Abstract:
A self-oscillation circuit includes a vibration unit having a vibrator, a positive feedback path which positively feeds back a signal based on vibration of the vibrator to the vibration unit, a negative feedback circuit which generates a pulse-width-modulated signal having a frequency lower than a vibration frequency of the vibrator, based on a comparison result between a value corresponding to an amplitude of the vibrator and a reference value, and a switch circuit which switches connection and disconnection of the positive feedback path to the vibration unit by the pulse-width-modulated signal.

Description:
BACKGROUND 
       [0001]    Technical Field 
         [0002]    The present invention relates to a self-oscillation circuit for oscillating a vibrator by a positive feedback circuit. 
         [0003]    Related Art 
         [0004]    A capacitive vibration type pressure/differential pressure sensor or the like includes a self-oscillation circuit for oscillating a vibrator at a resonance frequency.  FIG. 8  is a view showing a configuration example of a related-art capacitive vibration type self-oscillation circuit. As shown in  FIG. 8 , a self-oscillation circuit  500  includes a positive feedback circuit for oscillating a vibrator  511  and a negative feedback circuit for controlling oscillation amplitude of the vibrator  511 . 
         [0005]    The positive feedback circuit is formed in a loop passing through the vibrator  511 , a second fixed electrode  513 , an I/V converter  520 , an inverting amplifier  530  and a variable-gain amplifier  560  from a first fixed electrode  512 . Generally, the vibrator  511  is vacuum-sealed in order to increase a value of Q. 
         [0006]    The negative feedback circuit is formed in a circuit passing through an absolute value circuit  540  for detecting an absolute value of a signal outputted from the inverting amplifier  530 , an error amplifier  550 , and the variable-gain amplifier  560 . 
         [0007]    In the positive feedback circuit, the vibrator  511  is fixed to a GND potential, and a bias voltage VBLAS is applied to the first fixed electrode  512  and the second fixed electrode  513  via a DC voltage source. At this time, a charge corresponding to capacitance is charged between the vibrator  511  and the first fixed electrode  512 , and between the vibrator  511  and the second fixed electrode  513 . 
         [0008]    In addition to the bias voltage VBIAS, an output voltage VGAO of the variable-gain amplifier  560  is applied to the first fixed electrode  512 . The vibrator  511  vibrates in accordance with the potential change of the first fixed electrode  512 . 
         [0009]    As the vibrator  511  vibrates, the charging and discharging of the charge occurs, and a current output signal from the second fixed electrode  513  is inputted to the I/V converter  520  and is outputted as a voltage signal IVO. The voltage signal IVO is inverted and amplified in the inverting amplifier  530  and is outputted as a voltage signal MVO. The voltage signal INVO is amplified in the variable-gain amplifier  560  and is applied, as the voltage signal VGAO, to the first fixed electrode  512 . Such positive feedback circuit allows the vibrator  511  to vibrate at its own resonance frequency. 
         [0010]    In the negative feedback circuit, the amplitude of the voltage signal INVO outputted from the inverting amplifier  530  is detected by the absolute value circuit  540 . The absolute value circuit  540  can be configured by using a full-wave rectifier circuit or the like. A voltage signal ABSO outputted from the absolute value circuit  540  corresponds to oscillation amplitude of the vibrator  511 . 
         [0011]    A difference between the voltage signal ABSO and a reference voltage VCONT is detected, as an error signal ERRO, in the error amplifier  550 , and the gain of the variable-gain amplifier  560  is changed by the error signal ERRO. In the case of  FIG. 8 , the gain of the variable-gain amplifier  560  is increased when the amplitude of the vibrator  511  is small and the error signal ERRO is great, and the gain of the variable-gain amplifier  560  is decreased when the amplitude of the vibrator  511  is great and the error signal ERRO is small. As the gain of the variable-gain amplifier  560  is adjusted, the amplitude of the vibrator  511  is normally controlled to be constant. 
