Patent Application
20130187720
This application claims the priority benefit of Japan application serial no. 2012-010936, filed on Jan. 23, 2012. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.
This disclosure relates to an oscillator that outputs an oscillation signal with high frequency accuracy without affected by an ambient temperature. Especially, this disclosure relates to a temperature compensation type oscillator that includes a crystal resonator such as a Microelectromechanical Systems (MEMS) resonator as a crystal resonator where a change in the resonance frequency relative to a temperature is comparatively large.
A crystal controlled oscillator has been generally used for applications that require high frequency stability. Recently, use of a MEMS oscillator including a MEMS resonator has been considered. The MEMS resonator has been developed accompanied by miniaturization of semiconductor device fabrication technique. For example, a MEMS resonator is disclosed in “Crystal controlled oscillator manufacturers talk about comparison theory of ‘MEMS vs. crystal’, which becomes the subject of heated debates in oscillators, by Takeo Oita NIKKEI MICRODEVICES, No. 268, p 71-76 (October, 2007)”. This publication is hereinafter referred to as non-Patent Literature 1. The MEMS resonator includes a semiconductor (for example, silicon) or a piezoelectric body such as aluminum nitride (AlN), and an electrode or similar on the piezoelectric body, thus forming a crystal resonator. The piezoelectric body is minutely processed with high accuracy, for example, at a size of several μm to several tens μm. For example, a MEMS resonator made of silicon semiconductor includes a resonator and four electrodes that are fabricated using a semiconductor device fabrication technique. The resonator is a silicon layer processed to have a circular plate with a diameter of several tens and two beams extending in the diameter direction of the circular plate where the two beams hold the circular plate. The four electrodes are disposed extremely close to the resonator. The two beams function as movable parts relative to the main body of the resonator formed on the circular plate and supports the main body of the resonator without inhibiting a contour mode vibration of the main body of the resonator. The respective electrodes are disposed at positions where an outer peripheral of the main body of the resonator is equally divided by four. The electrodes are disposed with a gap of, for example, 100 nm from the outer peripheral surface of the resonator. Between each electrode and the main body of the resonator, an electrostatic capacity is formed. In this MEMS resonator, electrostatically-driving the resonator via the electrodes vibrates the resonator at its unique mechanical resonance frequency. The oscillation slightly changes an interval between the resonator and the electrodes, and the electrostatic capacity periodically changes at the resonance frequency. Accordingly, an electrode potential also vibrates at the resonance frequency. Thus, embedding the MEMS resonator into an oscillation circuit obtains an oscillator that outputs a signal corresponding to the resonance frequency of the MEMS resonator, namely, a MEMS oscillator.
The MEMS oscillator can be manufactured using semiconductor device fabrication technique only. This facilitates downsizing and allows fabrication at low cost. The MEMS oscillator also features toughness, and a resonator needs not to be fixedly secured on a container with an adhesive, thus ensuring long-term stability. With the MEMS resonator, in a frequency versus temperature characteristic, which is expressed as a rate of change of the resonance frequency depending on a temperature, a temperature coefficient of the second or higher order term is negligible. However, an absolute value of a primary temperature coefficient is, for example, extremely large compared to a quartz crystal resonator or similar. Therefore, in a MEMS oscillator where any temperature compensation has not been performed, the following problem occurs, an output frequency substantially changes as an ambient temperature changes. In the case where a non-piezoelectric material such as silicon is used for a MEMS resonator, a series equivalent capacity in an equivalent circuit of the crystal resonator is extremely small. Therefore, an output frequency hardly changes even if a value of a load capacitance connected to the crystal resonator is changed. Therefore, a method that compensates a frequency versus temperature characteristic by changing the load capacitance value corresponding to an ambient temperature cannot be employed. A known method changes a resonance frequency of a MEMS resonator by changing a bias voltage applied between the MEMS resonator and electrodes disposed around the MEMS resonator (see Japanese Unexamined Patent Application Publication No. 11-508418 and non-Patent Literature 1). However, this change in the resonance frequency due to the change in the bias voltage is not enough to compensate for the change in the resonance frequency due to an ambient temperature. In the case where a piezoelectric material is used for a MEMS resonator, for example, a MEMS resonator configured as Film Bulk Acoustic Resonator (FBAR) is used, changing a value of a load capacitance connected to the crystal resonator allows changing an output frequency to some extent (see Japanese Unexamined Patent Application Publication No. 2006-318478). In the following description, a temperature coefficient of a resonance frequency represents a primary temperature coefficient in the case where a frequency versus temperature characteristic of a resonance frequency is expressed as a rate of change from a resonance frequency at a reference temperature.
