Abstract:
A temperature compensated oscillator and a control method are provided. The oscillator includes a Micro Electro Mechanical Systems (MEMS) resonator group, a heating device, and a controller. The MEMS resonator group includes a first MEMS resonator and a second MEMS resonator. The first MEMS resonator outputs a main oscillation frequency according to a control signal. The second MEMS resonator outputs an auxiliary oscillation frequency according to a temperature of the second MEMS resonator. The heating device increases a temperature of the MEMS resonator group. The controller controls the heating device according to a difference between the main oscillation frequency and the auxiliary oscillation frequency. In the control method, at first, the MEMS resonator group is provided. Thereafter, a frequency difference between the main oscillation frequency and the auxiliary oscillation frequency is calculated. Then, the temperature of the MEMS resonator group is controlled according to the frequency difference.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to Taiwan Application Serial Number 103102724, filed on Jan. 24, 2014, which is herein incorporated by reference. 
       BACKGROUND 
       [0002]    1. Field of Invention 
         [0003]    The present disclosure relates to a temperature compensated oscillator and a control method thereof. More particularly, the present disclosure relates to a micro electro mechanical systems (MEMS) temperature compensated oscillator and a control method thereof. 
         [0004]    2. Description of Related Art 
         [0005]    An oscillator is an electronic device used to generate a periodic signal (such as a square wave or a sine wave). Currently, a common electric device, such as a signal generator, a frequency synthesizer, or a phase lock loop, uses an oscillator to provide periodic signals required for operation. 
         [0006]    A quartz oscillator is one of the most popular oscillators presently. Since the quartz oscillator has advantages of simple structure and low cost, the quartz oscillator is popularly used in various electronic products. However, due to the limitation of mechanical cutting operations and polishing operations used to process quartz crystals, it is not easy to fabricate a quartz element having a small size and a high frequency. Therefore, a trend of using a micro electro mechanical systems (MEMS) oscillator to replace the quartz oscillator is gradually developed. 
         [0007]    For fabricating the MEMS oscillator, a MEMS technology is first used to fabricate a resonator structure, and then a System in Package (SiP) technology is used to integrate a controller and the resonator structure in a single chip package. Since the MEMS resonator is formed from silicon, the processes for fabricating the MEMS oscillator are compatible with semiconductor processes. Also, the MEMS oscillator has various oscillation modes, and thus a high frequency element with a small size can be fabricated thereby. However, since being affected by a Temperature Coefficient of Young&#39;s Modulus (TCE), a Coefficient of Thermal Expansion (CTE), etc. of the MEMS resonator, the frequency of the MEMS resonator is drifted with temperature changes. Therefore, a temperature compensation design is needed to increase the stability of the frequency of the MEMS resonator. 
       SUMMARY 
       [0008]    An aspect of the present disclosure is to provide a temperature compensated oscillator and a control method thereof. The temperature compensated oscillator and the control method thereof use a MEMS resonator to sense an environment temperature, thereby controlling a work state of a heating device to maintain temperature of MEMS resonators of the temperature compensated oscillator at a predetermined temperature. 
         [0009]    According to an embodiment of the present disclosure, the temperature compensated oscillator includes a MEMS resonator group, a heating device, and a controller. The MEMS resonator group includes a first MEMS resonator and a second MEMS resonator. The first MEMS resonator is configured to output a first periodic signal in accordance with a control signal, wherein the first periodic signal has a main oscillation frequency. The second MEMS resonator is configured to output a second periodic signal in accordance with temperature of the second MEMS resonator, wherein the second periodic signal has an auxiliary oscillation frequency. The heating device is configured to increase temperature of the MEMS resonator group. The controller is configured to control the heating device in accordance with a difference between the main oscillation frequency and the auxiliary oscillation frequency. The controller includes a counter and a temperature control unit. The counter is configured to calculate a frequency difference between the main oscillation frequency and the auxiliary oscillation frequency. The temperature control unit is configured to control the heating device in accordance with the frequency difference. 
