is applied, and a signal having a constant frequency is output from the terminal XTB.

  The oscillation circuit of the present embodiment includes a variable capacitor VC700 connected in series with the load capacitor C700, and a variable capacitor VC701 connected in series with the load capacitor C701. A control terminal XCA capable of applying a control voltage A is provided between the load capacitor C700 and the variable capacitor VC700, and a control terminal XCB capable of applying a control voltage B is provided between the load capacitor C701 and the variable capacitor VC701. It has been. The capacity of the variable capacitor VC700 is controlled by the control voltage A, and the capacity of the variable capacitor VC701 is controlled by the control voltage B. By controlling the capacity of these variable capacitors VC700 and VC701, the frequency of the signal output from the output terminal XTB is changed.

  However, for example, when controlling the oscillation circuit with the same control voltage for the control terminal XCA and the control terminal XCB (when the control terminal XCA and the control terminal XCB are connected and used), the output terminal XTB includes the load capacitor C701 and the control terminal XCB. Are coupled to the input terminal XT via the bias resistor R751, the bias resistor R750 of the control terminal XCA, and the load capacitor C700, and a part of a signal (particularly a high frequency signal) output from the output terminal XTB is input terminal. Since it will return to XT, load resistance will fall. Therefore, in this embodiment, as shown in FIG. 7, a switch SW700 is provided in parallel with the variable capacitor VC700, a switch SW701 is provided in parallel with the variable capacitor VC701, and these switches SW700 and SW701 are controlled to be turned on and off. Since the terminals XT and XTB are grounded via the load capacitors C700 and C701 when the switches SW700 and SW701 are in the on state, the circuit is equivalent to the configuration shown in FIG. At this time, since there is no capacitive coupling between the output terminal XTB and the input terminal XT described above, a decrease in load resistance can be prevented.

  The oscillation circuit according to the present embodiment corresponds to the timer circuit 701 that counts a predetermined time (for example, 500 μsec) from the activation of the power supply Vcc of the oscillation circuit, the switches SW700 and SW701 described above, and the count time by the timer circuit 701. And a controller 703 that controls on / off of the switches SW700 and SW701. The controller 703 prevents the load resistance from decreasing by turning on the switches SW700 and SW701 until a predetermined time elapses after the activation of the power supply Vcc of the oscillation circuit (at the time of activation), and activates the power supply Vcc of the oscillation circuit. When a predetermined time elapses, the switches SW700 and SW701 are turned off.

(Fourth embodiment)
FIG. 8 is a circuit diagram showing the concept of the oscillation circuit of the fourth embodiment for oscillating a crystal resonator. As shown in FIG. 8, the oscillation circuit of the fourth embodiment is substantially the same as the oscillation circuit of the third embodiment except for the variable capacitance. In FIG. 8, the same reference numerals are assigned to components common to FIG. 7.

  In the fourth embodiment, a variable capacitor VC800 for controlling the frequency of the output signal is provided in series between the load capacitors C700 and C701. A control terminal XCA capable of applying a control voltage A is provided between the load capacitor C700 and the variable capacitor VC800, and a control terminal XCB capable of applying a control voltage B is provided between the load capacitor C701 and the variable capacitor VC800. Yes. The capacity of the variable capacitor VC800 is controlled by control voltages A and B. The frequency of the signal output from the output terminal XTB is changed by controlling the capacitance of the variable capacitor VC800.

  However, the output terminal XTB is capacitively coupled to the input terminal XT via the load capacitor C701, the control terminal XCB, the variable capacitor VC800, the control terminal XCA, and the load capacitor C700, and a signal (particularly, a signal output from the output terminal XTB). , A part of the high-frequency signal) returns to the input terminal XT, and the load resistance is lowered. Therefore, in the present embodiment, as shown in FIG. 8, the switch SW700 is provided at the middle point of the load capacitor C700 and the variable capacitor VC800, that is, the control terminal XCA, and the middle point of the load capacitor C701 and the variable capacitor VC800, that is, the control terminal. A switch SW701 is provided in the XCB, and these switches SW700 and SW701 are on / off controlled. Since the terminals XT and XTB are grounded via the load capacitors C700 and C701 when the switches SW700 and SW701 are in the on state, the circuit is equivalent to the configuration shown in FIG. At this time, since there is no capacitive coupling between the output terminal XTB and the input terminal XT described above, a decrease in load resistance can be prevented.