Non-Patent Literature 1 discloses a method for obtaining a constant output frequency using the MEMS oscillator without affected by an ambient temperature. The method (PLL division ratio compensation method) supplies an output from the MEMS oscillator to a frequency synthesizer circuit with a phase-locked loop (a PLL) circuit, and changes a division ratio in a frequency dividing circuit in the PLL circuit corresponding to an ambient temperature (Non-Patent Literature 1). Since this method switches a division ratio corresponding to an ambient temperature, this arises problems that an output frequency changes discontinuously, a phase noise characteristic is poor, and phase continuity in an output signal is not guaranteed. Accordingly, the MEMS oscillator where a temperature compensation is performed using a PLL division ratio compensation method is difficult to employ for, for example, a temperature compensation type crystal controlled oscillator.
Japanese Unexamined Patent Application Publication No. 2007-524303 (i.e., Patent Literature 3) discloses a method where many MEMS oscillators with respective different characteristics are prepared. This method selects some MEMS oscillators among the MEMS oscillators corresponding to an ambient temperature, and adds or subtracts outputs frequencies from the selected MEMS oscillators. Thus, this method obtains a temperature compensated output frequency.
Japanese Unexamined Patent Application Publication No. 2009-65601 (hereinafter referred to as Patent Literature 4) discloses a configuration that employs two MEMS resonators with different frequency versus temperature characteristics and different resonance frequencies. This configuration oscillates the respective MEMS resonators with separate oscillation circuits, and mixes oscillation signals from the MEMS resonators at a mixer circuit. Thus, an output frequency is not changed even if an ambient temperature changes. With a MEMS resonator using silicon, a known method changes a temperature coefficient by forming a silicon oxidized film on the surface of the MEMS resonator. The silicon oxidized film is disposed to perform a temperature compensation for a MEMS resonator alone. Here, assume that a first MEMS resonator has a resonance frequency of 150 MHz and a temperature coefficient of −10 ppm/° C., and a second MEMS resonator has a resonance frequency of 50 MHz and a temperature coefficient of −30 ppm/° C. Although temperature coefficients expressed by ppm differ between the both MEMS resonators, actual changes in frequency values of the both MEMS resonators in accordance with a temperature change match at −1.5 KHz/° C. Here, if an oscillation signal from a first MEMS resonator and an oscillation signal from a second MEMS resonator are mixed in a mixer circuit, and a difference frequency component between the both signals (100 MHz) are extracted, a contribution amount of frequency change due to a temperature change is offset. Thus, a signal of 100 MHz can be obtained without affected by a change in an ambient temperature.
Assume that a reference temperature T0 is, for example, 25° C., let resonance frequencies of the MEMS resonators 11 and 12 at the reference temperature be denoted as F1 and F2 (MHz). Let the temperature coefficients of the MEMS resonators 11 and 12 be denoted as A1 and A2 (ppm/° C.), respectively. Then, a resonance frequency f1(T) of the MEMS resonator 11 when an ambient temperature is T is expressed by f1(T)=F1+A1(T−T0)F1×10−6 (MHz). Similarity, a resonance frequency f2(T) of the MEMS resonator 12 at the same ambient temperature T is expressed by f2(T)=F2+A2(T−T0)F2×10−6 (MHz). Even if f1(T)≧f2(T) is assumed, the formula has universality. A difference frequency of f1(T)−f2(T) between the both resonance frequencies is expressed by f1(T)−f2(T)=F1−F2+(A1F1−A2F2) (T−T0)×10−6 (MHz). If F1/F2=A2/A1 is satisfied, A1F1=A2F2 is given and yields f1(T)−f2(T)=F1−F2 (MHz). The difference frequency between the both resonance frequencies is always expressed by F1−F2, without affected by an ambient temperature. Accordingly, an output frequency Fout, where a temperature is compensated, is obtained.
However, it is difficult to accurately form a silicon oxidized film or similar on the surface of the MEMS resonator. Also, it is difficult to control a temperature coefficient at a desired value. In the case where a film is formed thick on the surface of the MEMS resonator, a resonance of the MEMS resonator (a Q factor) becomes low. As a result, a frequency in an oscillation signal becomes unstable. Eventually, with the method disclosed in Patent Literature 4, it is difficult to constitute a MEMS oscillator that outputs a constant frequency without affected by a change in an ambient temperature.