         [0012]    Patent Document 1: International Publication WO 2011/102062 
         [0013]    When such self-oscillation circuit  500  is applied to an apparatus that requires low power consumption, such as, for example, two-wire type instrument, it is necessary to constitute the self-oscillation circuit by an ASIC where it is easy to achieve low power consumption. The reason is that it is difficult to satisfy the low power consumption specification when the self-oscillation circuit is configured by a discrete component. 
         [0014]    However, in the related-art self-oscillation circuit  500 , a gain of the variable-gain amplifier  560 , which is used in the positive feedback, is changed by the output of the negative feedback circuit. Therefore, the positive feedback circuit and the negative feedback circuit have a mutually dependent relationship, and hence, the interface therebetween becomes complicated. Accordingly, a strict adjustment between the positive feedback circuit and the negative feedback circuit is required. For example, when the design changes to the IN converter  520  and the inverting amplifier  530  occur, the design of the variable-gain amplifier  560  should be also changed. This causes an increase in the design man-hours and acts as a barrier to the ASIC. 
       SUMMARY 
       [0015]    Exemplary embodiments of the invention provide a self-oscillation circuit without using a variable-gain amplifier that complicates an interface between a positive feedback circuit and a negative feedback circuit. 
         [0016]    A self-oscillation circuit according to an exemplary embodiment, comprises: 
         [0017]    a vibration unit having a vibrator; 
         [0018]    a positive feedback path which positively feeds back a signal based on vibration of the vibrator to the vibration unit; 
         [0019]    a negative feedback circuit which generates a pulse-width-modulated signal having a frequency lower than a vibration frequency of the vibrator, based on a comparison result between a value corresponding to an amplitude of the vibrator and a reference value; and 
         [0020]    a switch circuit which switches connection and disconnection of the positive feedback path to the vibration unit by the pulse-width-modulated signal. 
         [0021]    The self-oscillation circuit may further comprise: 
         [0022]    a synchronization unit which synchronizes the signal based on the vibration of the vibrator with a switching timing of the switch circuit. 
         [0023]    The self-oscillation circuit may further comprise: 
         [0024]    a buffer provided in the positive feedback path, the buffer being switched between an enable state and a disable state by the pulse-width-modulated signal. 
         [0025]    The negative feedback circuit may generate the pulse-width-modulated signal such that the greater a difference between the value corresponding to the amplitude of the vibrator and the reference value is, the longer a pulse width is. 
         [0026]    The negative feedback circuit may include an error amplifier which compares the value corresponding to the amplitude of the vibrator with the reference value to output an error signal, and a PWM unit which performs pulse-width-modulation of the error signal. 
         [0027]    The PWM unit may include a triangular wave oscillator which outputs a triangular wave having a frequency lower than the vibration frequency of the vibrator, and a comparator which compares the error signal with the triangular wave to generate the pulse-width-modulated signal. 
         [0028]    The negative feedback circuit may include an AD converter which digitally converts the value corresponding to the amplitude of the vibrator, a digital error detection unit which compares the digitized value with the reference value to detect an error, a digital PWM unit which performs pulse width modulation of the detected error. 
         [0029]    The digital error detection unit may include a subtractor which calculates a difference between the digitized value and the reference value and a digital filter which controls the digital PWM unit according to the difference. 
         [0030]    According to the present invention, it is possible to achieve a self-oscillation circuit without using a variable-gain amplifier. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIG. 1  is a view showing a configuration of a self-oscillation circuit according to a first embodiment of the present invention. 
           [0032]      FIG. 2  is a waveform diagram for explaining an operation of a PWM unit. 
           [0033]      FIG. 3  is a view showing a configuration of a self-oscillation circuit according to a second embodiment of the present invention. 
           [0034]      FIG. 4  is a view showing a configuration of a self-oscillation circuit according to a third embodiment of the present invention. 
           [0035]      FIG. 5  is a waveform diagram for explaining an operation of a synchronization unit. 
           [0036]      FIGS. 6A to 6C  are views showing a modified example of each embodiment. 
           [0037]      FIG. 7  is a view showing a modified example of each embodiment. 
           [0038]      FIG. 8  is a view showing a configuration example of a related-art capacitive vibration type self-oscillation circuit. 