  As in the third embodiment, the oscillation circuit of the present embodiment includes a timer circuit 701 that counts a predetermined time (for example, 500 μsec) from the activation of the power supply Vcc of the oscillation circuit, the switches SW700 and SW701 described above, And a controller 703 that controls on / off of the switches SW700 and SW701 in accordance with the count time of the timer circuit 701. The controller 703 prevents the load resistance from decreasing by turning on the switches SW700 and SW701 until a predetermined time elapses after the activation of the power supply Vcc of the oscillation circuit (at the time of activation), and activates the power supply Vcc of the oscillation circuit. When a predetermined time elapses, the switches SW700 and SW701 are turned off.

(Fifth embodiment)
FIG. 9 is a circuit diagram showing the concept of the oscillation circuit of the fifth embodiment for oscillating a crystal resonator. As shown in FIG. 9, the oscillation circuit of the fifth embodiment includes an inverter 500 between terminals XT and XTB to which a crystal resonator (X′TAL) is connected. Further, load capacitors C500 and C501 are provided on the input side and the output side of the inverter 500, respectively, and a feedback resistor R500 is provided in parallel with the inverter 500. In the oscillation circuit, the crystal unit oscillates by the inverter 500 to which the power supply voltage Vcc is applied, and outputs a signal having a constant frequency from the terminal XTB.

  In the present embodiment, a varactor 510 composed of two MOS transistors M510 and M511 is provided between the load capacitor C500 and the load capacitor C501. The electrostatic capacity of the varactor 510 varies depending on a control terminal XCA provided between the load capacitor C500 and the varactor 510 and a control voltage applied from the XCB provided between the load capacitor C501 and the varactor 510. The anode side of the varactor 510 (back gates of the MOS transistors M510 and M511) is grounded via the switch SW500.

  The oscillation circuit of the present embodiment includes a timer circuit 501 that counts a predetermined time (for example, 500 μsec) from the activation of the power supply Vcc of the oscillation circuit, the switch SW500 described above, and the switch SW500 according to the count time by the timer circuit 501. And a controller 503 for controlling on / off. The controller 503 turns on the switch SW500 until a predetermined time elapses after activation of the power supply Vcc of the oscillation circuit (at the time of activation), and turns on the switch SW500 when a predetermined time elapses after activation of the power supply Vcc of the oscillation circuit. Switch to off state.

  FIG. 10 is an equivalent circuit between the control terminals XCA and XCB when the switch SW500 is in the ON state. FIG. 11 is an equivalent circuit between the control terminals XCA and XCB when the switch SW500 is in the OFF state. When the switch SW500 is closed, as shown in FIG. 10, the anode of the varactor 510 is grounded, and among the capacitive components C510 and C511 of the varactor 510, the capacitive component C511 parallel to the control terminals XCA and XCB is also grounded. . Since this is equivalent to the configuration shown in FIG. 2A, the capacitive coupling between the output terminal XTB and the input terminal XT is small. Therefore, at this time, it is possible to prevent a decrease in load resistance. On the other hand, when the switch SW500 is opened, as shown in FIG. 11, the anode of the varactor 510 is opened, and the capacitive component C511 of the varactor 510 is connected in parallel with the capacitive component C510. This is equivalent to the configuration shown in FIG.

<Other methods>
Next, an embodiment of an oscillation circuit using a method other than the above method 1 (a method for increasing the mutual conductance gm of the inverter) and the above method 2 (a method for reducing input / output capacitive coupling) will be described with reference to the drawings. I will explain.