         [0010]    According to another embodiment of the present disclosure, in the control method of the temperature compensated oscillator, at first, a MEMS resonator group is provided, in which the MEMS resonator group includes a first MEMS resonator and a second MEMS resonator. Then, the first MEMS resonator and the second MEMS resonator are drove to output first periodic signal and a second periodic signal, in which the first periodic signal has a main oscillation frequency, and the second periodic signal has an auxiliary oscillation frequency. Thereafter, a frequency difference between the main oscillation frequency and the auxiliary oscillation frequency is calculated. Then, a temperature control operation to control a heating device to adjust temperature of the MEMS resonator group is performed. 
         [0011]    It can be known from the above descriptions that the temperature compensated oscillator of the present disclosure includes two resonators, in which the first MEMS resonator is used to output the main oscillation frequency desired by a user, and the second MEMS resonator is used to sense the change of temperature and to output the auxiliary oscillation frequency accordingly. By receiving the difference between the main oscillation frequency and the auxiliary oscillation frequency, the controller can turn on or turn off the heater in accordance with the temperature change of the resonators, and thus the first MEMS resonator can work at the predetermined temperature. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    These and other features, aspects, advantages and embodiments of the present disclosure will become better understood with regard to the following accompanying drawings where: 
           [0013]      FIG. 1  shows a functional block diagram of a temperature compensated oscillator in accordance with embodiments of the present invention; 
           [0014]      FIG. 2  shows a top view of the temperature compensated oscillator in ordance with embodiments of the present invention; 
           [0015]      FIG. 3  shows relationships between temperature and output frequencies of the MEMS resonator group in accordance with the embodiments of the present invention; 
           [0016]      FIG. 4  shows a functional block of the controller of the embodiments of the present invention; 
           [0017]      FIG. 5  shows a flow chart of a control method of the temperature compensated oscillator in accordance with embodiments of the present invention; 
           [0018]      FIG. 6A  shows a functional diagram of a temperature compensated oscillator in accordance with embodiments of the present invention; and 
           [0019]      FIG. 6B  shows relationships between temperature and output frequencies of a MEMS resonator group in accordance with the embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Referring to  FIG. 1 .  FIG. 1  shows a functional block diagram of a temperature compensated oscillator  100  in accordance with embodiments of the present invention. The temperature compensated oscillator  100  includes a MEMS resonator group  110 , a heating device  120 , and a controller  130 . In the embodiments of the present invention, the heating device  120  is configured to increase temperature of the MEMS resonator group  110 , and the controller  130  is configured to control a work state of the heating device  120  in accordance with a difference of the frequencies output by the MEMS resonator group  110 . 
         [0021]    The MEMS resonator group  110  includes a first MEMS resonator  112  and a second MEMS resonator  114 . The first MEMS resonator  112  is configured to output a first periodic signal having a main oscillation frequency f 1 . The second MEMS resonator  114  is configured to output a second periodic signal in accordance with temperature of the second MEMS resonator  114 , in which the second periodic signal has an auxiliary oscillation frequency f 2 . In the present embodiment, the oscillator  100  is a temperature compensated MEMS oscillator, and thus the first MEMS resonator  112  and the second MEMS resonator  114  maintain the main oscillation frequency f 1  and the auxiliary oscillation frequency f 2  in accordance with voltage signals provided by internal driving circuits. However, the embodiments of the present invention are not limited thereto. 
         [0022]    In general, main material of MEMS resonators is silicon, and a temperature coefficient of frequency (TCF) of silicon is negative. In order to decrease temperature sensitivity of the main oscillation frequency f 1  the first MEMS resonator  112  is formed from composite material in which material having a positive TCF, such as SiO 2 , is embedded. However, the embodiments of the present invention are not limited thereto. 