       
    
    
     DETAILED DESCRIPTION 
       [0039]    Exemplary embodiments of the present invention will be described with reference to the drawings.  FIG. 1  is a view showing a configuration of a self-oscillation circuit  100  according to a first embodiment of the present invention. Meanwhile, the self-oscillation circuit of the present invention is not limited to a capacitive vibration type self-oscillation circuit, but can be applied to various self-oscillation circuits having a positive feedback circuit. 
         [0040]    As shown in  FIG. 1 , the self-oscillation circuit  100  of the first embodiment includes a positive feedback circuit for oscillating a vibrator  111 , and a negative feedback circuit for controlling oscillation amplitude of the vibrator  111 . 
         [0041]    The positive feedback circuit is formed in a loop passing through the vibrator  111 , a second fixed electrode  113 , an IN converter  120 , an inverting amplifier  130  and a SW circuit  170  from a first fixed electrode  112 . Generally, the vibrator  111  is vacuum-sealed in order to increase a value of Q. Meanwhile, the first fixed electrode  112 , the vibrator  111  and the second fixed electrode  113  constitute a vibration unit. A positive feedback path is configured by a path that extends from an output of the inverting amplifier  130  to an input of the first fixed electrode  112  via the SW circuit  170 . 
         [0042]    The negative feedback circuit is formed in a circuit passing through an absolute value circuit  140  for detecting an absolute value of a voltage signal INVO outputted from the inverting amplifier  130 , an error amplifier  150 , a PWM unit  160 , and the SW circuit  170 . 
         [0043]    The SW circuit  170  is switched and controlled by an output signal PWMO of the PWM unit  160 . Specifically, when the output signal PWMO is H, the voltage signal INVO outputted from the inverting amplifier  130  is fed back to the first fixed electrode  112  to form a positive feedback loop of the positive feedback circuit. Further, when the output signal PWMO is L, the positive feedback loop of the positive feedback circuit is released. 
         [0044]    In the positive feedback circuit, the vibrator  111  is fixed to a GND potential, and a bias voltage VBIAS is applied to the first fixed electrode  112  and the second fixed electrode  113  via a DC voltage source, regardless of the state of the SW circuit  170 . At this time, a charge corresponding to capacitance is charged between the vibrator  111  and the first fixed electrode  112 , and between the vibrator  111  and the second fixed electrode  113 . 
         [0045]    When the output signal PWMO is H, the positive feedback loop is formed by the SW circuit  170 . Therefore, in addition to the bias voltage VBLAS, the voltage signal INVO outputted from the inverting amplifier  130  is applied to the first fixed electrode  112  and the vibrator  111  vibrates according to the potential change of the first fixed electrode  112 . 
         [0046]    As the vibrator  111  vibrates, the charging and discharging of the charge occurs, and a current output signal from the second fixed electrode  113  is inputted to the IN converter  120  and is outputted as a voltage signal IVO. The voltage signal IVO is inverted and amplified in the inverting amplifier  130  and is outputted as the voltage signal INVO. Such positive feedback circuit allows the vibrator  111  to vibrate at its own resonance frequency. 
         [0047]    In the negative feedback circuit, the amplitude of the voltage signal INVO outputted from the inverting amplifier  130  is detected by the absolute value circuit  140 . The absolute value circuit  140  can be configured by using a full-wave rectifier circuit or the like. A voltage signal ABSO outputted from the absolute value circuit  140  corresponds to the oscillation amplitude of the vibrator  111 . 
         [0048]    A difference between the voltage signal ABSO and a reference voltage VCONT is detected, as an error signal ERRO, in the error amplifier  150 . The error signal ERRO is pulse-width-modulated by the PWM unit  160  and is outputted as a PWMO signal. 
         [0049]    As shown in  FIG. 2 , the error signal ERRO can be compared with a triangular wave (a saw-toothed wave) TRI by a comparator  161 , thereby generating the PWMO signal. At this time, as the frequency of the pulse width modulation, i.e., the frequency of the triangular wave, a frequency lower than the resonance frequency of the vibrator  111  is used. The reason is that a positive feedback loop formation period is sufficiently secured with respect to the vibration cycle and the vibrator  111  is stably oscillated. 