The conventional oscillator using a crystal resonator such as a MEMS resonator, where a resonance frequency substantially changes corresponding to a temperature change, has a problem such as the following. When a change in a resonance frequency due to a temperature is compensated to obtain an output at constant frequency without affected by an ambient temperature, it is difficult to obtain a good phase noise characteristic.
A need thus exists for a temperature compensation type oscillator which is not susceptible to the drawback mentioned above.
According to an aspect of this disclosure, there is provided an oscillator. The oscillator includes a first crystal resonator, a second crystal resonator, a first amplifier circuit for oscillation, a second amplifier circuit for oscillation, a mixer circuit, a frequency selection circuit, and a first frequency conversion circuit. The first amplifier circuit is combined with the first crystal resonator and configured to output a first oscillation signal. The second amplifier circuit is combined with the second crystal resonator and configured to output a second oscillation signal. The mixer circuit is configured to mix the first oscillation signal with the second oscillation signal. The frequency selection circuit is configured to: select a predetermined frequency component from outputs from the mixer circuit, and output the selected component. The first frequency conversion circuit is configured to perform frequency conversion of the first oscillation signal. The first oscillation signal after the frequency conversion in the first frequency conversion circuit is supplied to the mixer circuit. Assuming that resonance frequencies of the first and the second crystal resonators at a reference temperature are respectively F1 and F2, and temperature coefficients expressed as a rate of change corresponding to temperatures of the resonance frequencies of the first and the second crystal resonators are respectively A1 and A2, the relationship of F2/F1≠|A1/A2| is satisfied. A signal with a temperature compensated frequency is obtained from the frequency selection circuit.
The foregoing and additional features and characteristics of this disclosure will become more apparent from the following detailed description considered with the reference to the accompanying drawings, wherein:
The preferred embodiments of the disclosure will be described with referring to the drawings.
A temperature compensation type oscillator illustrated in
The oscillator includes two crystal resonators 21 and 22. The following is assumed here. Resonance frequencies of the crystal resonators 21 and 22 at a reference temperature T0 (for example, 25° C.) are F1 and F2 (MHz), respectively. The crystal resonators 21 and 22 have temperature coefficients of A1 and A2 (ppm/° C.), respectively. Here, let it be assumed that F1/F2≠|A2/A1|.
The crystal resonator 21 is interposed between an input and an output of an amplifier circuit 13 for oscillation. The amplifier circuit 13 vibrates the crystal resonator 21 as a resonance element to output an oscillation signal. The resonance frequency of the crystal resonator 21 has a temperature dependence expressed using a temperature coefficient A1. Hence, a frequency f1 (T) of the oscillation signal is also a function of a temperature T, and is expressed by f1(T)=F1+A1(T−T0)F1×10−6 (MHz). The oscillation signal from the amplifier circuit 13 is supplied to a frequency conversion circuit 23. The frequency conversion circuit 23 includes a frequency synthesizer circuit with a PLL circuit of a fractional-N type. Let a division ratio of the PLL circuit be denoted as n1. The frequency conversion circuit 23 multiplies a frequency f1(T) of an oscillation signal, which is output from the amplifier circuit 13, by n1. Then, the frequency conversion circuit 23 outputs the multiplied signal as a signal of the frequency f1′(T). The frequency conversion circuit 23 may employ a fractional multiplication circuit. Similarly, the crystal resonator 22 is interposed between an input and an output of an amplifier circuit 14 for oscillation. The amplifier circuit 14 oscillates the crystal resonator 22 as a resonance element and outputs an oscillation signal expressed by a frequency f2(T). The f2(T) is a function of temperature and is expressed by f2(T)=F2+A2(T−T0)F2×10−6 (MHz). The oscillation signal from the amplifier circuit 14 is supplied to the frequency conversion circuit 24, which includes a frequency synthesizer circuit with the PLL circuit of a fractional-N type where a division ratio is n2, or a fractional multiplication circuit. The frequency conversion circuit 24 multiplies the frequency f2(T) of the oscillation signal, which is output from the amplifier circuit 14, by n2 and outputs the multiplied signal as a signal of the frequency f2′(T). The frequency multiplication ratios n1 and n2 are generally expressed by a positive rational number.
