JP2013110511A - High stability oscillator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high stability oscillator which can be made compact easily while reducing the manufacturing cost, and can combine the short-term stability and the long-term stability of the oscillation output frequency.SOLUTION: The high stability oscillator outputting a signal of a fixed frequency comprises: a PLL circuit including a voltage controlled crystal oscillator 15 as an output oscillator; and an MEMS oscillator 11 having an MEMS vibrator. Output from the MEMS oscillator 11 is supplied, as a reference signal, to the phase comparator 12 of the PLL circuit. Long-term stability of the output frequency is maintained by the MEMS oscillator 11, and short-term stability of the output frequency is maintained by the voltage controlled crystal oscillator 15.

Description

  The present invention relates to an oscillator that can output a signal of a reference frequency with high stability.

  A crystal oscillator combining a crystal resonator and an oscillation circuit is widely used as a reference source for frequency and time. In general, a crystal oscillator has a structure in which a crystal resonator as a resonance circuit is inserted in a feedback loop of an amplifier in the oscillator, so that an output signal corresponding to the frequency of the natural vibration of the crystal resonator is stably output. In particular, the short-term stability at the output frequency is excellent. However, the crystal oscillator has a structure in which the crystal piece constituting the crystal unit is fixed to the container using an adhesive, and the long-term stability of the output frequency (against aging) due to the adhesive. There is a problem that stability is not good.

  On the other hand, an atomic oscillator (also called an atomic frequency standard or an atomic clock) is known as an oscillator that outputs a signal of a reference frequency with extremely high accuracy. The atomic oscillator generates a highly stable frequency signal based on the resonance absorption phenomenon of microwaves accompanying transition between hyperfine levels of atoms such as cesium (Cs) and rubidium (Rb). FIG. 2 is a block diagram showing a configuration of a conventional typical atomic oscillator.

  The atomic oscillator shown in FIG. 2 is a PLL (phase-locked loop) circuit in which a frequency signal of resonance absorption (ie, atomic resonance) based on transition between hyperfine levels of cesium or rubidium atoms is input as a reference signal. The PLL circuit includes a phase comparator 12, a charge pump 13 connected to the output of the phase comparator 12, a low-pass filter (LPF) 14 connected to the output of the charge pump 13, A voltage controlled crystal oscillator (VCXO) 15 whose oscillation frequency changes according to the output voltage of the low-pass filter 14 is provided, and the output of the voltage controlled crystal oscillator 15 is output to an external circuit as an output of the atomic oscillator. While being supplied, it is fed back to the phase comparator 12 via the frequency divider 16. The part that generates the frequency signal of the atomic resonance includes a microwave synthesizer 21 that generates a microwave signal based on the output frequency of the voltage-controlled crystal oscillator 15, and a microwave generated by the microwave synthesizer 21 such as cesium or rubidium. A physical package 22 configured to irradiate vapor to obtain a signal corresponding to resonance absorption, and a microcontroller 23 that outputs a reference signal generated based on the frequency of resonance absorption in the physical package 22 to the phase comparator 12. And. The physics package includes, for example, a cell in which vapors of cesium atoms and rubidium atoms are confined. For example, the microcontroller 23 performs analog / digital conversion on the signal from the physical package 22 to perform digital signal processing, performs digital / analog conversion on the result of the digital signal processing, and uses the reference signal as an analog signal. It is configured to output to the phase comparator 12.

  Although the atomic oscillator is excellent in frequency stability over a long period of time, it is configured to search for a frequency that maximizes absorption due to resonance absorption. Therefore, when the signal output from the physical package 22 is used alone, the atomic oscillator is short-term. It is not excellent in stability. In addition to a PLL circuit, an atomic oscillator requires a microwave synthesizer and a cell in which vapors of cesium and rubidium atoms are confined, and the oscillator has a wide range of components, making it smaller and lower in cost. It has the problem that it is difficult. It is also necessary to perform precise temperature control in the physical package.