         [0050]    The smaller the amplitude of the vibrator  111  is and the greater the error signal ERRO is, the longer the H pulse width of the PWMO signal in each cycle is. Further, the greater the amplitude of the vibrator  111  is and the smaller the error signal ERRO is, the shorter the H pulse width of the PWMO signal in each cycle is. 
         [0051]    When the PWMO signal is H, i.e., when the error signal ERRO is greater than the triangular wave the positive feedback loop is formed, and thus, the amplitude of the vibrator  111  is grown. On the other hand, when the PWMO signal is L, i.e., when the error signal ERRO is smaller than the triangular wave TRI, the positive feedback loop is released, and thus, the amplitude of the vibrator  111  is attenuated. As the growth and attenuation of the amplitude is repeated, the amplitude of the vibrator  111  is normally controlled to be constant. 
         [0052]    As described above, the Q of the vibrator  111  is generally designed to be relatively high. Therefore, with respect to the oscillation cycle of the vibrator  111 , the growth and attenuation of the amplitude is very gentle. Accordingly, even when the vibrator  111  is intermittently operated by the SW circuit  170 , the hunting width of the amplitude of the vibrator  111  can be reduced and the amplitude can be normally controlled to be almost constant. 
         [0053]    According to the self-oscillation circuit  100  of the first embodiment, the variable-gain amplifier that complicates the interface between the positive feedback circuit and the negative feedback circuit is not necessary, and the positive feedback circuit and the negative feedback circuit are disconnected, so that the interface adjustment between the circuits is simplified. That is, the characteristics on the positive feedback circuit side are uniquely determined by the design of the IN converter  120  and the inverting amplifier  130 , and the characteristics on the negative feedback circuit side are uniquely determined by the design of the error amplifier  150  and the PWM unit  160 . In this way, the positive feedback circuit and the negative feedback circuit can be independently adjusted. As a result, the design man-hours can be reduced and the ASIC is easily achieved. Furthermore, instead of the variable-gain amplifier that lacks of versatility, the PWM unit  160 , which can be configured by a versatile comparator and a versatile triangular wave oscillator, is used. Therefore, it is also possible to achieve the easy mounting, the low voltage and the low consumption power. 
         [0054]      FIG. 3  is a view showing a configuration of a self-oscillation circuit  200  according to a second embodiment of the present invention. As shown in  FIG. 3 , the self-oscillation circuit  200  of the second embodiment also includes a positive feedback circuit for oscillating the vibrator  111 , and a negative feedback circuit for controlling oscillation amplitude of the vibrator  111 . However, in the self-oscillation circuit  200  according to the second embodiment, the negative feedback circuit is digitized. 
         [0055]    Since the positive feedback circuit is similar to that of the first embodiment, the positive feedback circuit is denoted by the same reference numeral. Namely, the positive feedback circuit is formed in a loop passing through the vibrator  111 , the second fixed electrode  113 , the UV converter  120 , the inverting amplifier  130  and the SW circuit  170  from the first fixed electrode  112 . 
         [0056]    The negative feedback circuit is formed in a circuit passing through an AD converter  210  for digitally converting the voltage signal INVO outputted from the inverting amplifier  130 , a digital error detection unit  220  for comparing the digitized value with a reference value and detecting an error, a digital PWM unit  230  for performing the pulse width modulation of the detected error, and the SW circuit  170 . For example, the digital error detection unit  220  can be configured by a subtractor for calculating a difference between the digitized value and the digital reference voltage and a digital filter for controlling the digital PWM unit  230  according to the error. 
         [0057]    The SW circuit  170  is switched and controlled by the output signal PWMO of the digital PWM unit  230 . Specifically, the positive feedback loop of the positive feedback circuit is formed when the output signal PWMO is H, and the positive feedback loop of the positive feedback circuit is released when the output signal PWMO is L. 