The temperature compensation type oscillator includes a mixer circuit 15. The mixer circuit 15 inputs signals from the respective frequency conversion circuits 23 and 24, and generates signals with frequencies corresponding to a sum frequency and a difference frequency of the input signals. Assume that a frequency of the signal output from the mixer circuit 5 is fmix, fmix is expressed by fmix=f1′(T)±f2′(T). In the double sign (±), “+” corresponds to a sum frequency, and “−” corresponds to a difference frequency. Since f1′(T)=n1f1(T) and f2′(T) =n2f2(T) are given, the following formula is obtained.
f
mix
=f
1′(T)±f2′(T)
f
1′(T)±f2′(T)=n1F1±n2F2+(n1A1F1±n2A2F2) (T−T0)×10−6 (MHz)
From this formula, n1/n2=−(A2F2/A1F1) is satisfied insofar as values of the temperature coefficients A1 and A2 have different signs. Hence, n1A1F1+n2A2F2=0 is given by appropriately selecting the multiplication ratios n1 and n2, and the sum frequency is expressed by n1F1+n2F2 without affected by a temperature. That is, as a temperature compensated output frequency, a sum frequency component from the mixer circuit 15 may be used. Similarity, n1/n2=−(A2F2/A1F1) is satisfied insofar as values of the temperature coefficients A1 and A2 have the same sign. Hence, n1A1F1−n2A2F2=0 is given by appropriately selecting the multiplication ratios n1 and n2, and the difference frequency is expressed by n1F1−n2F2 without affected by a temperature. That is, as a temperature compensated output frequency, a difference frequency component from the mixer circuit 15 may be used.
The multiplication ratios n1 and n2 of the frequency conversion circuits 23 and 24 are determined as described above, based on F1, F2, A1, and A2, which are actually measured at the two crystal resonators 21 and 22. This holds either of the sum frequency or the difference frequency, which is output from the mixer circuit 15, at a constant frequency even if an ambient temperature changes. A frequency selection circuit 25 is disposed at an output of the mixer circuit 15. The frequency selection circuit 25 selects either of the sum frequency component or the difference frequency component for which a temperature is compensated as described above, and outputs the selected component as an output frequency Fout1. The frequency selection circuit 25 may include, for example, a low-pass filter (for selecting a difference frequency component), a high-pass filter (for selecting a sum frequency component), or a band-pass filter.
When fabricating a crystal resonator, it is difficult to precisely control a temperature coefficient, setting aside the question of the resonance frequency. With the oscillator illustrated in
The oscillator illustrated in
In the example illustrated in
To change a temperature coefficient in the MEMS resonator, for example, in the case where the MEMS resonator is made of a silicon material, the following method may be employed. This method forms a silicon oxidized film as a temperature compensation material on the surface of the crystal resonator, and controls a thickness of the silicon oxidized film. The resonance frequency of the MEMS resonator depends on a size of the crystal resonator, but it is difficult to substantially change the crystal resonator size due to fabrication reasons. For this, the resonance frequency of the MEMS resonator cannot be substantially changed in the same mode of vibration. To substantially change the resonance frequencies between the two MEMS resonators, for example, one MEMS resonator may be driven in a basic vibration mode while the other MEMS resonator may be driven in a spurious mode.
In the oscillator illustrated in
F
out1
=n
1
f
1(T)−f2(T)
n
i
f
1(T)−f2(T)=n1F1−F2+(n1A1F1−A2F2) (T−T0)×10−6 (MHz).
If n1=A2F2/A1F2 is given, Fout1 is expressed by Fout1=n1F1−F2, the temperature compensated output frequency Fout1 is thus obtained.
Exemplary drawings illustrated in
The oscillator according to the disclosure may further include a second frequency conversion circuit configured to perform a frequency conversion of the second oscillation signal. The oscillator may be configured such that the second oscillation signal, which is a signal after the frequency conversion is performed in the second frequency conversion circuit, may be supplied to a mixer circuit.