         [0023]    Although the first MEMS resonator  112  of this embodiment is formed from materials including the positive TCF material, a change of temperature of the first MEMS resonator  112  may slightly affect the main oscillation frequency f 1 . Therefore, the heating device  120  is used to maintain the temperature of the first MEMS resonator  112  at a predetermined working temperature (for example, 85° C.), and the second MEMS resonator  114  is used to sense temperature for controlling the heating device  120  according to the sensing result, so that the temperature of the first MEMS resonator  112  can be maintained at the predetermined working temperature. 
         [0024]    Referring to  FIG. 2 ,  FIG. 2  shows a top view of the temperature compensated oscillator  100  in accordance with embodiments of the present invention. The heater  120  includes first contact pads  122 , second contact pads  124 , and resistors  126 . The first contact pads are used to provide a first temperature control voltage V 1 . The second contact pads  124  are used to provide a second temperature control voltage V 2 . The resistors  126  are electrically connected between the first contact pads  122  and the second contact pads  124  to use a voltage difference between the first temperature control voltage V 1  and the second temperature control voltage V 2  to provide heat energy to the MEMS resonator group  110 . 
         [0025]    In this embodiment, the heat energy generated by the resistors  126  may be transmitted to a frame  116  of the MEMS resonator group  110  through connection bodies  128 , and then the frame  116  transmits the heat energy to the resonator group  110 . The connection bodies  128  are connected between the resistors  126  and the resonator group  110  and formed from electric insulation materials. In this embodiment, the connection bodies  128  are formed from SiO 2 , but embodiments of the present invention are not limited thereto. The resistors  126 , the connection bodies  128 , the first MEMS resonator  112 , and the second MEMS resonator  114  are suspend above a semiconductor substrate (not illustrated), so that a good heat isolation environment is provided for conveniently controlling the temperature of the first MEMS resonator  112 . In the package of the temperature compensated oscillator  100 , air can be drew out to form an vacuum environment in the package for obtaining a better heat isolation effect. 
         [0026]    In addition, the temperature compensated oscillator  100  further includes a proof mass voltage supply circuit  140  and gain stage circuits (not illustrated). The proof mass voltage supply circuit  140  is used to provide a proof mass voltage V p  to the first MEMS resonator  112  and the second MEMS resonator  114  for helping the first MEMS resonator  112  and the second MEMS resonator  114  start oscillating. The gain stage circuits include a first gain stage circuit and a second gain stage circuit. The first gain stage circuit is electrically connected to the first MEMS resonator  112  to form an oscillation circuit. The second gain stage circuit is electrically connected to the second MEMS resonator  114  to form another oscillation circuit. In the embodiments of the present invention, the first gain stage circuit and the first MEMS resonator  112  form a Pierce oscillator, and the second gain stage circuit and the second MEMS resonator  114  form another Pierce oscillator. However, the embodiments of the present invention are not limited thereto. In other embodiments of the present invention, the first gain stage circuit and the first MEMS resonator  112  may form a Copitts oscillator, and the second gain stage circuit and the second MEMS resonator  114  may form another Copitts oscillator. 
         [0027]    Referring to  FIG. 3 ,  FIG. 3  shows relationships between temperature and output frequencies of the MEMS resonator group  110  in accordance with the embodiments of the present invention, in which a curve C 1  represents a relationship between temperature and an output frequency of the first MEMS resonator  112 , and a curve C 2  represents a relationship between temperature and an output frequency of the second MEMS resonator  114 . As mentioned above, the first MEMS resonator  112  includes positive TCF material to decrease the temperature sensitivity of the main oscillation frequency f 1 . Therefore, compared with the curve C 1  of the first MEMS resonator  112 , the curve C 2  of the second MEMS resonator  114  has a slope having a greater absolute value, so as to make the temperature sensing more convenient. 