(Sixth embodiment)
FIG. 12 is a circuit diagram showing an oscillation circuit according to a sixth embodiment that oscillates a crystal resonator. As shown in FIG. 12, the oscillation circuit of the sixth embodiment includes an inverter made of CMOS between terminals XT and XTB to which a crystal resonator (X′TAL) is connected, and N constituting the CMOS. A constant current I300 is supplied between the channel MOS transistor M300 and the P channel MOS transistor M301 from a constant current source. Note that an inverter can also be configured by a single P-channel MOS transistor M301. Load capacitors C300 and C301 are provided on the input side and the output side of the inverter, respectively, and a feedback resistor R300 is provided in parallel with the two inverters. In the oscillation circuit, the crystal resonator oscillates by an inverter to which a voltage is applied, and outputs a signal having a constant frequency from the output terminal XTB.

  Furthermore, the oscillation circuit of the present embodiment is provided on a path from a timer circuit 301 that counts a predetermined time (for example, 500 μsec) from the start-up of the power supply Vcc of the oscillation circuit and a path from the power supply Vcc to the inverter configured with CMOS. A switch SW300 provided on the path from the power source Vcc to the MOS transistor M301 via the constant current source, and a controller 303 for controlling on / off of the switches SW300 and SW301 according to the count time by the timer circuit 301. Prepare. The switch SW300 is provided on the source side of the N-channel MOS transistor M300, and the switch SW301 is provided on the upstream side of the constant current source. In this embodiment, an on-off control of the switches SW300 and SW301 controls an inverter composed of CMOS (hereinafter referred to as “first inverter”) and an inverter composed of a single P-channel MOS transistor M301 (hereinafter referred to as “second inverter”). ") Is energized.

  Since the mutual conductance gm2 of the second inverter can be changed by adjusting the current I300 supplied from the constant current source to the second inverter, the mutual resistance gm2 can be increased to increase the negative resistance. . However, the use of the second inverter has the following problems. The oscillator has better noise characteristics as the amplitudes of the signal input to the inverter and the signal output from the inverter are larger. However, in the case of the second inverter, the input-side terminal, that is, the gate of the P-channel MOS transistor M301 is easily clamped at around 0.7V, so that the amplitude of the input / output signal cannot fully swing to the value of the power supply voltage. This phenomenon appears particularly when the second inverter is composed of a bipolar transistor. On the other hand, in the case of the first inverter composed of CMOS, the noise characteristic is good because the amplitude of the input / output signal fully swings to the value of the power supply voltage.

  Therefore, in the present embodiment, the switches SW300 and SW301 are controlled to be turned on and off so that the second inverter is energized at the time of startup and the first inverter is energized at the time of steady operation. Note that the switch corresponding to the inverter that is not energized is completely turned off.

  13 and 14 are specific circuit diagrams of the oscillation circuit shown in FIG. Similar to the second embodiment, the current source can be realized by a 1: 1 current mirror circuit. Further, the switch SW301 may be provided on the side of the MOS transistor M411 constituting the current mirror circuit as shown in FIG. 13, or may be provided on the side of the MOS transistor M410 constituting the current mirror circuit as shown in FIG. .

  As described above, according to the oscillation circuit of the present embodiment, the second inverter that provides a large negative resistance is used at the time of startup, and the first inverter that does not limit the amplitude of the input / output signal in the steady state. Use. That is, in this embodiment, an inverter suitable for the situation is used in terms of both negative resistance and noise characteristics.

  In the above embodiment, an oscillation circuit that oscillates a crystal resonator has been described as an example. However, the present invention is not limited to a crystal resonator, and can also be applied to an oscillation circuit that oscillates a resonator using a ceramic resonator or MEMS technology. .

  The oscillation circuit according to the present invention is useful as an oscillation circuit for a vibrator having two advantages of good starting characteristics and low power consumption.