  As an oscillator that is inferior to an atomic oscillator but has excellent long-term stability, there is a MEMS oscillator including a MEMS (Microelectromechanical Systems) vibrator. The MEMS vibrator has been developed with the recent miniaturization of semiconductor device manufacturing technology. For example, as described in Non-Patent Document 1, a semiconductor such as silicon or AlN (aluminum nitride). For example, a piezoelectric body such as the above is processed in a fine and high precision with a size of several μm to several tens of μm, and an electrode or the like is further arranged thereon to constitute a vibrator. For example, a MEMS vibrator made of a silicon semiconductor uses a semiconductor device manufacturing technique, and a resonator obtained by processing a silicon layer into a shape in which a disk having a diameter of several tens of μm is held by two beams extending in the diameter direction; It consists of four electrodes provided in close proximity to the resonator. The two beams function as a movable portion for the resonator body formed on the disc, and support the resonator body without hindering the contour vibration of the resonator body. The electrodes are arranged with a gap of, for example, 100 nm with respect to the outer peripheral surface of the resonator at each of the positions that divide the outer periphery of the resonator main body into four equal parts. A capacitance is formed. In such a MEMS vibrator, if the resonator is electrostatically driven through the electrode, the resonator vibrates at its mechanical natural frequency, and the distance between the resonator and the electrode changes minutely due to the vibration. The capacitance periodically changes at the natural frequency, and accordingly, the electrode potential also vibrates at the natural frequency. Therefore, by incorporating such a MEMS vibrator into an oscillation circuit, an oscillator that outputs a signal that matches the natural frequency of the MEMS vibrator, that is, a MEMS oscillator can be obtained.

  MEMS oscillators can be manufactured using only semiconductor device manufacturing technology, so it is easy to downsize and can be manufactured at low cost, and it is not necessary to fix the resonator using an adhesive. Will be excellent. However, the MEMS oscillator may have a large first-order coefficient absolute value in its frequency-temperature characteristics, which is inferior to a crystal oscillator in terms of short-term stability in oscillation output.

  In Patent Documents 1 and 2, a MEMS oscillator is used so that a signal of a desired frequency can be generated in a switchable manner on the premise of application within a predetermined frequency band between the UHF band and the GHz band. A frequency synthesizer is disclosed. Patent Documents 1 and 2 include a PLL circuit that uses a signal from a MEMS oscillator as a reference signal. In these synthesizers, a voltage controlled oscillator (VCO) having a large frequency variable range is used as an output oscillator of the PLL circuit, and a frequency divider capable of changing a frequency division ratio during operation is used. Furthermore, in order to ensure that the output frequency of the MEMS oscillator changes with temperature, a temperature sensor is provided, and the frequency division ratio of the frequency divider is also changed by the measurement value of the temperature sensor.

  However, the devices described in Patent Documents 1 and 2 have a problem that short-term stability at the output frequency is not good because a voltage-controlled oscillator having a large frequency variable range is used. In addition, when the frequency division ratio is automatically changed for temperature compensation, the output frequency changes discontinuously during operation, and it is required to supply a signal with a constant frequency stably. It will not be suitable for the intended use.

JP 2010-193240 A WO2009 / 063612

Takeo Oreda, "Crystal oscillator manufacturer talks about the comparison of MEMS vs. quartz", Nikkei Microdevices, No. 268, pp. 71-76 (October 2007 issue)

  As described above, a crystal oscillator is excellent in short-term stability, but is insufficient in terms of long-term stability or aging characteristics. On the other hand, the synthesizer using the MEMS oscillator has a problem in that although the long-term stability of the output frequency is excellent, the short-term stability is insufficient and a discontinuous change in the output frequency may occur. Although an atomic oscillator is extremely excellent in long-term stability, there is a problem that it is difficult to reduce the size and cost and that a high degree of temperature control is required.

  An object of the present invention is to provide a highly stable oscillator that can be easily downsized, can be manufactured at low cost, and has both short-term stability and long-term stability at an oscillation output frequency.

  A highly stable oscillator according to the present invention includes a PLL circuit having a voltage-controlled crystal oscillator as an output oscillator and a MEMS oscillator having a MEMS vibrator, and the output of the MEMS oscillator is supplied as a reference signal to a phase comparator of the PLL circuit. And output a signal having a constant frequency.

  In the present invention, as the MEMS vibrator, for example, one constituted using silicon or one constituted using a piezoelectric material such as AlN can be used.

  According to the present invention, by combining a MEMS oscillator having excellent long-term stability and a crystal oscillator having excellent short-term stability, it is possible to achieve both short-term stability and long-term stability, and to easily achieve downsizing and cost reduction. A stable oscillator is obtained.