In the case where two crystal resonators with respective resonance frequencies F1 and F2 and respective temperature coefficients A1 and A2 at a reference temperature T0 are used, an amplifier circuit for oscillation is provided for each crystal resonator, and a difference frequency between oscillation signals from respective amplifier circuits without a frequency conversion circuit is set as an output frequency, and F2/F1=A1/A2 is given, a constant output frequency can be obtained without affected by an ambient temperature. The relationship is not limited to the case where a crystal resonator is a MEMS resonator, but is generally applied insofar as the temperature coefficients A1 and A2 have the same sign (that is, A1A2>0). If the temperature coefficients of the two crystal resonators have different signs (A1A2<0), considering the sum frequency of the resonance frequencies f1(T) and f2(T), f1(T)+f2(T)=F1+F2 (MHz) is satisfied if F1A1+F2A2 =0, that is, F1/F2=−(A2/A1) is given. The sum frequency is always expressed by F1+F2 without affected by an ambient temperature. Thus, a constant output frequency, which is not affected by an ambient temperature, is obtained. In short, when the temperature coefficients A1 and A2 have the same sign and different signs, F1/F2>0 is given. Assume that |•| is a sign that denotes an absolute value. When F1/F2=|A2/A1| is given, a constant output frequency can be obtained without affected by an ambient temperature by obtaining a difference frequency or a sum frequency between the resonance frequencies f1(T) and f2(T) in a mixer circuit or similar. However, it is difficult to precisely control a temperature coefficient, especially in a crystal resonator. Eventually, it is difficult to obtain a temperature compensated output frequency where two crystal resonators are formed such that F1/F2=|A2/A1| is satisfied.
In contrast, the oscillator according to the disclosure, a frequency conversion circuit is disposed at an output of at least one side of the two amplifier circuits, and a frequency of the oscillation signal is converted in the frequency conversion circuit. In the case where a frequency synthesizer circuit with a PLL circuit, or a fractional multiplication circuit is used as a frequency conversion circuit, even if the frequency conversion circuit is passed through, the temperature coefficient of an oscillation signal does not change. For example, assume that the frequency conversion circuit is disposed at an output of the amplifier circuit for the first crystal resonator, and a division ratio of the PLL circuit there is regarded as n1. Considering a frequency at a reference temperature, the frequency conversion circuit converts the resonance frequency F1 into a frequency F1′(=n1F1) (that is, the frequency is multiplied by n1.) while the temperature coefficient A1 remains the same. Even if F2/F1≠|A1/A2| is given, F2/F1′=|A1/A2| is satisfied by appropriately setting the multiplication ratio n1 in the frequency conversion circuit. Accordingly, the difference frequency or the sum frequency between an oscillation signal from the frequency conversion circuit (the frequency F1′ at the reference temperature) and an oscillation signal from the other crystal resonator (the frequency F2 at the reference temperature) becomes constant without affected by the ambient temperature. In short, a temperature compensation type oscillator is configured without using a crystal resonator where a temperature coefficient is precisely controlled at a desired value. This applies when frequency conversion circuits are disposed at respective outputs of the two amplifier circuits for oscillation.
The oscillator according to the disclosure includes the frequency conversion circuit. A multiplication ratio in the frequency conversion circuit is determined corresponding to parameters unique to each crystal resonator (resonance frequency and a temperature coefficient at a reference temperature) and does not change corresponding to an ambient temperature. Thus, the oscillator according to the disclosure does not discontinuously change an output frequency. This guarantees phase continuity in an output signal, thus ensuring a good phase noise characteristic.
According to the disclosure, various crystal resonators such as a MEMS resonator and quartz crystal resonator may be employed as crystal resonators. Among two crystal resonators, one crystal resonator may be driven in a basic vibration mode, and the other crystal resonator may be driven in a spurious mode. In the case where the MEMS resonator is used as the crystal resonator, for example, the MEMS resonator may be made of a silicon material or a piezoelectric material.
With the oscillator according to the disclosure, the multiplication ratio in the frequency conversion circuit is determined such that a difference frequency or a sum frequency may be constant corresponding to the resonance frequencies F1 and F2 of the respective crystal resonators and the temperature coefficients A1 and A2 of the respective crystal resonators at a reference temperature without affected by an ambient temperature. However, an output frequency itself, which is obtained from the result, does not always match a desired frequency. To obtain a desired output frequency in the end, it is preferred that a frequency conversion circuit be further connected to an output of the frequency selection circuit, which selects a component of a predetermined frequency, that is, a component of a difference frequency or a sum frequency, from outputs from the mixer circuit and then outputs the selected component.
In the case where a temperature compensated output frequency is obtained using two crystal resonators, which have different resonance frequencies and different temperature coefficients, this disclosure describes the following effects. The use of the frequency conversion circuit, which holds a multiplication ratio using a parameter unique to each crystal resonator, eliminates the need for strict setting such as a temperature coefficient or each crystal resonator and also avoids difficulty in fabrication of the crystal resonator. Thus, an oscillator that features both good phase noise characteristic and a temperature characteristic is provided.
The principles, preferred embodiment and mode of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-010936 | Jan 2012 | JP | national |