         [0028]    In this embodiment, the controller  130  receives the main oscillation frequency f 1  output by the first MEMS resonator  112  and the auxiliary oscillation frequency  12  output by the second MEMS resonator  114 , and performs a temperature control operation in accordance with a frequency difference Δf between the main oscillation frequency f 1  and the auxiliary oscillation frequency f 2 . As shown in  FIG. 3 , it is represented that the temperature of the MEMS resonator group  110  is increased when the frequency difference Δf is increased. In contrast, it is represented that the temperature of the MEMS resonator group  110  is decreased when the frequency difference Δf is decreased. Therefore, in this embodiment, a frequency difference corresponding to the predetermined working temperature can be measured as a standard value of the frequency difference Δf in advance, and then the controller  130  can control the heating device  130  in accordance with the standard value of the frequency difference difference Δf. 
         [0029]    Referring to  FIG. 4 ,  FIG. 4  shows functional block of the controller  130  of the embodiments of the present invention. The controller  130  includes a counter  132 , a temperature control unit  136  and a digital-to-analog converter  138 . 
         [0030]    The counter  134  is electrically connected to the first MEMS resonator  112  and the second MEMS resonator  114  to receive the first periodic signal output by the first MEMS resonator  112  and the second periodic signal output by the second MEMS resonator  114 , and to calculate the frequency difference Δf between the main oscillation frequency f 1  and the auxiliary oscillation frequency  12 . The temperature control unit  136  is electrically connected to the counter  134  to output a first voltage control code V 1 _code and a second voltage control code V 2 _code to the digital-to-analog converter  138 . The digital-to-analog converter  138  is configured to respectively convert the first voltage control code V 1 _code and the second voltage control code V 2 _code to the first temperature control voltage V 1  and the second temperature control voltage V 2 , thereby using the heating device  120  to adjust the temperature of the MEMS resonator group  110 . In addition, it is noted that the digital-to-analog converter  138  can be removed if the temperature control unit  136  can output analog signals. 
         [0031]    Referring to  FIG. 5 ,  FIG. 5  shows a flow chart of a control method  500  of the temperature compensated oscillator in accordance with embodiments of the present invention. In the control method  500 , at first, a model establishing operation  510  is performed to calculate a temperature to frequency difference function representing the relationship between temperature and frequencies of the MEMS resonator group  110 , before the temperature compensated oscillator  100  starts working. In this embodiment, since the predetermined working temperature is 85° C. three frequency differences of the MEMS resonator group  110  at 0° C., 40° C., and 85° C. are measured in the model establishing operation  510 . Then, the three frequency differences are used to establish the temperature to frequency difference function in the model establishing operation  510 . In this embodiment, the temperature to frequency difference function is a second-order function, but the embodiments are not limited thereto. 
         [0032]    After the model establishing operation  510 , a standard value determination operation  520  is performed to find a frequency difference corresponding to the predetermined working temperature of the temperature compensated oscillator  100  ( 85  in this embodiment) in accordance with the temperature to frequency difference function, and to use the frequency difference as a standard value of temperature difference. Then, a driving operation  530  is performed to drive the MEMS resonator group  110  to start working. Thereafter, a frequency difference calculating operation  540  is performed to use the counter  134  to calculate a frequency difference Δf between the main oscillation frequency f 1  and the auxiliary oscillation frequency f 2 . 
         [0033]    Then, a temperature control operation  550  is performed to control the heating device  120  in accordance with the frequency difference to adjust temperature of the MEMS resonator group  110 . In the temperature control operation  550  of this embodiment, at first, a compensation value calculation operation  552  is performed to calculate a compensation temperature value in accordance with the frequency difference and the standard value of temperature difference. In this embodiment, a difference between the frequency difference and the standard value of frequency difference is calculated in the compensation value calculation operation  552 , but the embodiments of the present invention are not limited thereto. After the compensation value calculation operation  552 , a voltage calculation operation  554  is performed to calculate the first temperature control voltage V 1  and the second temperature control voltage V 2  needed for the heating device  120  in accordance with the compensation temperature value, and to transmit the first temperature control voltage V 1  and the second temperature control voltage V 2  to the heating device  120  for adjusting the temperature of the MEMS resonator group  110  to the predetermined working temperature. 