It is a block diagram which shows the oscillator of one Embodiment of this invention. It is a block diagram which shows an example of a structure of an atomic oscillator.

  Next, a preferred embodiment of the present invention will be described with reference to the drawings.

  The highly stable oscillator according to the embodiment of the present invention shown in FIG. 1 is configured to output a signal having a predetermined single frequency, and includes a MEMS oscillator 11 and an oscillation of the MEMS oscillator 11. And a PLL circuit for inputting an output signal as a reference signal. The single frequency is a relatively low frequency, for example, 100 MHz or less. The PLL circuit includes a phase comparator 12 to which a reference signal from the MEMS oscillator 11 is input, a charge pump 13 connected to the output of the phase comparator 12, and a low-pass filter (LPF) connected to the output of the charge pump 13. ) 14 and a voltage controlled crystal oscillator (VCXO) 15 whose oscillation frequency changes according to the output voltage of the low-pass filter 14. The output of the voltage controlled crystal oscillator 15 is supplied to the external circuit as the output of the high stability oscillator and is fed back to the phase comparator 12 via the frequency divider 16.

  In this highly stable oscillator, since the output frequency is a single frequency determined in advance, the voltage controlled crystal oscillator 15 based on a crystal oscillator excellent in short-term stability is used as the output oscillator of the PLL circuit. The frequency variable range in the voltage controlled crystal oscillator 15 is a narrow range in the vicinity of a predetermined output frequency. The frequency dividing ratio of the frequency divider 16 is set to a value determined according to the output frequency and the MEMS oscillator 11, and the frequency dividing ratio is kept constant at least during the operation of the high stability oscillator.

  In this highly stable oscillator, long-term stability is ensured by the MEMS oscillator 11 having excellent long-term stability, while short-term stability is enhanced by the voltage-controlled crystal oscillator 15. For example, by setting a long time constant of the PLL circuit (for example, setting the cut-off frequency of the low-pass filter 13 to be extremely low) or by intermittently executing PLL control, a short-term in this highly stable oscillator Both stability and long-term stability can be achieved. In order to intermittently execute the PLL control, a sample / hold circuit that is operated by a clock signal supplied from the outside for intermittent control is inserted, for example, between the low-pass filter 14 and the voltage-controlled crystal oscillator 15. It is possible. If a voltage controlled oscillator (VCO) of a type that does not use a crystal resonator is used instead of the voltage controlled crystal oscillator 15, the short-term stability at the oscillation frequency of the voltage controlled oscillator is not good. Even if a MEMS oscillator is combined, an oscillator having both short-term stability and long-term stability cannot be obtained.

  Here, the MEMS oscillator 11 used in the highly stable oscillator of this embodiment will be described. As described above, the MEMS vibrators constituting the MEMS oscillator 11 include those using a silicon semiconductor and those using a piezoelectric body. In general, a MEMS resonator using silicon has a higher equivalent series resistance when the natural frequency is increased, and it is difficult to start. In addition, the vibrator is too small to be suitable for manufacturing. On the other hand, a MEMS vibrator using a piezoelectric material has a feature that although the equivalent series resistance is small, if the natural frequency is lowered, the piezoelectric film becomes too thick to be suitable for manufacturing. Therefore, when a relatively low output frequency is to be obtained as a highly stable oscillator, it is preferable to use a silicon oscillator material as the MEMS vibrator of the MEMS oscillator 11, and to obtain a relatively high output frequency. In this case, it is preferable to use a piezoelectric material. As the MEMS vibrator using the piezoelectric material, for example, one configured as an FBAR (Film Bulk Acoustic Resonator) can be used.

    11 MEMS oscillator; 12 phase comparator; 13 charge pump; 14 low-pass filter (LPF); 15 voltage-controlled crystal oscillator (VCXO); 16 divider; 21 microwave synthesizer; 22 physical package;

Claims (3)

A PLL circuit including a voltage controlled crystal oscillator as an output oscillator;
A MEMS oscillator having a MEMS resonator;
With
A highly stable oscillator in which an output of the MEMS oscillator is supplied as a reference signal to a phase comparator of the PLL circuit and outputs a signal having a constant frequency.   The highly stable oscillator according to claim 1, wherein the MEMS vibrator is configured using silicon.   The highly stable oscillator according to claim 1, wherein the MEMS vibrator is configured using a piezoelectric material.

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