Circuit diagram showing an oscillation circuit that does not include a feedback resistor An oscillation circuit (a) with a load capacitance grounded and an oscillation circuit (b) with no load capacitance grounded Circuit diagram showing concept of oscillation circuit of first embodiment for oscillating crystal resonator Specific circuit diagram of the oscillation circuit shown in FIG. Circuit diagram showing concept of oscillation circuit of second embodiment for oscillating crystal resonator Specific circuit diagram of the oscillation circuit shown in FIG. Circuit diagram showing concept of oscillation circuit of third embodiment for oscillating crystal resonator Circuit diagram showing concept of oscillation circuit of fourth embodiment for oscillating crystal resonator Circuit diagram showing concept of oscillation circuit of fifth embodiment for oscillating crystal resonator Equivalent circuit between control terminals XCA and XCB when switch SW500 is on Equivalent circuit between control terminals XCA and XCB when switch SW500 is off Circuit diagram showing an oscillation circuit of a sixth embodiment for oscillating a crystal resonator Specific circuit diagram of the oscillation circuit shown in FIG. Specific circuit diagram of the oscillation circuit shown in FIG. Circuit diagram showing a conventional oscillation circuit that oscillates a crystal unit Circuit diagram showing a conventional oscillation circuit that oscillates a crystal unit Equivalent circuit of oscillation circuit Equivalent circuit of crystal unit

Explanation of symbols

500,700 Inverters 201, 301, 501, 701 Timer circuits 203, 303, 503, 703 Controller 510 Varactor
X'TAL Quartz crystal XT Input terminal XTB Output terminal XCA, XCB Control terminal

Claims (8)

An oscillation circuit for oscillating a vibrator,
An inverter having one transistor;
A feedback resistor connected in parallel with the inverter;
Capacitive elements provided on the input side and output side of the inverter;
A variable current source for supplying one of two types of currents having different levels to the inverter;
A timer circuit that counts a predetermined time from activation of the power supply of the oscillation circuit;
Of the two types of currents that can be supplied by the variable current source, the current having the larger level until the count time by the timer circuit elapses the predetermined time, and the level after the predetermined time elapses. A current control unit that controls the variable current source so as to supply a smaller current to the inverter;
An oscillation circuit comprising: The oscillation circuit according to claim 1,
The variable current source has two current sources connected in parallel,
The current control unit includes a switch provided between one of the two current sources and the inverter, and a switch control unit that performs on / off control of the switch according to a count time by the timer circuit. And
The switch control unit turns on the switch until the count time by the timer circuit elapses the predetermined time, and switches the switch to an off state after the predetermined time elapses. Oscillator circuit. The oscillation circuit according to claim 1,
The variable current source has one current source and a variable resistor,
The oscillation circuit according to claim 1, wherein the current control unit controls a resistance value of the variable resistor to change a level of a current supplied from the variable current source to the inverter. An oscillation circuit for oscillating a vibrator,
An inverter;
A feedback resistor connected in parallel with the inverter;
Capacitive elements provided on the input side and output side of the inverter;
A variable capacitive element provided in series with the capacitive element between the two capacitive elements;
A timer circuit that counts a predetermined time from activation of the power supply of the oscillation circuit;
A capacity control unit that controls the capacity between the input and output of the inverter according to the count time by the timer circuit;
An oscillation circuit comprising: The oscillation circuit according to claim 4,
The capacitance control unit includes a switch that is grounded via the capacitive element by being on-controlled, and a switch control unit according to a count time by the timer circuit,
The switch control unit turns on the switch until the count time by the timer circuit elapses the predetermined time, and switches the switch to an off state after the predetermined time elapses. Oscillator circuit. An oscillation circuit for oscillating a vibrator,
A first inverter having a first MOS transistor;
A second inverter having the first MOS transistor and the second MOS transistor constituting the CMOS;
A feedback resistor connected in parallel with the first inverter and the second inverter;
Capacitive elements provided on the input side and the output side of the first inverter and the second inverter,
A current source for supplying current to the first inverter;
A timer circuit that counts a predetermined time from activation of the power supply of the oscillation circuit;
The first inverter and the second inverter are energized until the count time by the timer circuit passes the predetermined time, and the second inverter is energized after the predetermined time. A current control unit for controlling the second inverter;
An oscillation circuit comprising: The oscillation circuit according to claim 6,
The current controller is
A first switch provided on a path from a power source to the first inverter via the current source;
A second switch provided on a path from the power source to the second inverter;
A switch control unit for controlling on / off of the first switch and the second switch according to a count time by the timer circuit;
An oscillation circuit comprising:   An oscillator comprising: a vibrator; and the oscillation circuit according to claim 1 that causes the vibrator to oscillate.

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