         [0034]    It can be known from the above description that the temperature compensated oscillator  100  and the control method  500  thereof use the frequency difference between the frequencies of the first MEMS resonator  112  and the second MEMS resonator  114  to determine if the temperature of the MEMS resonator group  110  is changed, and maintain the temperature of the MEMS resonator group  110  at the predetermined working temperature in accordance with the frequency difference. Since the second MEMS resonator  114  and the first MEMS resonator  112  can be fabricated in the same process, the temperature compensated oscillator  100  of the embodiments of the present invention has advantages of simple fabrication process and low cost. 
         [0035]    In addition, it is noted that the compensation value calculation operation  552  is performed by the temperature control unit  136 , but the embodiments of the present invention are not limited thereto. In other embodiments of the present invention, the counter  134  can be used to calculate the temperature compensation value, and to provide the temperature compensation value to the temperature control unit  136  to enable the temperature control unit  136  to calculate the first temperature control voltage V 1  and the second temperature control voltage V 2 . 
         [0036]    Referring to  FIG. 6A  and  FIG. 6B  simultaneously,  FIG. 6A  shows a functional diagram of a temperature compensated oscillator  600  in accordance with embodiments of the present invention, and  FIG. 6B  shows relationships between temperature and output frequencies of a MEMS resonator group  610  in accordance with the embodiments of the present invention, in which a curve C 3  represents a relationship between temperature and output frequency of a second MEMS resonator  614 . The temperature compensated oscillator  600  is similar to the oscillator  100 , but the difference is in that the oscillator  600  includes the MEMS resonator group  610  and a controller  630 . 
         [0037]    The MEMS resonator group  610  is similar to the MEMS resonator group  110 . The MEMS resonator group  610  includes the first MEMS resonator  112  and the second MEMS resonator  614 . The second MEMS resonator  614  is configured to output an auxiliary oscillation frequency f 3  and includes material having a positive TCF. As shown in  FIG. 6B , in this embodiment, the positive TCF material enables a slope of the curve C 3  be greater than that of the curve C 1  of the first MEMS resonator  112 . Therefore, it is represented that the temperature of the MEMS resonator group  610  is decreased when the frequency difference Δf is increased. In contrast, it is represented that the temperature of the MEMS resonator group  610  is increased when the frequency difference Δf is decreased. 
         [0038]    The controller  630  is similar the controller  130 , but the difference is in that the controller  630  controls the heating device  120  in different ways. In this embodiment, the controller  630  adjusts the temperature of the MEMS resonator group  610  by turning on or turning off the heating device  120 . For example, it is represented that the temperature of the MEMS resonator group  610  is too love when the difference between the main oscillation frequency f 1  and the auxiliary oscillation frequency f 3  is greater than the standard value of frequency difference, and thus the controller  630  turn on the heating device  120  for increasing the temperature of the MEMS resonator group  610 . For another example, it is represented that the temperature of the MEMS resonator group  610  is too high, when the difference between the main oscillation frequency f 1  and the auxiliary oscillation frequency f 3  is smaller than the standard value of frequency difference, and thus the controller  630  turn off the heating device  120  for decreasing the temperature of the MEMS resonator group  610 . 
         [0039]    It can be known from the above descriptions that temperature compensated oscillator  600  of the embodiments of the present invention adjusts the temperature of the MEMS resonator group  610  by turning on or turning off the heating device  120 . Compared with the control method of the oscillator  100 , the control method of the temperature compensated oscillator  600  is simpler. In addition, in the embodiments of the present invention, when the slope of the temperature-to-frequency curve of the first MEMS resonator is different from that of the second MEMS resonator, the temperature compensated oscillator of the embodiments of the present invention can use the frequency difference of the MEMS resonators to determine if the temperature of the MEMS resonator groups is increased or decreased, and to control the heating device accordingly. 
         [0040]    Although the present disclosure has been described above as in some embodiments, it is not used to limit the present disclosure. It will be intended to those skilled in the art that various modifications and variations can be made to the structure of the present disclosure without departing from the scope or spirit of the disclosure. Therefore, the scope of the disclosure is to be defined solely by the appended claims.