         [0058]    Although the negative feedback circuit of the self-oscillation circuit  200  of the second embodiment is digitized, a basic operation principle of the self-oscillation circuit  200  is similar to the self-oscillation circuit  100  of the first embodiment. Since, in addition to the characteristics of the self-oscillation circuit  100  of the first embodiment, the self-oscillation circuit  200  of the second embodiment is configured such that an analog circuit element is omitted, the design man-hours can be further reduced. Further, since the integration degree of the analog circuit in the ASIC is lowered, the production cost can be reduced. 
         [0059]      FIG. 4  is a view showing a configuration of a self-oscillation circuit  300  according to a third embodiment of the present invention. As shown in  FIG. 4 , the self-oscillation circuit  300  of the third embodiment has a configuration that a synchronization unit  180  is additionally provided to the self-oscillation circuit  100  of the first embodiment. Meanwhile, the synchronization unit  180  may be additionally provided to the self-oscillation circuit  200  of the second embodiment. 
         [0060]    The synchronization unit  180  includes a comparator and a D-FF and is configured such that the SW circuit  170  is switched at the timing when the voltage signal INVO (AC component) outputted from the inverting amplifier  130  is changed from negative to positive. Specifically, at the timing when the voltage signal INVO (AC component) is changed from negative to positive, i.e., when a voltage signal SWO is equal to the bias voltage VBIAS, the comparator outputs a CMPO2 signal (voltage rise) to operate the D-FF. Further, the SW circuit  170  is switched by a D-FFQ signal outputted from the D-FF. It is noted that the switching may be performed at the timing when the voltage signal INVO is changed from positive to negative or at the timing of every half cycle. 
         [0061]    When the synchronization unit  180  is not provided, the switching of the SW circuit  170  is not synchronized with the vibration of the vibrator  111 . Therefore, at the time of the switching of the SW circuit  170 , sudden variation occurs in the voltage signal SWO applied to the first fixed electrode  112 , and hence, the voltage signal SWO is often disturbed. 
         [0062]    On the contrary, when the synchronization unit  180  is provided, the switching of the SW circuit  170  is synchronized with the vibration of the vibrator  111 , as shown in  FIG. 5 . Namely, the SW circuit  170  is switched when the voltage signal SWO is equal to the bias voltage VBIAS. As a result, at the time of the switching of the SW circuit  170 , sudden variation does not occur in the voltage signal SWO, and hence, the disturbance of the voltage signal SWO can be reduced. 
         [0063]    Meanwhile, in each of the above-described embodiments, the bias voltage VBIAS is applied to the first fixed electrode  112  via the SW circuit  170 , as shown in  FIG. 6A . In this case, both an H terminal and an L terminal of the SW circuit  170  are connected to a DC voltage source. 
         [0064]    On the contrary, the bias voltage VBIAS may be applied to the first fixed electrode  112  without passing through the SW circuit  170 , as shown in  FIG. 6B . In this case, the L terminal of the SW circuit  170  may be connected to the DC voltage source or may not be connected to a DC voltage source by being floated, as shown in  FIG. 6C . 
         [0065]    By the way a somewhat large parasitic capacitance (e.g., about 30 pF) is present between the first fixed electrode  112  and the GND. Therefore, it is necessary to supply the inverting amplifier  130  with a current for driving the parasitic capacitance. This is a factor of increasing the consumption current. 
         [0066]    In each of the above-described embodiments, it is sufficient to drive the parasitic capacitance only during the period in which the positive feedback loop is formed. Therefore, it is possible to perform an operation of increasing the drive capacity when the signal PWMO is H and decreasing the drive capacity when the signal PWMO is L. 
         [0067]    Therefore, for example, as shown in  FIG. 7 , a buffer  190  having an enable function is placed between the inverting amplifier  130  and the SW circuit  170 . Only when the signal PWMO is H, the buffer  190  is in an enable state to secure the drive capacity. Further, when the signal PWMO is L, the buffer  190  is in a disable state and the current is not consumed in the buffer  190 . In this way, it is possible to reduce the consumption power. The buffer  190  having the enable function can be applied to all of the above-described embodiments.